Distribution and diversity of the vectors of Rift Valley fever along the livestock movement routes in the northeastern and coastal regions of Kenya

Expanding Understanding of Urban Rift Valley Fever Risk and Associated Vector Ecology at Slaughterhouses in Kisumu, Kenya

Rift Valley fever virus (RVFV) is an adaptable arbovirus that can be transmitted by a wide variety of arthropods. Widespread urban transmission of RVFV has not yet occurred, but peri-urban outbreaks of RVFV have recently been documented in East Africa. We previously reported low-level exposure in urban communities and highlighted the risk of introduction via live animal influx. We deployed a slaughtered animal testing framework in response to an early warning system at two urban slaughterhouses and tested animals entering the meat value chain for anti-RVFV IgG and IgM antibodies. We simultaneously trapped mosquitoes for RVFV and bloodmeal testing. Out of 923 animals tested, an 8.5% IgG seroprevalence was identified but no evidence of recent livestock exposure was detected. Mosquito species abundance varied greatly by slaughterhouse site, which explained 52% of the variance in blood meals. We captured many Culex spp., a known RVFV amplifying vector, at one of the sites (p < 0.001), and this species had the most diverse blood meals. No mosquito pools tested positive for RVFV antigen using a rapid VecTOR test. These results expand understanding of potential RVF urban disease ecology, and highlight that slaughterhouses are key locations for future surveillance, modelling, and monitoring efforts.

Parous rate and longevity of anophelines mosquitoes in bure district, northwestern Ethiopia

The intensity of malaria transmission is measured by parous rate, daily survival rate, human blood meal frequency, sporozoite rate, and entomological inoculation rates. Female parous status is a key index of vector competence, adult vector longevity, recruitment rate of adult, and the length of a gonotrophic cycle. Hence, the present study was aimed to investigate the parous rate and the longevity of Anopheles mosquitoes in Bure District, Northwestern Ethiopia. Parous rate was estimated as the number of mosquitoes with parous ovaries divided by the number of females dissected multiplied by 100. Mosquito life expectancy (longevity as d) was estimated by. One way- ANOVA was applied to confirm the presence of parous rate difference in the villages (p < 0.05). A total of 952 unfed hosts-seeking Anopheles mosquitoes was dissected for parous rate determination. The overall parous rate of An. arabiensis in the district was 52.0%, and the highest parous rate was recorded in Shnebekuma than other villages (F 2, 33 = 6.974; p = 0.003). Similarly, the parous rate of An. cinereus showed significant variation among villages (F 2, 33 = 5.044, p = 0.012) and the highest rate (63.0%) was recorded in Bukta. The mean longevity of An. funestus, An. arabiensis, An. coustani, An. squamosus, An. pharoensis, and An. cinereus was 6.5 days, 4.6 days, 3.5 days, 3.7 days, 2.7 days, and 2.2 days, respectively. The longevity of each species was not sufficient to complete the life cycle of malaria parasite for malaria transmission throughout the year because P. falciparum requires from 12–14 day.

Large herbivore loss has complex effects on mosquito ecology and vector-borne disease risk

Loss of biodiversity can affect transmission of infectious diseases in at least two ways: by altering host and vector abundance or by influencing host and vector behaviour. We used a large herbivore exclusion experiment to investigate the effects of wildlife loss on the abundance and feeding behaviour of mosquito vectors and to explore consequences for vector-borne disease transmission. Large herbivore loss affected both mosquito abundance and blood-feeding behaviour. For Aedes mcintoshi, the dominant mosquito species in our study and a primary vector of Rift Valley fever virus (RVFV), abundance decreased with large herbivore loss, while blood feeding on humans increased. Despite an elevated human biting rate in the absence of large herbivores, we estimated that the potential for RVFV transmission to humans doubles in the presence of large herbivores. These results demonstrate that multiple effects of biodiversity loss on vectors can lead to counterintuitive outcomes for human disease risk.

