Inter-epidemic transmission of Rift Valley fever in livestock in the Kilombero River Valley, Tanzania: a cross-sectional survey

Widening geographic range of Rift Valley fever disease clusters associated with climate change in East Africa

BACKGROUND

Recent epidemiology of Rift Valley fever (RVF) disease in Africa suggests growing frequency and expanding geographic range of small disease clusters in regions that previously had not reported the disease. We investigated factors associated with the phenomenon by characterising recent RVF disease events in East Africa.

METHODS

Data on 100 disease events (2008-2022) from Kenya, Uganda and Tanzania were obtained from public databases and institutions, and modelled against possible geoecological risk factors of occurrence including altitude, soil type, rainfall/precipitation, temperature, normalised difference vegetation index (NDVI), livestock production system, land-use change and long-term climatic variations. Decadal climatic variations between 1980 and 2022 were evaluated for association with the changing disease pattern.

RESULTS

Of 100 events, 91% were small RVF clusters with a median of one human (IQR, 1-3) and three livestock cases (IQR, 2-7). These clusters exhibited minimal human mortality (IQR, 0-1), and occurred primarily in highlands (67%), with 35% reported in areas that had never reported RVF disease. Multivariate regression analysis of geoecological variables showed a positive correlation between occurrence and increasing temperature and rainfall. A 1°C increase in temperature and a 1-unit increase in NDVI, one months prior were associated with increased RVF incidence rate ratios of 1.20 (95% CI 1.1, 1.2) and 1.93 (95% CI 1.01, 3.71), respectively. Long-term climatic trends showed a significant decadal increase in annual mean temperature (0.12-0.3°C/decade, p<0.05), associated with decreasing rainfall in arid and semi-arid lowlands but increasing rainfall trends in highlands (p<0.05). These hotter and wetter highlands showed increasing frequency of RVF clusters, accounting for 76% and 43% in Uganda and Kenya, respectively.

CONCLUSION

These findings demonstrate the changing epidemiology of RVF disease. The widening geographic range of disease is associated with climatic variations, with the likely impact of wider dispersal of virus to new areas of endemicity and future epidemics.

Widening geographic range of Rift Valley fever disease clusters associated with climate change in East Africa

Background

Recent epidemiology of Rift Valley fever (RVF) disease in Africa suggests growing frequency and expanding geographic range of small disease clusters in regions that previously had not reported the disease. We investigated factors associated with the phenomenon by characterizing recent RVF disease events in East Africa.

Methods

Data on 100 disease events (2008 - 2022) from Kenya, Uganda, and Tanzania were obtained from public databases and institutions, and modeled against possible geo-ecological risk factors of occurrence including altitude, soil type, rainfall/precipitation, temperature, normalized difference vegetation index (NDVI), livestock production system, land-use change, and long-term climatic variations. Decadal climatic variations between 1980-2022 were evaluated for association with the changing disease pattern.

Results

Of 100 events, 91% were small RVF clusters with a median of one human (IQR, 1-3) and 3 livestock cases (IQR, 2-7). These clusters exhibited minimal human mortality (IQR 0-1), and occurred primarily in highlands (67%), with 35% reported in areas that had never reported RVF disease. Multivariate regression analysis of geo-ecological variables showed a positive correlation between occurrence and increasing temperature and rainfall. A 1oC increase in temperature and 1-unit increase in NDVI, 1-3 months prior were associated with increased RVF incidence rate ratios (IRR) of 1.20 (95% CI 1.1,1.2) and 9.88 (95% CI 0.85, 119.52), respectively. Long-term climatic trends showed significant decadal increase in annual mean temperature (0.12 to 0.3oC/decade, P<0.05), associated with decreasing rainfall in arid and semi-arid lowlands but increasing rainfall trends in highlands (P<0.05). These hotter and wetter highlands showed increasing frequency of RVF clusters, accounting for 76% and 43% in Uganda and Kenya, respectively.

Conclusion

These findings demonstrate the changing epidemiology of RVF disease. The widening geographic range of disease is associated with climatic variations, with the likely impact of wider dispersal of virus to new areas of endemicity and future epidemics.

Key questions

What is already known on this topic?: Rift Valley fever is recognized for its association with heavy rainfall, flooding, and El Niño rains in the East African region. A growing body of recent studies has highlighted a shifting landscape of the disease, marked by an expanding geographic range and an increasing number of small RVF clusters.What this study adds: This study challenges previous beliefs about RVF, revealing that it predominantly occurs in small clusters rather than large outbreaks, and its association with El Niño is not as pronounced as previously thought. Over 65% of these clusters are concentrated in the highlands of Kenya and Uganda, with 35% occurring in previously unaffected regions, accompanied by an increase in temperature and total rainfall between 1980 and 2022, along with a rise in the annual number of rainy days. Notably, the observed rainfall increases are particularly significant during the short-rains season (October-December), aligning with a secondary peak in RVF incidence. In contrast, the lowlands of East Africa, where typical RVF epidemics occur, display smaller and more varied trends in annual rainfall.How this study might affect research, practice, or policy: The worldwide consequence of the expanding RVF cluster is the broader dispersion of the virus, leading to the establishment of new regions with virus endemicity. This escalation heightens the risk of more extensive extreme-weather-associated RVF epidemics in the future. Global public health institutions must persist in developing preparedness and response strategies for such scenarios. This involves the creation and approval of human RVF vaccines and therapeutics, coupled with a rapid distribution plan through regional banks.

Seroprevalence of Rift Valley Fever virus in one-humped camels (Camelus dromedaries) in Egypt

Rift Valley fever (RVF) is a mosquito-borne viral disease that affects a variety of domestic animals, including cattle, sheep, goats, and camels, and has zoonotic potential. Although the rift valley fever virus (RVFV) is usually asymptomatic in camels, it can induce abortion in some pregnant animals. In the current study, a serosurvey was carried out to investigate the prevalence of RVFV antibodies and related risk factors in camels from four Egyptian governorates. A total of 400 serum samples were examined for anti-RVFV antibodies using a competitive enzyme-linked immunosorbent assay (c-ELISA). The results revealed that the overall prevalence of RVF among examined camels was 21.5% and the disease was more prevalent in Kafr ElSheikh governorate in Nile Delta of Egypt. In addition, the age group of camels with more than 5 years (OR=4.49, 95%CI: 1.39-14.49), the female sex (OR=3.38, 95%CI: 1.51-7.58), the emaciated animals (OR=1.52, 95%CI: 0.86-2.66), the summer season's infection (OR=5.98, 95%CI: 1.79-19.93), the presence of mosquitoes (OR= 2.88, 95%CI: 1.39-5.95), and the absence of mosquitoes control (OR=3.97, 95%CI: 2.09-7.57) were identified as risk factors for RVFV infection. The results of this study support knowledge on the risk factors for RVFV infection and demonstrate that camels raising in Egypt have RVFV antibodies. Quarantine measures or vaccination program should be implemented to reduce the likelihood of RVFV introduction, dissemination among susceptible animals, and ultimately transmission to humans.

Ecological and subject-level drivers of interepidemic Rift Valley fever virus exposure in humans and livestock in Northern Kenya

Nearly a century after the first reports of Rift Valley fever (RVF) were documented in Kenya, questions on the transmission dynamics of the disease remain. Specifically, data on viral maintenance in the quiescent years between epidemics is limited. We implemented a cross-sectional study in northern Kenya to determine the seroprevalence, risk factors, and ecological predictors of RVF in humans and livestock during an interepidemic period. Six hundred seventy-six human and 1,864 livestock samples were screened for anti-RVF Immunoglobulin G (IgG). Out of the 1,864 livestock samples tested for IgG, a subset of 1,103 samples was randomly selected for additional testing to detect the presence of anti-RVFV Immunoglobulin M (IgM). The anti-RVF virus (RVFV) IgG seropositivity in livestock and humans was 21.7% and 28.4%, respectively. RVFV IgM was detected in 0.4% of the livestock samples. Participation in the slaughter of livestock and age were positively associated with RVFV exposure in humans, while age was a significant factor in livestock. We detected significant interaction between rainfall and elevation's influence on livestock seropositivity, while in humans, elevation was negatively associated with RVF virus exposure. The linear increase of human and livestock exposure with age suggests an endemic transmission cycle, further corroborated by the detection of IgM antibodies in livestock.

Contemporary epidemiological data of Rift Valley fever virus in humans, mosquitoes and other animal species in Africa: A systematic review and meta-analysis

Rift Valley fever (RVF) is a severe zoonotic mosquito-borne disease that represents an important threat to human and animal health, with major public health and socioeconomic impacts. This disease is endemic throughout many African countries and the Arabian Peninsula. This systematic review with meta-analysis was conducted to determine the RVF prevalence in humans, mosquitoes and other animal species in Africa. The review also provides contemporary data on RVF case fatality rate (CFR) in humans. In this systematic review with meta-analysis, a comprehensive literature search was conducted on the PubMed, Embase, Web of Science and Global Index Medicus databases from January 2000 to June 2022 to identify relevant studies. Pooled CFR and prevalence estimates were calculated using the random-effects model. Subgroup analysis and sensitivity analysis were performed, and the I2 -statistic was used to investigate a potential source of heterogeneity. A total of 205 articles were included in the final analysis. The overall RVF CFR in humans was found to be 27.5% [95% CI = 8.0-52.5]. The overall pooled prevalence was 7.8% [95% CI = 6.2-9.6] in humans and 9.3% [95% CI = 8.1-10.6] in animals, respectively. The RVF prevalence in individual mosquitoes ranged from 0.0% to 25%. Subgroup analysis showed substantial heterogeneity with respect to geographical regions and human categories. The study shows that there is a correspondingly similar prevalence of RVF in human and animals; however, human CFR is much higher than the observed prevalence. The lack of a surveillance programme and the fact that this virus has subclinical circulation in animals and humans could explain these observations. The implementation of a One Health approach for RVF surveillance and control would be of great interest for human and animal health.

Evidence of Rift Valley Fever Virus Circulation in Livestock and Herders in Southern Ghana

Rift Valley fever (RVF) is a re-emerging zoonotic disease of domestic ruminants and humans. While neighbouring countries have reported outbreaks of RVF, Ghana has not yet identified any cases. The aim of this study was to determine whether RVF virus (RVFV) was circulating in livestock and herders in the southern part of Ghana, to estimate its seroprevalence, and to identify associated risk factors. The study surveyed 165 livestock farms randomly selected from two districts in southern Ghana. Serum samples of 253 goats, 246 sheep, 220 cattle, and 157 herdsmen were tested to detect IgG and IgM antibodies against RVFV. The overall seroprevalence of anti-RVF antibodies in livestock was 13.1% and 30.9% of farms had RVFV seropositive animals. The species-specific prevalence was 24.1% in cattle, 8.5% in sheep, and 7.9% in goats. A RVFV IgG seroprevalence of 17.8% was found among the ruminant herders, with 8.3% of all herders being IgM positive. RVFV was shown, for the first time, to have been circulating in southern Ghana, with evidence of a recent outbreak in Kwahu East; however, it was clinically undetected despite significant recent human exposure. A One Health approach is recommended to better understand RVF epidemiology and socio-economic impact in Ghana.

Small-scale field evaluation of transfluthrin-treated eave ribbons and sandals for the control of malaria vectors in rural Tanzania

BACKGROUND

Early-evening and outdoor-biting mosquitoes may compromise the effectiveness of frontline malaria interventions, notably insecticide-treated nets (ITNs). This study aimed to evaluate the efficacy of low-cost insecticide-treated eave ribbons and sandals as supplementary interventions against indoor-biting and outdoor-biting mosquitoes in south-eastern Tanzania, where ITNs are already widely used.

METHODS

This study was conducted in three villages, with 72 households participating (24 households per village). The households were divided into four study arms and assigned: transfluthrin-treated sandals (TS), transfluthrin-treated eave ribbons (TER), a combination of TER and TS, or experimental controls. Each arm had 18 households, and all households received new ITNs. Mosquitoes were collected using double net traps (to assess outdoor biting), CDC light traps (to assess indoor biting), and Prokopack aspirators (to assess indoor resting). Protection provided by the interventions was evaluated by comparing mosquito densities between the treatment and control arms. Additional tests were done in experimental huts to assess the mortality of wild mosquitoes exposed to the treatments or controls.

RESULTS

TERs reduced indoor-biting, indoor-resting and outdoor-biting Anopheles arabiensis by 60%, 73% and 41%, respectively, while TS reduced the densities by 18%, 40% and 42%, respectively. When used together, TER & TS reduced indoor-biting, indoor-resting and outdoor-biting An. arabiensis by 53%, 67% and 57%, respectively. Protection against Anopheles funestus ranged from 42 to 69% with TER and from 57 to 74% with TER & TS combined. Mortality of field-collected mosquitoes exposed to TER, TS or both interventions was 56-78% for An. arabiensis and 47-74% for An. funestus.

CONCLUSION

Transfluthrin-treated eave ribbons and sandals or their combination can offer significant household-level protection against malaria vectors. Their efficacy is magnified by the transfluthrin-induced mortality, which was observed despite the prevailing pyrethroid resistance in the study area. These results suggest that TER and TS could be useful supplementary tools against residual malaria transmission in areas where ITN coverage is high but additional protection is needed against early-evening and outdoor-biting mosquitoes. Further research is needed to validate the performance of these tools in different settings, and assess their long-term effectiveness and feasibility for malaria control.

Seroprevalence and Associated Risk Factors of Rift Valley Fever in Livestock from Three Ecological Zones of Malawi

The epidemiology of Rift Valley fever (RVF) is poorly understood in Malawi. Here, a cross-sectional study was conducted (March-June 2020) to investigate the seroprevalence and potential risk factors of RVF virus (RVFV) in cattle, goats, and sheep in three ecological zones of Malawi. A total of 1523 serum samples were tested for anti-RVFV IgG and IgM antibodies by ELISA. Additionally, a questionnaire survey was used to assess potential RVF risk factors. The overall seroprevalence was 17.14% (261/1523; 95% CI = 15.33-19.11) for individual livestock and 33.24% (120/361; 95% CI = 28.18-38.11) for the livestock herd. Seroprevalence was significantly high in sheep (25.68%, 95% CI = 19.31-33.26) compared with cattle (21.35%, 95% CI = 18.74-24.22) and goats (7.72%, 95% CI = 5.72-10.34), (p = 0.047). At the individual livestock level, the risk was elevated in female livestock (OR: 1.74, 95% CI = 1.08-12.82) (p = 0.016), while at the herd level, areas receiving approximately 1001-1500 mm of rainfall (OR: 2.47, 95% CI = 1.14-5.37) (p = 0.022), areas of rainfall amount greater than approximately 1600 mm (OR: 2.239, 95% CI = 1.07-8.82) (p = 0.023), and mixed species herds (OR: 10.410, 95% CI = 3.04-35.59) (p = 0.001), were significant risk factors. The detection of IgM antibodies confirmed active circulation of RVFV in Malawi. Therefore, monitoring of RVF in animals, humans, and vectors using a "One Health" approach, along with community sensitization among the high-risk populations, could help mitigate the threat posed by this zoonotic disease in Malawi.

Inter-epidemic Rift Valley fever virus infection incidence and risks for zoonotic spillover in northern Tanzania

Rift Valley fever virus (RVFV) is a mosquito-borne pathogen that has caused epidemics involving people and animals across Africa and the Arabian Peninsula. A number of studies have found evidence for the circulation of RVFV among livestock between these epidemics but the population-level incidence of infection during this inter-epidemic period (IEP) is rarely reported. General force of infection (FOI) models were applied to age-adjusted cross-sectional serological data to reconstruct the annual FOI and population-level incidence of RVFV infection among cattle, goats, and sheep in northern Tanzania from 2009 through 2015, a period without reported Rift Valley fever (RVF) cases in people or animals. To evaluate the potential for zoonotic RVFV spillover during this period, the relationship between village-level livestock RVFV FOI and human RVFV seropositivity was quantified using multi-level logistic regression. The predicted average annual incidence was 72 (95% Credible Interval [CrI] 63, 81) RVFV infections per 10,000 animals and 96 (95% CrI 81, 113), 79 (95% CrI 62, 98), and 39 (95% CrI 28, 52) per 10,000 cattle, sheep, and goats, respectively. There was variation in transmission intensity between study villages, with the highest estimated village-level FOI 2.49% (95% CrI 1.89, 3.23) and the lowest 0.12% (95% CrI 0.02, 0.43). The human RVFV seroprevalence was 8.2% (95% Confidence Interval 6.2, 10.9). Human seropositivity was strongly associated with the village-level FOI in livestock, with the odds of seropositivity in an individual person increasing by around 1.2 times (95% CrI 1.1, 1.3) for each additional annual RVFV seroconversion per 1,000 animals. A history of raw milk consumption was also positively associated with human seropositivity. RVFV has circulated at apparently low levels among livestock in northern Tanzania in the period since the last reported epidemic. Although our data do not allow us to confirm human RVFV infections during the IEP, a strong association between human seropositivity and the FOI in cattle, goats, and sheep supports the hypothesis that RVFV circulation among livestock during the IEP poses a risk for undetected zoonotic spillover in northern Tanzania. We provide further evidence for the likely role of raw milk consumption in RVFV transmission from animals to people.

A statistical calibration tool for methods used to sample outdoor-biting mosquitoes

Background

Improved methods for sampling outdoor-biting mosquitoes are urgently needed to improve surveillance of vector-borne diseases. Such tools could potentially replace the human landing catch (HLC), which, despite being the most direct option for measuring human exposures, raises significant ethical and logistical concerns. Several alternatives are under development, but detailed evaluation still requires common frameworks for calibration relative to HLC. The aim of this study was to develop and validate a statistical framework for predicting human-biting rates from different exposure-free alternatives.

Methods

We obtained mosquito abundance data (Anopheles arabiensis, Anopheles funestus and Culex spp.) from a year-long Tanzanian study comparing six outdoor traps [Suna Trap (SUN), BG Sentinel (BGS), M-Trap (MTR), M-Trap + CDC (MTRC), Ifakara Tent Trap-C (ITT-C) and Mosquito Magnet-X Trap (MMX)] and HLC. Generalised linear models were developed within a Bayesian framework to investigate associations between the traps and HLC, taking intra- and inter-specific density dependence into account. The best model was used to create a calibration tool for predicting HLC-equivalents.

Results

For An. arabiensis, SUN catches had the strongest correlation with HLC (R² = 19.4), followed by BGS (R² = 17.2) and MTRC (R² = 13.1) catches. The least correlated catch was MMX (R² = 2.5). For An. funestus, BGS had the strongest correlation with the HLC (R² = 53.4), followed by MTRC (R² = 37.4) and MTR (R² = 37.4). For Culex mosquitoes, the traps most highly correlated with the HLC were MTR (R² = 45.4) and MTRC (R² = 44.2). Density dependence, both between and within species, influenced the performance of only BGS traps. An interactive Shiny App calibration tool was developed for this and similar applications.

Conclusion

We successfully developed a calibration tool to assess the performance of different traps for assessing outdoor-biting risk, and established a valuable framework for estimating human exposures based on the trap catches. The performance of candidate traps varied between mosquito taxa; thus, there was no single optimum. Although all the traps tested underestimated the HLC-derived exposures, it was possible to mathematically define their representativeness of the true biting risk, with or without density dependence. The results of this study emphasise the need to aim for a consistent and representative sampling approach, as opposed to simply seeking traps that catch the most mosquitoes.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05403-7.

Seroepidemiology of selected transboundary animal diseases in goats in Zambia

Transboundary pathogens of goats present significant constraints to the livelihoods of millions of farmers in countries such as Zambia. Consequently, this study aimed to investigate the seroprevalence of Mycoplasma capricolum subsp. capripneumoniae (Mccp), foot and mouth disease virus (FMDV), Brucella spp., Crimean Congo haemorrhagic fever virus (CCHFV), and Rift Valley fever virus (RVFV) in Zambian goats. Another aim was to identify associations between seroprevalence and different predictor variables, such as trade and border proximity. From September to October 2019, 962 serum samples were collected from goats in seven Zambian districts, four of which have an international border while the remaining three do not. A questionnaire survey was conducted with each household, focusing on trade routines, management strategies and herd disease history. Animal-level seroprevalence adjusted for herd-level clustering was 8.2 % (95 % confidence interval [CI] 7.5-9.0) for Mccp, 12.9% (95% CI 12.0-13.7) for FMDV, 13.0 % (95% CI 12.1-13.9) for Brucella spp., 3.3 % (95% CI 2.8-3.7) for CCHFV, and 0.4 % (95 % CI 0.3-0.7) for RVFV. The association between herd-level seroprevalence and border proximity and trade appeared negligible, with the exception of selling goats at least twice a year which was identified as a potential risk factor for Brucella spp. (OR 4.1, 95 % CI 1.1-16.0, p = 0.040). In addition, a positive association between herd-level seroprevalence of FMDV and a herd size of 21 goats or more (OR 3.3, 95 % CI 1.0-11.1, p = 0.049) was detected. Also, positive associations between animal-level seroprevalence of Brucella spp. and increasing age (OR 7.7, 95 % CI 1.5-40.7, p = 0.016), and CCHFV and keeping pigs in the household (OR 2.7, 95 % CI 1.0-7.1, p = 0.044), were found. For FMDV (OR 3.8, 95 % CI 1.4-10.9, p = 0.011) and Brucella spp. (OR 4.5, 95 % CI 1.2-17.3, p = 0.031) on the other hand, animal-level seroprevalence was significantly higher in households without pigs. To the best of the authors' knowledge, this is the first study to describe the presence of antibodies for CCPP and CCHF in the Zambian goat population. While the association between seroprevalence and trade and border proximity generally appeared negligible, it is recommended that their influence is further evaluated in future studies, preferably through in-depth longitudinal studies incorporating impacts of different biosecurity measures and trade variations, linked to for example seasonality and trade peaks.