Diversity and Abundance of Potential Vectors of Rift Valley Fever Virus in the North Region of Cameroon

Simple Summary

Rift Valley fever (RVF) is a mosquito-borne disease caused by the Rift Valley fever virus (RVFV) transmitted by various genera of mosquitoes usually classified into primary vectors and secondary vectors. The former, belonging to the genus Aedes, are known for their ability to lay drought resistant eggs that can maintain the virus on dry soil for many years in geomorphic structures in the form of shallow depressions. After heavy rains, mosquitoes hatch from these eggs, some of which are infected and transmit the virus to neighboring animals. The secondary vectors, mainly mosquitoes of the genera Culex, Anopheles, and Mansonia, can colonize these sites, reproduce in abundance, and subsequently spread RVFV. Although the northern regions of Cameroon host more than half of the country’s cattle, sheep, and goat populations, there is a dearth of information on the occurrence and transmission of RVFV and its vectors. The very common transhumance of animals during periods of drought leads to contact between domestic and wild animals and creates opportunities for cross-transmission of the virus. It also increases the possibilities of exposure of herds to vectors, in particular at water points. In addition, rare heavy rainfall, flooding, and irrigation-based agricultural practices in these regions provide conditions for vector proliferation and increase the risk of the spread of vector-borne diseases, including RVF. Therefore, this study aimed to determine species diversity and spatial distribution of potential RVFV vectors in the North Region of Cameroon. The study revealed the presence of potential primary and secondary vectors of RVFV with an abundance and a diversity varying according to the ecological sites studied. This presence of potential vectors with their variable number per trap, per night, or per site may create areas of variable risk for disease transmission to susceptible hosts. Molecular analysis (PCR) tests for RVFV RNA research and viral isolation methods on these vectors to determine their role in the epidemiology and control of RVF cannot be overemphasized.

Abstract

Rift Valley fever (RVF) is a major viral zoonosis transmitted by mosquitoes. The virus is endemic in most parts of sub-Saharan Africa and can affect humans, livestock, and wild ungulates. Knowledge of the biology of vectors of Rift Valley fever virus (RVFV) is essential for the establishment of effective control measures of the disease. The objective of this study was to determine the species diversity and relative abundance of potential RVFV vectors in the North Region of Cameroon. Adult mosquitoes were trapped during the wet and dry seasons from December 2017 to January 2019 with “EVS Light” traps with CO₂ baits placed at selected sites. The captured mosquitoes were identified using dichotomous keys according to standard procedures. The abundance was calculated with regard to site, zone, and collection season. A total of 27,851 mosquitoes belonging to four genera (Aedes, Anopheles, Mansonia, and Culex) and comprising 31 species were caught (including 22 secondary vectors (98.05%) and nine primary vectors (1.94%). The total number of mosquitoes varied significantly depending on the locality (p-value < 0.001). The average number of mosquitoes collected per trap night was significantly higher in irrigated areas (p-value < 0.001), compared to urban and non-irrigated areas. The study revealed the presence of potential primary and secondary vectors of RVFV with varying abundance and diversity according to locality and ecological site in the North Region of Cameroon. The results showed that the genus Mansonia with the species Ma. uniformis and Ma. africana formed the dominant taxon (52.33%), followed by the genera Culex (45.04%) and Anopheles (2.61%). The need for molecular analysis (PCR) tests for RVFV RNA research and viral isolation methods on these vectors to determine their role in the epidemiology and control of RVF cannot be overemphasized.

High risk for human exposure to Rift Valley fever virus in communities living along livestock movement routes: A cross-sectional survey in Kenya

Introduction

Multiple outbreaks of Rift Valley Fever (RVF) with devastating effects have occurred in East Africa. These outbreaks cause disease in both livestock and humans and affect poor households most severely. Communities living in areas practicing nomadic livestock movement may be at higher risk of infection. This study sought to i) determine the human exposure to Rift Valley fever virus (RVFV) in populations living within nomadic animal movement routes in Kenya; and ii) identify risk factors for RVFV infection in these communities.

Methods

A cross-sectional descriptive study design was used. Samples were collected from the year 2014 to 2015 in a community-based sampling exercise involving healthy individuals aged ≥18 years from Isiolo, Tana River, and Garissa counties. In total, 1210 samples were screened by ELISA for the presence of immunoglobulin IgM and IgG antibodies against RVFV. Positive results were confirmed by plaque reduction neutralization test.

Results

Overall, IgM and IgG prevalence for all sites combined was 1.4% (95% CI 0.8–2.3%) and 36.4% (95% CI 33.8–39.2%), respectively. Isiolo County recorded a non-significant higher IgG prevalence of 38.8% than Garissa 35.9% and Tana River 32.2% (Chi square = 2.5, df = 2, p = 0.287). Males were significantly at higher risk of infection by RVFV than females (OR = 1.67, 95% CI 1.17–2.39, p<0.005). Age was significantly associated with RVFV infection (Wald Chi = 94.2, df = 5, p<0.0001). Individuals who had regular contact with cattle (OR = 1.38, 95%CI 1.01–1.89) and donkeys (OR = 1.38, 95%CI 1.14–1.67), or contact with animals through birthing (OR = 1.69, 95%CI 1.14–2.51) were significantly at a greater risk of RVFV infection than those who did not.