An unusually long Rift valley fever inter-epizootic period in Zambia: Evidence for enzootic virus circulation and risk for disease outbreak

Rift valley fever (RVF) is a mosquito-borne disease of animals and humans. Although RVF outbreaks are usually reported at 5-15-year intervals in sub-Saharan Africa, Zambia has experienced an unusually long inter-epizootic/-epidemic period of more than three decades. However, serological evidence of RVF virus (RVFV) infection in domestic ruminants during this period underscores the need for comprehensive investigation of the mechanisms of virus perpetuation and disease emergence. Mosquitoes (n = 16,778) captured from eight of the ten provinces of Zambia between April 2014 and May 2019 were pooled (n = 961) and screened for RVFV genome by a pan-phlebo RT-PCR assay. Aedes mosquito pools (n = 85) were further screened by nested RT-PCR assay. Sera from sheep (n = 13), goats (n = 259) and wild ungulates (n = 285) were screened for RVFV antibodies by ELISA while genome detection in pooled sera (n = 276) from domestic (n = 248) and wild ungulates (n = 37) was performed by real-time RT-PCR assay. To examine the association between the long inter-epizootic period and climatic variables, we examined El Niño-Southern Oscillation indices, precipitation anomalies, and normalized difference vegetation index. We then derived RVF risk maps by exploring climatic variables that would favor emergence of primary RVFV vectors. While no RVFV genome could be detected in pooled mosquito and serum samples, seroprevalence was significantly high (OR = 8.13, 95% CI [4.63–14.25]) in wild ungulates (33.7%; 96/285) compared to domestic ruminants (5.6%; 16/272). Retrospective analysis of RVF epizootics in Zambia showed a positive correlation between anomalous precipitation (La Niña) and disease emergence. On risk mapping, whilst northern and eastern parts of the country were at high risk, domestic ruminant population density was low (< 21 animals/km²) in these areas compared to low risk areas (>21 animals/km²). Besides evidence of silent circulation of RVFV and the risk of disease emergence in some areas, wildlife may play a role in the maintenance of RVFV in Zambia.

Rift valley fever (RVF) is an important mosquito borne disease of humans and ruminants. RVF is endemic in sub-Saharan Africa where disease outbreaks are frequently reported in humans and animals. For RVF outbreaks to occur, there has to be presence of the host (animals or humans) and favorable climatic conditions that favor emergence of RVF virus vectors (mosquitoes). Understanding this complex interaction enables us to predict future disease outbreaks and map RVF high risk areas. Above normal rainfall and floods will usually trigger RVF outbreaks. In this study, we conducted surveillance for RVF in mosquitoes, domestic and wild ruminants. We then examined climatic conditions that may have been responsible for past RVF outbreaks and the unusually (>30 years) long inter-epizootic period. Finally, we derived RVF risk maps for Zambia using a combination of climatic variables. We found evidence of RVF infection in both wild and domestic ruminants while there was a positive correlation between past RVF outbreaks and above-normal rainfall. On risk mapping, we found favorable conditions (i.e. high rainfall and risk of flooding) for RVF emergence in northern and eastern parts of the country. Studies like this are important in planning surveillance and disease control programs.

Crossing the Line: Seroprevalence and Risk Factors for Transboundary Animal Diseases Along the Tanzania-Zambia Border

Transboundary pathogens pose a threat to livelihood security in countries such as Zambia and Tanzania. This study aimed to investigate the seroprevalence of peste des petits ruminants virus (PPRV), foot and mouth disease virus (FMDV), sheep and goat pox virus (SGPV), Rift Valley fever virus (RVFV) and Brucella spp. in sheep and goats along the Tanzania-Zambia border. Another aim was to assess the association between certain predictor variables and seroprevalence, focusing on trade and proximity to an international border, to a town and to the Tanzania-Zambia highway. During September-October 2018, 486 serum samples from small ruminants in Zambia and 491 in Tanzania were collected and analyzed using enzyme-linked immunosorbent assays (ELISA). A questionnaire focused on management strategies was administered to each household. The animal-level seroprevalence in Zambia was 0.21% [95% confidence interval (CI) (0.01–1.14) for PPRV, 1.03% (95% CI 0.33–2.39) for FMDV, 0% (95% CI 0–0.76) for SGPV, 2.26% (95% CI 1.14–4.01) for RVFV and 1.65% (95% CI 0.71–3.22) for Brucella spp.]. In Tanzania, animal-level seroprevalence was 2.85% (95% CI 1.57–4.74) for PPRV, 16.9% (95% CI 13.7–20.5) for FMDV, 0.20% (95% CI 0.01–1.13) for SGPV, 3.26% (95% CI 1.87–5.24) for RVFV and 20.0% (95% CI 14.5–26.5) for Brucella spp. For PPRV (OR 6.83, 95% CI 1.37–34.0, p = 0.019) and FMDV (OR 5.68, 95% CI 1.58–20.3, p = 0.008), herds situated more than 30 km from an international border were more likely to be seropositive, while being located 10–30 km (OR 4.43, 95% CI 1.22–16.1 p = 0.024) from a border was identified as a risk factor for Brucella spp. For FMDV (OR 79.2, 95% CI 4.52–1388.9, p = 0.003), being situated within 30 km from a town was associated with seropositivity. Furthermore, contact with wild ruminants (OR 18.2, 95% CI 1.36–244), and the presence of sheep in the household (OR 5.20, 95% CI 1.00–26.9, p = 0.049), was associated with seropositivity for PPRV, and FMDV. No significant associations between trade or distance to the Tan-Zam highway and seroprevalence were found. We recommend that the impact of trade and proximity to borders, towns and roads should be further evaluated in larger studies, ideally incorporating aspects such as temporal trade fluctuations.

Over 100 Years of Rift Valley Fever: A Patchwork of Data on Pathogen Spread and Spillover

During the past 100 years, Rift Valley fever virus (RVFV), a mosquito-borne virus, has caused potentially lethal disease in livestock, and has been associated with significant economic losses and trade bans. Spillover to humans occurs and can be fatal. Here, we combined data on RVF disease in humans (22 countries) and animals (37 countries) from 1931 to 2020 with seroprevalence studies from 1950 to 2020 (n = 228) from publicly available databases and publications to draw a more complete picture of the past and current RVFV epidemiology. RVFV has spread from its original locus in Kenya throughout Africa and into the Arabian Peninsula. Throughout the study period seroprevalence increased in both humans and animals, suggesting potentially increased RVFV exposure. In 24 countries, animals or humans tested positive for RVFV antibodies even though outbreaks had never been reported there, suggesting RVFV transmission may well go unnoticed. Among ruminants, sheep were the most likely to be exposed during RVF outbreaks, but not during periods of cryptic spread. We discuss critical data gaps and highlight the need for detailed study descriptions, and long-term studies using a one health approach to further convert the patchwork of data to the tale of RFV epidemiology.

Prioritizing smallholder animal health needs in East Africa, West Africa, and South Asia using three approaches: Literature review, expert workshops, and practitioner surveys

Managing the health needs of livestock contributes to reducing poverty and improving the livelihoods of smallholder and pastoralist livestock keepers globally. Animal health practitioners, producers, policymakers, and researchers all must prioritize how to mobilize limited resources. This study employed three approaches to prioritize animal health needs in East and West Africa and South Asia to identify diseases and syndromes that impact livestock keepers. The approaches were a) systematic literature review, b) a series of expert workshops, and c) a practitioner survey of veterinarians and para-veterinary professionals. The top constraints that emerged from all three approaches include endo/ ectoparasites, foot and mouth disease, brucellosis, peste des petits ruminants, Newcastle disease, and avian influenza. Expert workshops additionally identified contagious caprine pleuropneumonia, contagious bovine pleuropneumonia, mastitis, and reproductive disorders as constraints not emphasized in the literature review. Practitioner survey results additionally identified nutrition as a constraint for smallholder dairy and pastoralist small ruminant production. Experts attending the workshops agreed most constraints can be managed using existing veterinary technologies and best husbandry practices, which supports a shift away from focusing on individual diseases and new technologies towards addressing systemic challenges that limit access to veterinary services and inputs. Few research studies focused on incidence/ prevalence of disease and impact, suggesting better incorporation of socio-economic impact measures in future research would better represent the interests of livestock keepers.

Fine-scale distribution of malaria mosquitoes biting or resting outside human dwellings in three low-altitude Tanzanian villages

BACKGROUND

While malaria transmission in Africa still happens primarily inside houses, there is a substantial proportion of Anopheles mosquitoes that bite or rest outdoors. This situation may compromise the performance of indoor insecticidal interventions such as insecticide-treated nets (ITNs). This study investigated the distribution of malaria mosquitoes biting or resting outside dwellings in three low-altitude villages in south-eastern Tanzania. The likelihood of malaria infections outdoors was also assessed.

METHODS

Nightly trapping was done outdoors for 12 months to collect resting mosquitoes (using resting bucket traps) and host-seeking mosquitoes (using odour-baited Suna® traps). The mosquitoes were sorted by species and physiological states. Pooled samples of Anopheles were tested to estimate proportions infected with Plasmodium falciparum parasites, estimate proportions carrying human blood as opposed to other vertebrate blood and identify sibling species in the Anopheles gambiae complex and An. funestus group. Environmental and anthropogenic factors were observed and recorded within 100 meters from each trapping positions. Generalised additive models were used to investigate relationships between these variables and vector densities, produce predictive maps of expected abundance and compare outcomes within and between villages.

RESULTS

A high degree of fine-scale heterogeneity in Anopheles densities was observed between and within villages. Water bodies covered with vegetation were associated with 22% higher densities of An. arabiensis and 51% lower densities of An. funestus. Increasing densities of houses and people outdoors were both associated with reduced densities of An. arabiensis and An. funestus. Vector densities were highest around the end of the rainy season and beginning of the dry seasons. More than half (14) 58.3% of blood-fed An. arabiensis had bovine blood, (6) 25% had human blood. None of the Anopheles mosquitoes caught outdoors was found infected with malaria parasites.

CONCLUSION

Outdoor densities of both host-seeking and resting Anopheles mosquitoes had significant heterogeneities between and within villages, and were influenced by multiple environmental and anthropogenic factors. Despite the high Anopheles densities outside dwellings, the substantial proportion of non-human blood-meals and absence of malaria-infected mosquitoes after 12 months of nightly trapping suggests very low-levels of outdoor malaria transmission in these villages.

Sero-prevalence of brucellosis, Q-fever and Rift Valley fever in humans and livestock in Somali Region, Ethiopia

Information on zoonotic diseases in humans and livestock are limited in pastoral/agro-pastoral communities in Ethiopia. A multi-stage cross sectional cluster design study was implemented with the aim to establish the seroprevalence of zoonotic diseases including brucellosis, Q-fever and Rift Valley fever (RVF) in humans and livestock in Adadle Woreda of the Somali Region, Ethiopia. Blood samples were collected from humans and livestock and tested by relevant serological tests. For brucellosis, Rose Bengal test (RBT) and indirect ELISA was used for screening and confirmatory diagnosis respectively. Indirect and competitive ELISA were also used for Q-fever and RVF respectively. The individual seropositivity of Q-fever in livestock was 9.6% (95% CI 5.9-15.1) in cattle, 55.7% (95% CI 46.0-65.0) in camels, 48.8% (95% CI 42.5-55.0) in goats, and 28.9% (95% CI 25.0-33.2) in sheep. In humans, seropositivity of Q-fever was 27.0% (95% CI 20.4-34.0), with prevalence in males of 28.9% vs 24.2% in females (OR = 1.3; 95% CI 0.6-2.5). Camel seropositivity of Q-fever was significantly associated with age (OR = 8.1; 95% CI 2.8-23.7). The individual apparent seroprevalence of RVF was 13.2% (95% CI 8.7-18.8) in humans, 17.9% (95% CI 11.0-27.8) in cattle, 42.6% (95% CI 34.8-50.7) in camels, 6.3% (95% CI 3.3-11.6) in goats and 7.4% (95% CI 4.7-11.5) in sheep. Camels had the highest seropositivity of both Q-fever and RVF. Only a weak correlation was observed between human and livestock seropositivity for both Q-fever and RVF. Only cattle and camels were seropositive for brucellosis by iELISA. The individual seroprevalence of brucellosis was 2.8(0.9-6.4) in humans, 1.5% (95% CI 0.2-5.2) in cattle and 0.6% (95% CI 0.0-3.2) in camels. This study showed the importance of zoonoses in Somali Region and is the first published study to describe RVF exposure in humans and livestock in the country. Even though human exposure to RVF virus was reported, public health sector of Somali Region has not given attention to such zoonoses. Collaboration between public and animal health sectors for further investigation on these zoonoses using the One Health concept is indispensable.

DNA barcoding of mosquitoes collected through a nationwide survey in 2011 and 2012 in Malawi, Southeast Africa

We conducted a nationwide survey of mosquito distribution in Malawi from November 2011 to April 2012, and from July to September 2012. Using dried specimens of mosquito adults collected during the survey, we analyzed their cytochrome c oxidase subunit I (COI) gene sequences, prepared specimens, and registered the genetic information (658 bp) of 144 individuals belonging to 51 species of 10 genera in GenBank. Using the obtained genetic information, we analyzed the degree of intraspecific variation and investigated the various species from morphological and genetic perspectives. Moreover, we conducted phylogenetic analysis of the medically important species distributed from Africa to Asia and explored their geographical differentiation. Results showed that individuals morphologically classified as Culex univittatus complex included a individual of Cx. perexiguus which, to date, have not been reported in southern Africa. Furthermore, Mansonia uniformis, distributed in Africa and Asia, was revealed to belong to genetically distinct populations, with observed morphological differences of the samples suggesting that they are separate species. The results of genetic analysis further suggested that Cx. ethiopicus is not a synonym of Cx. bitaeniorhynchus, but that it is an independent species; although, in this study, the only definite morphological difference observed was in the shape of the wing scales. Further morphological and genetic investigation of individuals of these species, including larvae, is highly recommended.

Detection of Rift Valley Fever virus inter-epidemic activity in Kilimanjaro Region, North Eastern Tanzania

Background

Rift Valley Fever virus (RVFV) is a zoonotic arbovirus of public health impact infecting livestock, wildlife, and humans mainly in Africa and other parts of the world. Despite its public health importance, mechanisms of RVFV maintenance during interepidemic periods (IEPS) remain unclear.

Objective

We aimed to examine comparatively exposure to RVFV between humans and goats and RVFV infection between humans, goats and mosquitoes.

Methods

A cross sectional study was performed in the Lower Moshi area of the Kilimanjaro region from March to June 2020. RVFV exposure was determined by detecting IgG/IgM to RVFV using a competitive enzyme linked immunosorbent assay whereas infection was determined by real time quantitative polymerase chain reaction (RT-qPCR) assay.

Results

Results show that the male gender was related to RVFV seropositivity (χ² = 5.351; p=0.030). Being 50 years and above was related to seropositivity (χ² =14.430; p=0.006) whereas bed net use, larger numbers of persons living in the same house (>7 persons) and RVFV seropositivity in goats were related to higher seropositivity to RVFV among humans χ² =6.003; p=0.021, χ² =23.213; p < 0.001 and χ² =27.053; p < 0.001), respectively. By the use of RT-qPCR, goats exhibited the highest RVFV infection rate of 4.1%, followed by humans (2.6%), Ae. aegypti (2.3%), and Cx. pipiens complex(1.5%). Likewise, a higher proportion of goats (23.3%) were RVFV seropositive as compared with humans (13.2%).

Conclusion

Our findings suggest the Lower Moshi area as a potential hotspot for Rift Valley Fever (RVF), posing the danger of being a source of RVFV spread to other areas. Goats had the highest infection rate, suggesting goats as important hosts for virus maintenance during IEPs. We recommend the implementation of strategies that will warrant active RVF surveillance through the identification of RVF hotspots for targeted control of the disease.

Seroprevalence of Rift Valley Fever and West Nile Fever in Cattle in Gambella Region, South West Ethiopia

INTRODUCTION

Rift Valley fever (RVF) and West Nile fever (WNF) are re-emerging mosquito-borne zoonotic diseases that cause public health and economic crises. Ethiopia shares borders with South Sudan and Kenya, where these diseases are often documented. The free movement of animals and humans across these borders expects to increase the spread of these diseases. The current study was conducted to assess the occurrence of these diseases in the Gambella region of Ethiopia.

METHODOLOGY

We collected a total of 368 cattle serum samples from the Lare district on the border of South Sudan and measured the presence of IgG antibody against RVF and WNF virus infections using enzyme-linked immunosorbent assays (ELISA).

RESULTS

The prevalence of anti-RVF virus IgG antibody was 7.6% (95% CI: 5.3-10.82%), while that of anti-WNF virus IgG antibody was 5.4% (95% CI: 3.52-8.29%). In this study higher seroprevalence of IgG antibodies to RVF virus infection was observed comparing to the WNF virus in cattle. There was no significant association between the prevalence and the cattle age, sex or sampled locations.

CONCLUSION

The detection of IgG antibody to RVF and WNF virus infections in the Gambella region warrants further study of active case findings and the dynamics of transmission.

Serosurvey on Sheep Unravel Circulation of Rift Valley Fever Virus in Nigeria

Rift Valley fever is an arboviral zoonoses causing severe morbidity and mortality among humans and animals in many African countries. A cross-sectional study in populations of sheep reared around the Gidan-Waya Forest Reserve located in Jema'a LGA of Kaduna State, Nigeria to determine the serological evidence of exposure to Rift Valley fever virus (RVFV) using a commercial competitive enzyme-linked immunosorbent assay. Of the 200 sheep sampled, 9 (4.5%; 95 CI 2.23-8.33) were positive for antibodies to the RVFV. The detection of antibodies suggests a covert circulation among the sheep and may be indicative of a subclinical infection.

Seropositivity and associated intrinsic and extrinsic factors for Rift Valley fever virus occurrence in pastoral herds of Nigeria: a cross sectional survey

Background

Rift Valley fever (RVF) is a vector-borne emerging zoonotic disease of animals and humans, characterized by socioeconomic losses to livestock farmers and global public health threat. The study determined RVFV seroprevalence in cattle, assessed pastoralists’ knowledge about RVF, and factors that influence its occurrence in pastoral cattle herds of Nigeria. A cross-sectional study was conducted in pastoral herds of North-central Nigeria from 2017 to 2018. Data were collected using serology and questionnaire tools. Descriptive statistics were used to analyze the obtained data. Categorical variables were presented as proportions and their associations determined by Chi-square tests. Associations of risk factors were analyzed by univariable and multivariable logistic regressions analyses at 95% confidence level.

Results

The overall IgM seropositivity of RVFV in pastoral cattle herds was 5.6%. This was higher in nomadic herds (7.4%) than in agro-pastoral herds (3.8%). All animal demographic characteristics of age, sex and breeds were not significantly (p > 0.05) associated with RVFV occurrence in pastoral herds. All the 403 pastoralists selected participated in the study, with the majorities of them being male, married and have no formal education. Majority of the pastoralists had low knowledge levels about zoonotic RVFV infection. All identified socio-ecological factors significantly (p < 0.05) influenced RVFV occurrence in herds. Mosquitoes availability in cattle environment (OR = 7.81; 95% CI: 4.85, 12.37), presence of rivers and streams at grazing fields (OR = 10.80; 95% CI: 6.77, 17.34), high rainfall (OR = 4.30; 95% CI: 2.74, 6.59), irrigated rice fields (OR = 5.14; 95% CI: 3.21, 7.79), bushy vegetation (OR = 6.11; 95% CI: 3.96, 9.43), animal movement (OR = 2.2; 95% CI: 1.45, 3.25), and seasons (OR = 2.34; 95% CI: 1.55, 3.51) were more likely to influenced RVFV occurrence in cattle herds.

Conclusions

Results of this study had illustrated recent circulation of RVFV in pastoral cattle herds in Nigeria and needs urgent interventions. The surveyed pastoralists had low knowledge level about RVF while the socio-ecological factors significantly influenced RVFV occurrence in herds. To address these gaps, pastoralists should be educated on clinical manifestations and modes of transmission of the disease in animals and humans, and mitigation measures. Adequate knowledge about RVF epidemiology will assure food security and public health.

Rift Valley fever in northern Senegal: A modelling approach to analyse the processes underlying virus circulation recurrence

Rift Valley fever (RVF) is endemic in northern Senegal, a Sahelian area characterized by a temporary pond network that drive both RVF mosquito population dynamics and nomadic herd movements. To investigate the mechanisms that explain RVF recurrent circulation, we modelled a realistic epidemiological system at the pond level integrating vector population dynamics, resident and nomadic ruminant herd population dynamics, and nomadic herd movements recorded in Younoufere area. To calibrate the model, serological surveys were performed in 2015–2016 on both resident and nomadic domestic herds in the same area. Mosquito population dynamics were obtained from a published model trained in the same region. Model comparison techniques were used to compare five different scenarios of virus introduction by nomadic herds associated or not with vertical transmission in Aedes vexans. Our serological results confirmed a long lasting RVF endemicity in resident herds (IgG seroprevalence rate of 15.3%, n = 222), and provided the first estimation of RVF IgG seroprevalence in nomadic herds in West Africa (12.4%, n = 660). Multivariate analysis of serological data suggested an amplification of the transmission cycle during the rainy season with a peak of circulation at the end of that season. The best scenario of virus introduction combined yearly introductions of RVFV from 2008 to 2015 (the study period) by nomadic herds, with a proportion of viraemic individuals predicted to be larger in animals arriving during the 2^nd half of the rainy season (3.4%). This result is coherent with the IgM prevalence rate (4%) found in nomadic herds sampled during the 2^nd half of the rainy season. Although the existence of a vertical transmission mechanism in Aedes cannot be ruled out, our model demonstrates that nomadic movements are sufficient to account for this endemic circulation in northern Senegal.