Conclusion

This study demonstrated that although the Isiolo County has been classified as being at medium risk for RVF, virus infection appeared to be as prevalent in humans as in Tana River and Garissa, which have been classified as being at high risk. Populations in these counties live within nomadic livestock movement routes and therefore at risk of being exposed to the RVFV. Interventions to control RVFV infections therefore, should target communities living along livestock movement pathways.

Rift Valley fever (RVF) is a neglected mosquito-borne zoonotic disease that causes major outbreaks and economic harm to human and ruminants health leading to increased poverty within affected communities. RVF is caused by RVF virus (RVFV) affecting humans and a wide range of ruminants. The virus is transmitted through bites from mosquitoes and exposure to blood, body fluids, or tissues of infected ruminants. It was first isolated in Kenya in 1930 and several outbreaks have been recorded in many countries in sub-Saharan Africa. We studied pastoralist communities living along livestock migratory routes. Migratory livestock do move long distances in search of water and pasture and may be at higher risk of exposure to RVFV. We also determined risk factors for RVFV infection by studying age, gender, contact with animals through birthing, and occupation. Prevention and control of RVFV infection can target significant risk factors to prevent spread and re-occurrence of outbreaks.

Relative Distribution, Diversity, and Bloodmeal Sources of Mosquitoes and Known Vectors of Rift Valley Fever Phlebovirus in Three Differing Ecosystems in Bura, Tana River County, Kenya

Environmental modifications disturb the equilibrium of mosquito populations, altering the risk of mosquito-borne diseases. Mosquito distribution, diversity, and bloodmeal sources were examined to compare Rift Valley fever (RVF) risk among irrigated, riverine, and pastoral ecosystems in Bura, Tana River County, Kenya, between September 2014 and June 2015. Thirty-eight households and 21 irrigation fields were selected for the study. Mosquitoes were trapped with carbon dioxide-impregnated CDC traps, one trap per household and three traps per irrigated field, and morphologically identified using taxonomic keys. Host DNA was extracted from engorged females and cytochrome b genes amplified by PCR to identify sources of bloodmeals. A total of 21,015 mosquitoes were collected; 5742 within households in the 3 ecosystems and 15,273 within irrigated fields. Mosquitoes collected within irrigated fields belonged to 8 genera and 37 species, while those from households within the irrigation scheme belonged to 6 genera and 29 species. Collections from riverine and pastoral households belonged to five and four genera, respectively. The most abundant genera in the irrigated fields were Aedes (21%) and Mansonia (22%), while Anopheles (43%) was the most abundant within households. Most mosquitoes in riverine and pastoral households belonged to Anopheles (76%) and Aedes (65%) genera, respectively. Seasonal variation driven by rainfall was evidenced by spikes in mosquito numbers within irrigated and riverine ecosystems. Host species identification revealed that goats and humans were the main sources of bloodmeal. There was an overall increase in mosquito abundance and diversity as a result of the presence of the irrigated ecosystem in this county, and an increased availability of highly RVF-susceptible hosts as a result of the establishment and concentration of residential areas, promoting potential vector-host contacts. These results highlight the impact of anthropogenic changes on mosquito ecology, potentially heightening the risk of transmission and maintenance of RVF in this region.

Mosquitoes of Etiological Concern in Kenya and Possible Control Strategies

Kenya is among the most affected tropical countries with pathogen transmitting Culicidae vectors. For decades, insect vectors have contributed to the emergence and distribution of viral and parasitic pathogens. Outbreaks and diseases have a great impact on a country's economy, as resources that would otherwise be used for developmental projects are redirected to curb hospitalization cases and manage outbreaks. Infected invasive mosquito species have been shown to increasingly cross both local and global boarders due to the presence of increased environmental changes, trade, and tourism. In Kenya, there have been several mosquito-borne disease outbreaks such as the recent outbreaks along the coast of Kenya, involving chikungunya and dengue. This certainly calls for the implementation of strategies aimed at strengthening integrated vector management programs. In this review, we look at mosquitoes of public health concern in Kenya, while highlighting the pathogens they have been linked with over the years and across various regions. In addition, the major strategies that have previously been used in mosquito control and what more could be done to reduce or combat the menace caused by these hematophagous vectors are presented.