Rift Valley fever (RVF) is one of the most important vector borne disease in Africa, seriously affecting the health of domestic ruminants and humans and leading to severe economic consequences. This disease is endemic in northern Senegal, a Sahelian area characterized by a temporary pond network that drive both RVF mosquito population dynamics and nomadic herd movements. Two non-exclusive mechanisms may support this endemicity: recurrent introductions of the virus by nomadic animals, and vertical transmission of the virus (i.e. from infected female mosquito to eggs) in local Aedes populations. The authors followed resident and nomadic domestic herds for 1 year. They used the data thus obtained to model a realistic epidemiological system at the pond level integrating vector population dynamics, resident and nomadic ruminant herd population dynamics. They found that the best scenario explaining RVF remanence combined yearly introductions of RVFV by nomadic herds, with a viraemic proportion predicted to be larger in animals arriving during the 2^nd half of the rainy season, which is consistent with an amplification of virus circulation in the area during the rainy season. Although the existence of a vertical transmission mechanism in Aedes cannot be ruled out, their results demonstrates that nomadic movements are sufficient to account for this endemic circulation in northern Senegal.

Rift Valley Fever: risk of persistence, spread and impact in Mayotte (France)

Rift Valley fever (RVF) is a vector-borne disease transmitted by different mosquito species, especially Aedes and Culex genus, to animals and humans. In November 2018, RVF re-emerged in Mayotte (France) after 11 years. Up to the end of October 2019, 126 outbreaks in animals and 143 human cases were reported. RVF mortality was 0.01%, and the number of abortions reported in polymerase chain reaction (PCR)-positive ruminants was fivefold greater than the previous 7 years. Milk loss production in 2019 compared to 2015-2018 was estimated to be 18%, corresponding to an economic loss of around €191,000 in all of Mayotte. The tropical climate in Mayotte provides conditions for the presence of mosquitoes during the whole year, and illegal introductions of animals represent a continuous risk of (re)introduction of RVF. The probability of RVF virus (RVFV) persisting in Mayotte for 5 or more years was estimated to be < 10% but could be much lower if vertical transmission in vectors does not occur. Persistence of RVF by vertical transmission in Mayotte and Réunion appears to be of minor relevance compared to other pathways of re-introduction (i.e. animal movement). However, there is a high uncertainty since there is limited information about the vertical transmission of some of the major species of vectors of RVFV in Mayotte and Réunion. The only identified pathways for the risk of spread of RVF from Mayotte to other countries were by infected vectors transported in airplanes or by wind currents. For the former, the risk of introduction of RVF to continental France was estimated to 4 × 10-6 epidemic per year (median value; 95% CI: 2 × 10-8; 0.0007), and 0.001 epidemic per year to Réunion (95% CI: 4 × 10-6; 0.16). For the latter pathway, mosquitoes dispersing on the wind from Mayotte between January and April 2019 could have reached the Comoros Islands, Madagascar, Mozambique and, possibly, Tanzania. However, these countries are already endemic for RVF, and an incursion of RVFV-infected mosquitoes would have negligible impact.

Patterns of Rift Valley fever virus seropositivity in domestic ruminants in central South Africa four years after a large outbreak

Rift Valley fever (RVF) is a mosquito-borne viral zoonosis showing complex epidemiological patterns that are poorly understood in South Africa. Large outbreaks occur in the central interior at long, irregular intervals, most recently in 2010–2011; however, the level of herd immunity of ruminant livestock, a key determinant of outbreaks, is unknown. During 2015–2016 a cross-sectional study on 234 randomly-selected farms investigated the prevalence, patterns of, and factors associated with, antibodies to RVF virus (RVFV) in livestock in an area heavily affected by that outbreak. A RVFV inhibition ELISA was used to screen 977 cattle, 1,549 sheep and 523 goats and information on potential risk factors was collected using a comprehensive questionnaire. The estimated RVFV seroprevalence, adjusted for survey design, was 42.9% in cattle, 28.0% in sheep and 9.3% in goats, showing a high degree of farm-level clustering. Seroprevalence increased with age and was higher on private vs. communal land, on farms with seasonal pans (temporary, shallow wetlands) and perennial rivers and in recently vaccinated animals. Seropositivity amongst unvaccinated animals born after the last outbreak indicates likely viral circulation during the post-epidemic period. The current level of herd immunity in livestock may be insufficient to prevent another large outbreak, should suitable conditions recur.

Circulation of Rift Valley Fever Virus Antibody in Cattle during Inter-Epizootic/Epidemic Periods in Selected Regions of Tanzania

Tanzania is one of the sub-Saharan countries that have experienced a number of Rift Valley fever (RVF) outbreaks at intervals of 10-20 years since the first isolation of the virus during the early 1930s. Recent studies have reported serological evidence of inter epizootic/epidemic period circulation of RVF virus (RVFV) in livestock and humans. The aim of this study was to conduct a cross-sectional survey in Tanzania during 2015/16 to further explore the possibility that RVFV was circulating among cattle during the Inter epizootic/epidemic period. A total of 443 cattle samples were collected in Manyara, Dodoma, Singida, and Mbeya regions of Tanzania. The samples were tested for RVFV antibodies using a commercial ELISA kit and a plaque reduction neutralization test. Serum samples were also tested for RVFV viral RNA by an reverse transcription polymerase chain reaction (RT-PCR) assay. An overall RVFV seroprevalence rate of 7.7% (34/443) was detected by ELISA among cattle in all four regions. The Mbeya region cattle had the highest seroprevalence of 26.4% (23/87), followed by Dodoma 5.9% (10/171) and lastly Singida 0.9% (1/101). Of 33 ELISA antibody-positive samples, only 0.2% (1/443) had IgM antibody. Of 36 ELISA antibody-positive and doubtful samples, 32 were positive for neutralizing antibody with titers between 10 and > 10,240. None of the samples were positive for RVFV viral RNA by RT-PCR. The detection of RVFV antibodies in cattle suggested that these animals were involved in an enzootic cycle during the interepidemic period and that the high antibody titers may confer protection of cattle against RVFV.

Mechanisms of inter-epidemic maintenance of Rift Valley fever phlebovirus

Rift Valley fever phlebovirus (RVFV) is an arthropod-borne virus that has caused substantial epidemics throughout Africa and in the Arabian Peninsula. The virus can cause severe disease in livestock and humans and therefore the control and prevention of viral outbreaks is of utmost importance. The epidemiology of RVFV has some particular characteristics. Unexpected and significant epidemics have been observed in spatially and temporally divergent patterns across the African continent. Sudden epidemics in previously unaffected areas are followed by periods of long-term apparent absence of virus and sudden, unpredictable reoccurrence in disparate regions. Therefore, the elucidation of underlying mechanisms of viral maintenance is one of the largest gaps in the knowledge of RVFV ecology. It remains unknown whether the virus needs to be reintroduced before RVF outbreaks can occur, or if unperceived viral circulation in local vertebrates or mosquitoes is sufficient for maintenance of the virus. To gain insight into these knowledge gaps, we here review existing data that describe potential mechanisms of RVFV maintenance, as well as molecular and serological studies in endemic and non-endemic areas that provide evidence of an inter- or pre-epidemic virus presence. Basic and country-specific mechanisms of RVFV introduction into non-endemic countries are summarized and an overview of studies using mathematical modeling of RVFV persistence is given.

Seroprevalence and Associated Risk Factors of Rift Valley Fever in Domestic Small Ruminants in the North Region of Cameroon

Rift Valley fever (RVF) is a zoonotic vector borne infectious disease of major medical and veterinary importance particularly in sub-Saharan Africa. However, there is dearth of epidemiological knowledge of the disease in Cameroon. We conducted a cross-sectional study (January 2016–January 2017) to investigate the seroprevalence and potential risk factors of Rift Valley fever virus (RVFV) in sheep and goats in the North region of Cameroon. Stratified sampling approach was used to select herds where sera were collected from 680 randomly selected small ruminants (355 goats and 325 sheep) in eight localities (Kismatari, Lagdo, Pitoa, Garoua, Bocklé, Dembo, Poli and Touboro) within three administrative divisions (Bénoué, Mayo-Rey and Faro) in the North region. Anti-RVFV antibodies were detected using a competitive Enzyme-Linked Immunosorbent Assay (ELISA), while a capture ELISA was used for the detection of specific RVFV-Immunoglobulin M (Ig-M) antibodies. We evaluated the associated potential risk factors of RVF in small ruminants based on observations of animal-related intrinsic and extrinsic factors in combination with serological results. The results revealed that 3.4% (95% confidence interval (CI): 2.2–5.1%) of sampled animals and 24.6% (95% CI: 15.1–37.1%) of 65 sampled herds were seropositive for anti-RVFV antibodies and no difference in seropositivity between sheep and goats at individual animal as well as at herd levels was observed. Localities along hydrographic or large water banks such as Kismatari (OR: 14.333, (95% CI: 1.436–145.088)) and Pitoa (OR = 11.467 (95% CI: 1.249–50.306)) were significantly associated to RVFV antibody seroprevalence in a simple logistic regression. In addition, the multiple regression analysis showed that age and access to water points significantly influenced RVFV antibody seroprevalence in small ruminants. This study revealed that anti-RVFV antibodies are present in sheep and goats in the North region of Cameroon. It highlights the likely endemic circulation of RVFV in the considered localities despite the absence of clinical cases reported in animals or humans. Under these conditions, it is necessary to set up an early warning, surveillance and control strategy based on epizootic risk.

Seroprevalence of Rift Valley fever in cattle of smallholder farmers in Kwilu Province in the Democratic Republic of Congo

Rift Valley fever (RVF) is a zoonotic mosquito-borne disease caused by RVF virus (RVFV) that causes abortions and high mortalities in livestock and is also associated with acute and fatal disease in humans. In the Democratic Republic of Congo (DRC), information on the epidemiology of RVF is limited, particularly among cattle reared by smallholder farmers. This cross-sectional study was conducted to investigate the seroprevalence of RVF in cattle raised by smallholder farmers in Kwilu Province of DRC, which has not yet reported an RVF epidemic. A total of 677 cattle sera were collected from four territories and tested for anti-RVFV antibodies using immunofluorescent assay and enzyme-linked immunosorbent assay. The overall seroprevalence of anti-RVFV IgG was 6.5% (44/677) (95% CI 4.81-8.7). There was a statistically significant difference in the seroprevalence among the territories (χ² = 28.79, p < 0.001). Territory seroprevalences were as follows: Idiofa 14.08% (95% CI 9.78-19.76), Bulungu 4.14% (95% CI 1.83-8.68), Gungu 3.21% (95% CI 1.41-6.78), and Masi-Manimba 1.19% (95% CI 0.06-7.37). Seroprevalence differed significantly among age categories (p = 0.0017) and ecosystem (p < 0.001). The seroprevalence of animals aged between 1 and 2 years was 20.0% (95% CI 8.4-39.13) and was higher than group aged <1 year, between 2 and 3 years, and > 3 years. Forest area (18.92% (95% CI 12.35-27.7)) had higher seropositivity than savannah area (4.06% (95% CI 2.65-6.12)). Sex difference was not significant (χ² = 0.14, p = 0.704). These findings indicate that cattle in Kwilu Province had been exposed to RVFV, which represents a significant risk for both livestock and human health.