Host species and site of collection shape the microbiota of Rift Valley fever vectors in Kenya

The composition and structure of microbial communities associated with mosquitoes remain poorly understood despite their important role in host biology and potential to be harnessed as novel strategies for mosquito-borne disease control. We employed MiSeq sequencing of the 16S rRNA gene amplicons to characterize the bacterial flora of field-collected populations of Aedes mcintoshi and Aedes ochraceus, the primary vectors of Rift Valley fever (RVF) virus in Kenya. Proteobacteria (53.5%), Firmicutes (22.0%) and Actinobacteria (10.0%) were the most abundant bacterial phyla accounting for 85.5% of the total sequences. Non-metric multi-dimensional scaling plots based on Bray-Curtis dissimilarities revealed a clear grouping of the samples by mosquito species, indicating that the two mosquito species harbored distinct microbial communities. Microbial diversity, richness and composition was strongly influenced by the site of mosquito collection and overall, Ae. ochraceus had significantly higher microbial diversity and richness than Ae. mcintoshi. Our findings suggest that host species and site of collection are important determinants of bacterial community composition and diversity in RVF virus vectors and these differences likely contribute to the spatio-temporal transmission dynamics of RVF virus.

Knowledge of the microbial communities associated with disease vectors can be exploited for symbiotic control of vector-borne diseases. Here, we characterized and compared the bacterial communities of field-caught populations of Aedes mcintoshi and Aedes ochraceus, the primary vectors of Rift Valley fever (RVF) virus in Kenya. We show that the two mosquito species harbor distinct microbial communities whose diversity and richness are heavily influenced by the site of collection. Because some bacterial species are known to influence vector susceptibility to pathogens, differences in bacterial communities between the two mosquito species is likely one of the primary factors accounting for the spatial and temporal variation in transmission dynamics of RVF virus.

The occurrence, diversity and blood feeding patterns of potential vectors of dengue and yellow fever in Kacheliba, West Pokot County, Kenya

INTRODUCTION

Yellow fever (YF) and dengue (DEN) viruses are important re-emerging mosquito-borne viruses sharing similar vectors and reservoirs. The last documented YF outbreak in Kenya occurred in 1992-95. However, YF virus is re-emerging in bordering countries including Uganda, Ethiopia and South Sudan with the potential for spread to the neighboring regions in Kenya. Dengue is endemic in Kenya with outbreaks being detected in various towns in the north and the coast. This study reports on the Aedes (Stegomyia) mosquito species occurrence, diversity, and blood feeding patterns, as means of measuring the risk of transmission of YF and DEN in Kacheliba sub-county, West Pokot County, which borders previous YF outbreak areas in eastern Uganda.

METHODOLOGY

Adult mosquitoes were collected using CO2-baited BG Sentinel traps at three time points during the rainy season. Mosquitoes were identified to the species level. Species abundance during the three sampling periods were compared, with emphasis on Aedes aegypti and other Stegomyia species, using generalized linear models that included mosquito diversity. Individually blood-fed mosquitoes were analyzed by DNA amplification of the 12S rRNA gene followed by sequencing to determine the source of blood meal.

RESULTS

Overall, 8605 mosquitoes comprising 22 species in 5 genera were collected. Sampled Stegomyia species included Ae. aegypti (77.3%), Ae. vittatus (11.4%), Ae. metallicus (10.2%) and Ae. unilineatus (1.1%). Ae. aegypti dominated the blood-fed specimens (77%, n = 68) and were found to have fed mostly on rock hyraxes (79%), followed by goats (9%), humans and cattle (each 4%), with a minor proportion on hippopotamus and rock monitor lizards (each comprising 1%).

CONCLUSION

Our findings reveal the presence of important Stegomyia species, which are known potential vectors of YF and DEN viruses. In addition, evidence of more host feeding on wild and domestic animals (hyrax and goat) than humans was observed. How the low feeding on humans translates to risk of transmission of these viruses, remains unclear, but calls for further research including vector competence studies of the mosquito populations for these viruses. This forms part of a comprehensive risk assessment package to guide decisions on implementation of affordable and sustainable vaccination (YF) and vector control plans in West Pokot County, Kenya.

Structural drivers of vulnerability to zoonotic disease in Africa

This paper argues that addressing the underlying structural drivers of disease vulnerability is essential for a ‘One Health’ approach to tackling zoonotic diseases in Africa. Through three case studies—trypanosomiasis in Zimbabwe, Ebola and Lassa fever in Sierra Leone and Rift Valley fever in Kenya—we show how political interests, commercial investments and conflict and securitization all generate patterns of vulnerability, reshaping the political ecology of disease landscapes, influencing traditional coping mechanisms and affecting health service provision and outbreak responses. A historical, political economy approach reveals patterns of ‘structural violence’ that reinforce inequalities and marginalization of certain groups, increasing disease risks. Addressing the politics of One Health requires analysing trade-offs and conflicts between interests and visions of the future. For all zoonotic diseases economic and political dimensions are ultimately critical and One Health approaches must engage with these factors, and not just end with an ‘anti-political’ focus on institutional and disciplinary collaboration.

This article is part of the themed issue ‘One Health for a changing world: zoonoses, ecosystems and human well-being’.

Ecological distribution and population dynamics of Rift Valley fever virus mosquito vectors (Diptera, Culicidae) in Senegal

Background

Many zoonotic infectious diseases have emerged and re-emerged over the last two decades. There has been a significant increase in vector-borne diseases due to climate variations that lead to environmental changes favoring the development and adaptation of vectors. This study was carried out to improve knowledge of the ecology of mosquito vectors involved in the transmission of Rift Valley fever virus (RVFV) in Senegal.

Methods

An entomological survey was conducted in three Senegalese agro-systems, Senegal River Delta (SRD), Senegal River Valley (SRV) and Ferlo, during the rainy season (July to November) of 2014 and 2015. Mosquitoes were trapped using CDC light traps set at ten sites for two consecutive nights during each month of the rainy season, for a total of 200 night-traps. Ecological indices were calculated to characterize the different populations of RVFV mosquito vectors. Generalized linear models with mixed effects were used to assess the influence of climatic conditions on the abundance of RVFV mosquito vectors.

Results

A total of 355,408 mosquitoes belonging to 7 genera and 35 species were captured in 200 night-traps. RVFV vectors represented 89.02% of the total, broken down as follows: Ae. vexans arabiensis (31.29%), Cx. poicilipes (0.6%), Cx. tritaeniorhynchus (33.09%) and Ma. uniformis (24.04%). Comparison of meteorological indices (rainfall, temperature, relative humidity), abundances and species diversity indicated that there were no significant differences between SRD and SRV (P = 0.36) while Ferlo showed significant differences with both (P < 0.001). Mosquito collection increased significantly with temperature for Ae. vexans arabiensis (P < 0.001), Cx. tritaeniorhynchus (P = 0.04) and Ma. uniformis (P = 0.01), while Cx. poicilipes decreased (P = 0.003). Relative humidity was positively and significantly associated with the abundances of Ae. vexans arabiensis (P < 0.001), Cx. poicilipes (P = 0.01) and Cx. tritaeniorhynchus (P = 0.007). Rainfall had a positive and significant effect on the abundances of Ae. vexans arabiensis (P = 0.005). The type of biotope (temporary ponds, river or lake) around the trap points had a significant effect on the mosquito abundances (P < 0.001).

Conclusions

In terms of species diversity, the SRD and SRV ecosystems are similar to each other and different from that of Ferlo. Meteorological indices and the type of biotope (river, lake or temporary pond) have significant effects on the abundance of RVFV mosquito vectors.

Electronic supplementary material

The online version of this article (10.1186/s13071-017-2591-9) contains supplementary material, which is available to authorized users.

Datasets for mapping pastoralist movement patterns and risk zones of Rift Valley fever occurrence

Rift Valley fever (RVF) is a zoonotic disease affecting humans and animals. It is caused by RVF virus transmitted primarily by Aedes mosquitoes. The data presented in this article propose environmental layers suitable for mapping RVF vector habitat zones and livestock migratory routes. Using species distribution modelling, we used RVF vector occurrence data sampled along livestock migratory routes to identify suitable vector habitats within the study region which is located in the central and the north-eastern part of Kenya. Eleven herds monitored with GPS collars were used to estimate cattle utilization distribution patterns. We used kernel density estimator to produce utilization contours where the 0.5 percentile represents core grazing areas and the 0.99 percentile represents the entire home range. The home ranges were overlaid on the vector suitability map to identify risks zones for possible RVF exposure. Assimilating high spatial and temporal livestock movement and vector distribution datasets generates new knowledge in understanding RVF epidemiology and generates spatially explicit risk maps. The results can be used to guide vector control and vaccination strategies for better disease control.