Serological evidence of Rift Valley fever virus infection in slaughtered ruminants in Nigeria

The risk of exposure of slaughterhouse workers to Rift Valley fever (RVF) virus-infected animals in Nigeria was assessed by determining the prevalence of anti-RVF IgM in cattle, goats and sheep slaughtered in a major abattoir in Ibadan, Nigeria. Blood samples were collected from 290 animals in Bodija Municipal abattoir, Ibadan, Nigeria in January and February 2017 and analyzed for the presence of RVF virus using IgM Enzyme-Linked Immunosorbent Assay (ELISA) and Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) for detection of the virus RNA. Descriptive statistics was used to analyze data. Overall, an IgM prevalence of 0.7% (2/290) was found among the blood samples of the animals, suggesting recent exposure to the virus. Antibody was detected in the sera from a cow and one goat. RVF virus RNA was not detected in the 2 IgM positive blood samples. There was no statistically significant relationship between RVF IgM infection and some variables of the animals, including age, sex and breed (p ≥ 0.05). Results of this study indicate active RVF virus transmission in domestic livestock in Nigeria. The study emphasizes the need to embark on monitoring of human and animal populations to prevent outbreak of the virus in the country.

High seroconversion rate to Rift Valley fever virus in cattle and goats in far northern KwaZulu-Natal, South Africa, in the absence of reported outbreaks

Background

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease characterized in South Africa by large epidemics amongst ruminant livestock at very long, irregular intervals, mainly in the central interior. However, the presence and patterns of occurrence of the virus in the eastern parts of the country are poorly known. This study aimed to detect the presence of RVF virus (RVFV) in cattle and goats in far northern KwaZulu-Natal province and to estimate the prevalence of antibodies to the virus and the incidence rate of seroconversion.

Methodology

Cross-sectional studies were performed in communally farmed cattle (n = 423) and goats (n = 104), followed by longitudinal follow-up of seronegative livestock (n = 253) 14 times over 24 months, representing 160.3 animal-years at risk. Exposure to RVFV was assessed using an IgG sandwich ELISA and a serum neutralization test (SNT) and seroconversion was assessed using SNT. Incidence density was estimated and compared using multivariable Poisson models and hazard of seroconversion was estimated over time.

Principal findings

Initial overall seroprevalence was 34.0% (95%CI: 29.5–38.8%) in cattle and 31.7% (95%CI: 22.9–41.6%) in goats, varying by locality from 18–54%. Seroconversions to RVFV based on SNT were detected throughout the year, with the incidence rate peaking during the high rainfall months of January to March, and differed considerably between years. Overall seroconversion rate in cattle was 0.59 per animal-year (95% CI: 0.46–0.75) and in goats it was 0.41 per animal-year (95% CI: 0.25–0.64), varying significantly over short distances.

Conclusions/Significance

The high seroprevalence in all age groups and evidence of year-round viral circulation provide evidence for a hyperendemic situation in the study area. This is the first study to directly estimate infection rate of RVFV in livestock in an endemic area in the absence of reported outbreaks and provides the basis for further investigation of factors affecting viral circulation and mechanisms for virus survival during interepidemic periods.

Rift Valley fever (RVF) is a mosquito-transmitted viral disease that may cause large epidemics in domestic livestock and in humans. Although currently largely confined to Africa, it is of international concern due to its ability to spread and become established in areas where suitable mosquito vectors occur. Outbreaks occur sporadically, associated with conditions favourable for proliferation of mosquito populations, such as high rainfall and flooding, yet their location and timing remain difficult to predict. In other areas there is evidence that RVF virus is endemic and may circulate without causing outbreaks. However, the location and extent of such areas is poorly known, as is the transmission dynamics of the virus in those areas. In this paper, we report the existence of such an area of endemic RVF virus transmission on the tropical coastal plain of South Africa bordering Mozambique, where we found a high rate of exposure of domestic cattle and goats to the naturally circulating virus over a two-year period, with no outbreaks being reported. Research in such areas will help us to assess the potential for spread of the virus to other areas and also to better understand the behaviour of the virus during periods between epidemics.

Safety and immunogenicity of Rift Valley fever MP-12 and arMP-12ΔNSm21/384 vaccine candidates in goats (Capra aegagrus hircus) from Tanzania

Vaccination of domestic ruminants is considered to be an effective strategy for protecting these animals against Rift Valley fever (RVF), but available vaccines have limitations. Therefore, the aim of this study was to determine the safety and immunogenicity of RVF virus (RVFV) mutagenesis passage 12 (MP-12) and arMP-12ΔNSm21/384 vaccine candidates in goats (Capra aegagrus hircus) in Tanzania. Goats were vaccinated intramuscularly with RVFV MP-12 or arMP-12ΔNSm21/384, and then on Day 87 post-vaccination (PV) all animals were revaccinated using the RVFV MP-12 vaccine candidate. Serum samples were collected from the animals before and after vaccination at various intervals to test for RVFV using a Vero cell culture assay and reverse transcription polymerase chain reaction and for RVFV-neutralising antibody using a plaque reduction neutralisation assay. Serum samples collected before vaccination on Days -14 and 0, and on Days 3, 4 and 5 PV were negative for RVFV and neutralising antibody. All animals remained healthy, and viremia was not detected in any of the animals. Rift Valley fever virus antibody was first detected on Day 5 PV at a 1:10 dilution in five of five animals vaccinated with the MP-12 vaccine and in five of eight animals vaccinated with arMP-12ΔNSm21/384. Titres then increased and were sustained at 1:40 to 1:640 through to Day 87 PV. All animals that were revaccinated on Day 87 PV with MP-12 developed antibody titres ranging from 1:160 to as high as 1:10 240 on Days 14 and 21 PV. Although the antibody titres for goats vaccinated with RVF MP-12 were slightly higher than titres elicited by the arMP-12ΔNSm21/384 vaccine, these findings demonstrated that both vaccines are promising candidates for the prevention of RVF among Tansanian goats.

Rift Valley fever: An open-source transmission dynamics simulation model

Rift Valley fever (RVF) is one of the major viral zoonoses in Africa, affecting humans and several domestic animal species. The epidemics in eastern Africa occur in a 5-15 year cycle coinciding with abnormally high rainfall generally associated to the warm phase of the El Niño event. However, recently, evidence has been gathered of inter-epidemic transmission. An open-source, easily applicable, accessible and modifiable model was built to simulate the transmission dynamics of RVF. The model was calibrated using data collected in the Kilombero Valley in Tanzania with people and cattle as host species and Ædes mcintoshi, Æ. ægypti and two Culex species as vectors. Simulations were run over a period of 27 years using standard parameter values derived from two previous studies in this region. Our model predicts low-level transmission of RVF, which is in line with epidemiological studies in this area. Emphasis in our simulation was put on both the dynamics and composition of vector populations in three ecological zones, in order to elucidate the respective roles played by different vector species: the model output did indicate the necessity of Culex involvement and also indicated that vertical transmission in Ædes mcintoshi may be underestimated. This model, being built with open-source software and with an easy-to-use interface, can be adapted by researchers and control program managers to their specific needs by plugging in new parameters relevant to their situation and locality.

Association between Rift Valley fever virus seroprevalences in livestock and humans and their respective intra-cluster correlation coefficients, Tana River County, Kenya

We implemented a cross-sectional study in Tana River County, Kenya, a Rift Valley fever (RVF)-endemic area, to quantify the strength of association between RVF virus (RVFv) seroprevalences in livestock and humans, and their respective intra-cluster correlation coefficients (ICCs). The study involved 1932 livestock from 152 households and 552 humans from 170 households. Serum samples were collected and screened for anti-RVFv immunoglobulin G (IgG) antibodies using inhibition IgG enzyme-linked immunosorbent assay (ELISA). Data collected were analysed using generalised linear mixed effects models, with herd/household and village being fitted as random variables. The overall RVFv seroprevalences in livestock and humans were 25.41% (95% confidence interval (CI) 23.49–27.42%) and 21.20% (17.86–24.85%), respectively. The presence of at least one seropositive animal in a household was associated with an increased odds of exposure in people of 2.23 (95% CI 1.03–4.84). The ICCs associated with RVF virus seroprevalence in livestock were 0.30 (95% CI 0.19–0.44) and 0.22 (95% CI 0.12–0.38) within and between herds, respectively. These findings suggest that there is a greater variability of RVF virus exposure between than within herds. We discuss ways of using these ICC estimates in observational surveys for RVF in endemic areas and postulate that the design of the sentinel herd surveillance should consider patterns of RVF clustering to enhance its effectiveness as an early warning system for RVF epidemics.

Serological evidence of inter-epizootic/inter-epidemic circulation of Rift Valley fever virus in domestic cattle in Kyela and Morogoro, Tanzania

Background

Tanzania is among the Rift Valley fever (RVF) epizootic/endemic countries in sub Saharan Africa, where RVF disease outbreaks occur within a range of 3 to 17-year intervals. Detection of Rift Valley fever virus (RVFV) antibodies in animals in regions with no previous history of outbreaks raises the question of whether the disease is overlooked due to lack-of effective surveillance systems, or if there are strains of RVFV with low pathogenicity. Furthermore, which vertebrate hosts are involved in the inter-epidemic and inter-epizootic maintenance of RVFV? In our study region, the Kyela and Morogoro districts in Tanzania, no previous RVF outbreaks have been reported.

Methodology

The study was conducted from June 2014 to October 2015 in the Kyela and Morogoro districts, Tanzania. Samples (n = 356) were retrieved from both the local breed of zebu cattle (Bos indicus) and Bos indicus/Bos Taurus cross breed. RVFV antibodies were analyzed by two enzyme-linked immunosorbent assay (ELISA) approaches. Initially, samples were analyzed by a RVFV multi-species competition ELISA (cELISA), which detected both RVFV IgG and IgM antibodies. All serum samples that were positive with the cELISA method were specifically analysed for the presence of RVFV IgM antibodies to trace recent infection. A plaque reduction neutralization assay (PRNT₈₀) was performed to determine presence of RVFV neutralizing antibodies in all cELISA positive samples.

Findings

Overall RVFV seroprevalence rate in cattle by cELISA in both districts was 29.2% (104 of 356) with seroprevalence rates of 33% (47/147) in the Kyela district and 27% (57/209) in the Morogoro district. In total, 8.4% (30/356) of all cattle sampled had RVFV IgM antibodies, indicating current disease transmission. When segregated by districts, the IgM antibody seroprevalence was 2.0% (3/147) and 12.9% (27/209) in Kyela and Morogoro districts respectively. When the 104 cELISA positive samples were analyzed by PRNT₈₀ to confirm that RVFV-specific antibodies were present, the majority (89%, 93/104) had RVFV neutralising antibodies.

Conclusion

The results provided evidence of widespread prevalence of RVFV antibody among cattle during an inter-epizootic/inter-epidemic period in Tanzania in regions with no previous history of outbreaks. There is a need for further investigations of RVFV maintenance and transmission in vertebrates and vectors during the long inter-epizootic/inter-epidemic periods.

The RVFV maintenance between inter-epizootic/inter-epidemic periods is not fully understood, despite the widely hypothesized belief of maintenance via transovarially infected Aedes mosquito eggs. Increasing serological evidence however, suggests that there could be continuous virus circulation throughout these periods in domestic ruminants, wild animals and humans both in areas with and without known history of RVF outbreaks. In some countries, RVFV antibodies have been demonstrated in livestock raised in areas where no clinical disease has ever been reported. However, in Tanzania, RVFV antibodies in livestock have been demonstrated only in areas with history of RVF outbreaks, raising the question of whether the disease is not present, is overlooked due to lack of effective surveillance systems, or whether there are strains of RVFV with low pathogenicity that do not cause detectable clinical cases in non-outbreak areas. We report here inter-epizootic/inter-epidemic RVFV antibody prevalence in non-vaccinated cattle from areas with no previous RVF outbreak in Tanzania and demonstrate recent virus circulation by detection of IgM antibodies. The differences in RVFV seroprevalence in different study locations suggest local factors that favour the virus amplification and transmission within those areas.

Seroprevalence of Rift Valley fever virus in cattle in the Democratic Republic of the Congo

This study aimed at assessing the serological and virological status of Rift Valley fever virus (RVFV) in cattle from four climatically diverse zones of the Democratic Republic of the Congo (DRC). A total of 1675 sera samples collected between 2014 and 2015 from cattle without clinical manifestation of RVF infection were tested using competitive and capture enzyme ELISA to detect both IgG and IgM. RT-PCR was used for the detection of nucleic acid of RVFV. Out of the 1675 cattle sera tested, 203 were found to be IgG-positive, giving an overall true seroprevalence of 12.37% (95% CI 10.86-14.05). This seroprevalence varied between the four zones with a seroprevalence of 16.16% (95% CI 12.86-20.12), 14.70% (95% CI 11.72-18.29), 10.82% (95% CI 7.19-14.19), and 7.34% (95% CI 5.13-10.41) recorded in cattle sampled in the mountainous, humid savannah, dry savannah, and forest zones, respectively (p < 0.05, χ² = 17.26). A higher true seroprevalence of 14.58% (95% CI 9.3-22.13) was found in animals aged 1 year compared to 10.43% (95% CI 8.12-13.30) and 13.16% (95% CI 11.19-15.42) in groups aged between 2-3 and > 3 years, respectively, although the difference was not statistically significant (p > 0.05, χ² = 2.95). Similarly, no statistically significant difference (p > 0.05, χ² = 0.04) was found between the sexes of the animals. Among the IgG-positive samples screened for anti-RVFV IgM, only 1.47% (3/203) was IgM-positive. One of the IgM-positive samples was positive by RT-PCR. These findings reveal country-wide distribution of RVF in the DRC for the first time.

Systematic literature review of Rift Valley fever virus seroprevalence in livestock, wildlife and humans in Africa from 1968 to 2016

Background

Rift Valley fever virus (RVFV) is a zoonotic arbovirus that causes severe disease in livestock and humans. The virus has caused recurrent outbreaks in Africa and the Arabian Peninsula since its discovery in 1931. This review sought to evaluate RVFV seroprevalence across the African continent in livestock, wildlife and humans in order to understand the spatio-temporal distribution of RVFV seroprevalence and to identify knowledge gaps and areas requiring further research. Risk factors associated with seropositivity were identified and study designs evaluated to understand the validity of their results.

Methodology

The Preferred Reporting of Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were used to produce a protocol to systematically search for RVFV seroprevalence studies in PubMed and Web of Science databases. The Strengthening the Reporting of Observational studies in Epidemiology (STROBE) statement guided the evaluation of study design and analyses.

Principal findings

A total of 174 RVFV seroprevalence studies in 126 articles fulfilled the inclusion criteria. RVFV seroprevalence was recorded in 31 African countries from 1968 to 2016 and varied by time, species and country. RVFV seroprevalence articles including either livestock and humans or livestock and wildlife seroprevalence records were limited in number (8/126). No articles considered wildlife, livestock and human seroprevalence concurrently, nor wildlife and humans alone. Many studies did not account for study design bias or the sensitivity and specificity of diagnostic tests.

Conclusions

Future research should focus on conducting seroprevalence studies at the wildlife, livestock and human interface to better understand the nature of cross-species transmission of RVFV. Reporting should be more transparent and biases accounted for in future seroprevalence research to understand the true burden of disease on the African continent.

Rift Valley fever virus (RVFV) is a vector-borne virus that infects wildlife and livestock, and can subsequently spread to humans. Due to the nature of the disease it has the potential to cause substantial economic and public health impacts. Rift Valley Fever (RVF) has been identified in Africa and the Arabian Peninsula, but has the potential to spread more widely. This systematic review assessed the distribution of RVF in livestock and humans in Africa by collating all the relevant studies we could find, extracting the data and critically evaluating them. Understanding when and where RVF has occurred in Africa and why some animals and humans get disease helps target control strategies and, in particular, those that reduce spread from livestock to humans. Furthermore, by evaluating past studies we can ensure that future ones are more robust and reproducible, so they can help us better understand the disease.

Seroprevalence and Virus Activity of Rift Valley Fever in Cattle in Eastern Region of Democratic Republic of the Congo

Rift Valley fever (RVF) is a zoonotic disease that is characterized by periodic and severe outbreaks in humans and animals. Published information on the occurrence of RVF in domestic animals is very scarce in the Democratic Republic of the Congo (DRC). To assess possible circulation of Rift Valley fever virus (RVFV) in cattle in the eastern province of DRC, 450 sera collected from cattle in North Kivu, South Kivu, and Ituri provinces were analyzed using the enzyme-linked immunosorbent assay (ELISA), for the detection of viral Immunoglobulin (Ig) G and M, and reverse transcriptase polymerase chain reaction (RT-PCR), for detection of viral RVF RNA. A cumulative anti-RVF IgG prevalence of 6.22% (95% CI 4.25–8.97) was recorded from the three provinces sampled. In North Kivu and Ituri provinces the anti-RVF IgG prevalence was 12.67% [95% CI 7.80–19.07] and 6% [95% CI 2.78–11.08], respectively, while all the sera collected from South Kivu province were negative for anti-RVF IgG antibodies. Anti-RVF IgM prevalence of 1.8% was obtained among sampled animals in the three provinces. None of the positive anti-RVF IgM samples (n=8) was positive for viral RVFV RNA using RT-PCR. Our findings suggest that RVFV is widely distributed among cattle in eastern province of DRC particularly in North Kivu and Ituri provinces although the epidemiological factors supporting this virus circulation remain unknown in these areas.

Sero-prevalence and spatial distribution of Rift Valley fever infection among agro-pastoral and pastoral communities during Interepidemic period in the Serengeti ecosystem, northern Tanzania

Background

In the past two decades, Rift Valley Fever (RVF) outbreaks have been reported twice in Tanzania, with the most recent outbreak occurring in 2006/07. Given the ecology and climatic factors that support mosquito vectors in the Serengeti ecosystem, we hypothesized a continued transmission of RVF virus (RVFV) during interepidemic periods. This study was carried out to determine sero-prevalence, spatial distribution and factors associated with RVF in at-risk agro-pastoral and pastoral communities in the Serengeti Ecosystem in northern Tanzania.

Methods

A cross sectional study was carried out to establish the general exposure to RVFV by detecting anti–RVFV IgG and anti–RVFV IgM using ELISA techniques. The health facilities where human subjects were blood sampled concurrent with interviews included Bunda District Designated Hospital, Wasso DDH, Endulen hospital, Arash, Malambo, Olbabal, and Piyaya dispenaries (Ngorongoro district) and Nyerere DDH (Serengeti district) respectively. In addition, human subjects from Lamadi ward (Busega district) were recruited while receiving medical service at Bunda DDH. We conducted logistic regression to assess independent risk factor and mapped the hotspot areas for exposure to RVFV.

Results

A total of 751 subjects (males = 41.5%; females = 58.5%) with a median age of 35.5 years were enrolled at out-patient clinics. Of them, 34 (4.5, 95%CI 3.3–6.3%) tested positive for anti–RVFV IgG. Of the 34 that tested positive for anti–RVFV IgG, six (17.6%) tested positive for anti–RVFV IgM. Odds of exposure were higher among pastoral communities (aOR 2.9, 95% C.I: 1.21–6.89, p < 0.01), and agro-pastoral communities residing in Ngorongoro District (aOR 1.8, 95% C.I 1.14–3.39, p = 0.03). Hotspot areas for exposure to RVFV were Malambo, Olbalbal and Piyaya wards in Ngorongoro district, and Lamadi ward in Busega district.

Conclusions

The study found both previous and recent exposure of RVFV in humans residing in the Serengeti ecosystem as antibodies against both IgG and IgM were detected. Detection of anti-RVF IgM suggests an ongoing transmission of RVFV in humans during inter-epidemic periods. Residents of Ngorongoro district were most exposed to RVFV compared to Bunda and Serengeti districts. Therefore, the risk of exposure to RVFV was higher among pastoral communities compared to farming communities.

The impact of water on health and ill-health in a sub-Saharan African wetland: Exploring both sides of the coin

Wetlands are a source of water out of which humans derive their livelihoods in Sub-Saharan Africa. They are often over-utilized and expose humans to disease-causing infectious agents. This calls for an evaluation of the role of water, sanitation and hygiene (WASH) and their effects in disease prevention and transmission in wetlands. A health risk assessment based on syndromic surveillance of self-reported abdominal complaints and fever gathered from a rural wetland in semiarid Kenya is presented with symptoms serving as proxies for real health threats in wetlands. The incidence of abdominal complaints was significantly higher for those using unimproved water sources compared to improved water users (odds ratio 7.5; 95% CI 2.59-26.9; p=0.001). Drainage of stagnant water near the house (odds ratio 0.2; 95% CI 0.08-0.54; p=0.002) and sanitary hygiene (odds ratio 0.4; 95% CI 0.71-0.97; p=0.056) were associated with reduced risk of abdominal complaints. Drainage of water was also associated with reduced risk of fever (odds ratio 0.3; 95% CI 0.02-0.59; p=0.002) and so was the use of mosquito nets (odds ratio 0.6; 95% CI 0.39-0.02; p=0.063). Usage of wetlands in the afternoon, e.g. for irrigated agriculture, increased the incidence of fever (odds ratio 1.5; 95% CI 0.91-2.33; p=0.040). Overall, there appears a greater likelihood of reducing pathogen exposure in the domestic than in the occupational domain or in the proximity to the wetland. We show that WASH, environmental hygiene and human behaviour are risk factors associated with the contraction of diseases characterized by abdominal complaints (e.g. diarrhoea) and fever (e.g. malaria) in wetlands. The same factors also have the potential to promote human health in the context of wetlands. We demonstrate the applicability of syndromic approaches in surveillance-scarce areas and emphasize the importance of adopting an integrated health-based wetland management that considers WASH and incorporates strategies based on grassroots level risk assessments.

Contracting infectious diseases in Sub-Saharan African wetlands: A question of use? A review

Worldwide the pressure on water is increasing. In parts of Sub-Saharan Africa (SSA), natural wetlands constitute the only accessible water resources, providing water free of charge, agricultural potential and livelihoods in otherwise uninhabitable landscapes, which is why they are being used extensively. The degradation and contamination of water which result from the use of wetlands has the potential to spread disease-causing microorganisms and provide increased breeding habitats for disease vectors, Despite this importance, case studies are lacking and knowledge gaps remain about whether and how different kinds of wetland use influence the exposure to health risks and transmission of infectious diseases. This descriptive literature review aimed at identifying publications from peer-reviewed journals and book chapters that (i) address water-related infectious diseases in SSA wetlands and (ii) link those diseases to use-related exposures. The resulting overview includes 27 publications and shows that depending on the type of use, people in wetlands are exposed to different risk factors and water-related infectious diseases. Exposure to infectious agents depends on occupational characteristics, and time spent in wetlands. Disease transmission is driven by users' contact to water, characteristics of pathogens and vectors of disease. The amount of available literature varies significantly. Whereas several publications have linked crop production and the domestic use of wetland water to contraction of diseases, fewer are available on health risks identified with pastoralism in wetlands and other uses. Some risk factors are well researched, such as irrigation schemes favouring schistosomiasis prevalence. For others, including proximity of pastoralists to their livestock and the associated trachoma risk, knowledge remains limited. This review establishes connections of selected diseases with different transmission pathways that are linked to specific risk factors, transmission pathways and resulting diseases. All of these have been integrated into a detailed conceptual framework which simplifies the complexity of the relationships, while at the same time identifying missing links which might provide stimulus for future research tackling the potential research gaps. It concludes that socio-cultural and behavioural considerations regarding the wetland users are not sufficiently evaluated and should receive increased attention in future investigations.

Inter-epidemic Rift Valley fever virus seroconversions in an irrigation scheme in Bura, south-east Kenya

Rift Valley fever (RVF) is an acute mosquito-borne viral zoonosis whose outbreaks are often associated with prolonged rainfall and flooding, during which large numbers of vectors emerge. Recent studies into the inter-epidemic maintenance of RVF virus (RVFV) suggest that both vertical transmission in vectors and direct transmission between hosts act in combination with predisposing factors for persistence of the virus. A comparative longitudinal survey was carried out in Tana River County, Kenya, in irrigated, riverine and pastoral ecosystems from September 2014-June 2015. The objectives were to investigate the possibility of low-level RVFV transmission in these ecosystems during an inter-epidemic period (IEP), examine variations in RVFV seroprevalence in sheep and goats and determine the risk factors for transmission. Three hundred and sixteen small ruminants were selected and tested for immunoglobulin G antibodies against RVFV nucleoprotein using a competitive ELISA during six visits. Data on potential risk factors were also captured. Inter-epidemic RVFV transmission was evidenced by 15 seroconversions within the irrigated and riverine villages. The number of seroconversions was not significantly different (OR = 0.66, CI = 0.19-2.17, p = .59) between irrigated and riverine areas. No seroconversions were detected in the pastoral ecosystem. This study highlights the increased risk of inter-epidemic RVFV transmission posed by irrigation, through provision of necessary environmental conditions that enable vectors access to more breeding grounds, resting places and shade, which favour their breeding and survival.

Drivers for Rift Valley fever emergence in Mayotte: A Bayesian modelling approach

Rift Valley fever (RVF) is a major zoonotic and arboviral hemorrhagic fever. The conditions leading to RVF epidemics are still unclear, and the relative role of climatic and anthropogenic factors may vary between ecosystems. Here, we estimate the most likely scenario that led to RVF emergence on the island of Mayotte, following the 2006-2007 African epidemic. We developed the first mathematical model for RVF that accounts for climate, animal imports and livestock susceptibility, which is fitted to a 12-years dataset. RVF emergence was found to be triggered by the import of infectious animals, whilst transmissibility was approximated as a linear or exponential function of vegetation density. Model forecasts indicated a very low probability of virus endemicity in 2017, and therefore of re-emergence in a closed system (i.e. without import of infected animals). However, the very high proportion of naive animals reached in 2016 implies that the island remains vulnerable to the import of infectious animals. We recommend reinforcing surveillance in livestock, should RVF be reported is neighbouring territories. Our model should be tested elsewhere, with ecosystem-specific data.

Serological Evidence for the Circulation of Rift Valley Fever Virus in Domestic Small Ruminants in Southern Gabon

Rift Valley fever (RVF) is a zoonotic disease, which caused several epidemics in humans in many countries of Africa. Using an inhibition enzyme-linked immunosorbent assay (ELISA), real-time reverse transcription PCR, and nested one-step reverse transcription PCR, we conducted a cross-sectional study in populations of sheep and goats from the Mongo County in 2014 to determine the circulation of the Rift Valley fever virus (RVFV) in small ruminants from this area. From a total of 201 small ruminants (95 sheep and 106 goats), the overall IgG seroprevalence against the RVFV was 6.47% (13/201). No RVFV RNA was detected in the animal plasmas. Logistic regression analysis showed that age, species, sex, and locality were not the significant risk factors. The findings of this study highlight the risk of RVF for domestic ruminants bred in this region and for the human rural population living in contact with these animals and they emphasize the need to develop adequate control measures to limit this threat.

The Role of Hyalomma Truncatum on the Dynamics of Rift Valley Fever: Insights from a Mathematical Epidemic Model

To date, our knowledge of Rift Valley fever (RVF) disease spread and maintenance is still limited, as flooding, humid weather and presence of biting insects such as mosquitoes, have not completely explained RVF outbreaks. We propose a model that includes livestock, mosquitoes and ticks compartments structured according to their questing and feeding behaviour in order to study the possible role of ticks on the dynamics of RVF. To quantify disease transmission at the initial stage of the epidemic, we derive an explicit formula of the basic reproductive number, [Formula: see text]. Using the concept of Metzler matrix, we state necessary conditions for global asymptotic stability of the disease-free equilibrium. Results suggest that although host-ticks interactions may serve as disease reservoirs or disease amplifiers, the Aedes reproductive number should be kept under unity if disease post-epizootics activities are to be controlled. Results of both local and global sensitivity analysis of selected model parameters indicate that [Formula: see text] is more sensitive to the ticks attachment and detachment rates, probability of transmission from ticks to host and from host to ticks, length of infection in livestock and ticks death rate. Furthermore, when comparing the mean value of [Formula: see text] with that from previous studies which did not include ticks we found that our [Formula: see text] is very much larger resulting in an increase in the exponential phase of an outbreak. These findings suggest that if ticks are capable of transmitting the virus, they may be contributing to disease outbreaks and endemicity.

Rift Valley fever virus infections in Egyptian cattle and their prevention

Rift Valley fever virus (RVFV) causes consistently severe outbreaks with high public health impacts and economic losses in livestock in many African countries and has also been introduced to Saudi Arabia and Yemen. Egypt with its four large outbreaks in the last 40 years represents the northernmost endemic area of RVFV. The purpose of this study was to provide an insight into the current anti-RVFV antibody status in immunized as well as non-immunized dairy cattle from the Nile Delta of Egypt. During 2013-2015, a total of 4,167 dairy cattle from four governorates including Dakahlia, Damietta, Gharbia and Port Said were investigated. All cattle were born after 2007 and therewith after the last reported Egyptian RVFV outbreak in 2003. The samples derived from vaccinated animals from 26 different dairy farms as well as non-immunized cattle from 27 different smallholding flocks. All samples were examined following a three-part analysis including a commercially available competition ELISA, an in-house immunofluorescence assay and a virus neutralization test. Additionally, a subset of samples was analysed for acute infections using IgM ELISA and real-time reverse transcriptase PCR. The results indicated that the RVFV is still circulating in Egypt as about 10% of the non-immunized animals exhibited RVFV-specific antibodies. Surprisingly, the antibody prevalence in immunized animals was not significantly higher than that in non-vaccinated animals which points out the need for further evaluation of the vaccination programme. Due to the substantial role of livestock in the amplification and transmission of RVFV, further recurrent monitoring of the antibody prevalence in susceptible species is highly warranted.

Seroprevalence of Rift Valley fever in cattle along the Akagera-Nyabarongo rivers, Rwanda

Rift Valley fever (RVF) virus is caused by a zoonotic arbovirus that is endemic to eastern and southern Africa. It has also been reported in West and North Africa, Madagascar and the Arabian Peninsula. The virus is transmitted by mosquitoes, but people can also become infected while handling blood or other body fluids of animals and humans with RVF. In 2007, there was a large outbreak of RVF in Kenya, Tanzania, Sudan and Somalia. Outbreaks were also reported in South Africa in 2008-2011. The epidemiology of RVF and factors for disease occurrence in Rwanda are neither clear nor documented. Therefore, we conducted a crosssectional study from December 2012 to March 2013 to generate baseline information on RVF in cattle. Purposive sampling of cattle (n = 595) was done in six districts, and serum samples were screened with competitive enzyme-linked immunosorbent assay (ELISA). We performed a statistical analysis on the generated data, and risk factors associated with RVF seroprevalence were determined by a simple logistic regression. Overall, RVF seroprevalence was 16.8% (95% confidence interval [CI] [13.8% - 20.0%]). The highest seroprevalence was recorded in Kirehe district (36.9%) followed by Ngoma (22.3%), and the least was recorded in Nyagatare (7.9%). RVF was more likely to occur in adult cattle (19.9% [odds ratio {OR} = 1.88, 95% CI {0.98-3.61}]) compared to young cattle (10.5% [OR = 0.47, 95% CI {0.26-0.83}]). Pure exotic or cross-breeds were significantly exposed to RVF virus (seroprevalence 22.9% [OR = 4.26, 95% CI {1.82-9.99}]) in comparison to 14.1% (OR = 0.55, 95% CI [0.35-0.86]) in local breeds. Sex differences were not statistically significant. These findings indicated that cattle have been exposed to RVF virus in six districts in Rwanda with a significant risk in adult, exotic or cross-breeds in Kirehe district.