Rift Valley Fever Seroprevalence in Coastal Kenya

Rift Valley fever virus (RVFV) causes severe disease in both animals and humans, resulting in significant economic and public health damages. The objective of this study was to measure RVFV seroprevalence in six coastal Kenyan villages between 2009 and 2011, and characterize individual-, household-, and community-level risk factors for prior RVFV exposure. Sera were tested for anti-RVFV IgG via enzyme-linked immunosorbent assay. Overall, 51 (1.8%; confidence interval [CI₉₅] 1.3-2.3) of 2,871 samples were seropositive for RVFV. Seroprevalence differed significantly among villages, and was highest in Jego Village (18/300; 6.0%; CI₉₅ 3.6-9.3) and lowest in Magodzoni (0/248). Adults were more likely to be seropositive than children (P < 0.001). Seropositive subjects were less likely to own land or a motor vehicle (P < 0.01), suggesting exposure is associated with lower socioeconomic standing (P = 0.03). RVFV exposure appears to be low in coastal Kenya, although with some variability among villages.

Seroepidemiological Survey of Rift Valley Fever Virus in Ruminants in Garissa, Kenya

INTRODUCTION

Rift Valley fever (RVF) is a vector-borne zoonotic disease caused by phlebovirus in the family Bunyaviridae. In Kenya, major outbreaks occurred in 1997-1998 and 2006-2007 leading to human deaths, huge economic losses because of livestock morbidity, mortality, and restrictions on livestock trade.

AIM

This study was conducted to determine RVF seroprevalence in cattle, sheep, and goats during an interepidemic period in Garissa County in Kenya.

METHODS

In July 2013, we performed a cross-sectional survey and sampled 370 ruminants from eight RVF-prone areas of Garissa County. Rift Valley fever virus (RVFV) antibodies were detected using a multispecies competitive enzyme-linked immunosorbent assay. Mixed effect logistic regression models were used to determine the association between RVF seropositivity and species, sex, age, and location of the animals.

RESULTS

A total of 271 goats, 87 sheep, and 12 cattle were sampled and the overall immunoglobulin G seroprevalence was 27.6% (95% CI [23-32.1]). Sheep, cattle, and goats had seroprevalences of 32.2% (95% CI [20.6-31]), 33.3% (95% CI [6.7-60]), and 25.8% (95% CI [22.4-42]), respectively. Seropositivity in males was 31.8% (95% CI [22.2-31.8]), whereas that of females was 27% (95% CI [18.1-45.6]).

CONCLUSIONS

The high seroprevalence suggests RVFV circulation in domestic ruminants in Garissa and may be indicative of a subclinal infection. These findings provide evidence of RVF disease status that will assist decision-makers to flag areas of high risk of RVF outbreaks and prioritize the implementation of timely and cost-effective vaccination programs.

Rift Valley fever vector diversity and impact of meteorological and environmental factors on Culex pipiens dynamics in the Okavango Delta, Botswana

Background

In Northern Botswana, rural communities, livestock, wildlife and large numbers of mosquitoes cohabitate around permanent waters of the Okavango Delta. As in other regions of sub-Saharan Africa, Rift Valley Fever (RVF) virus is known to circulate in that area among wild and domestic animals. However, the diversity and composition of potential RVF mosquito vectors in that area are unknown as well as the climatic and ecological drivers susceptible to affect their population dynamics.

Methods

Using net traps baited with carbon dioxide, monthly mosquito catches were implemented over four sites surrounding cattle corrals at the northwestern border of the Okavango Delta between 2011 and 2012. The collected mosquito species were identified and analysed for the presence of RVF virus by molecular methods. In addition, a mechanistic model was developed to assess the qualitative influence of meteorological and environmental factors such as temperature, rainfall and flooding levels, on the population dynamics of the most abundant species detected (Culex pipiens).

Results

More than 25,000 mosquitoes from 32 different species were captured with an overabundance of Cx. pipiens (69,39 %), followed by Mansonia uniformis (20,67 %) and a very low detection of Aedes spp. (0.51 %). No RVF virus was detected in our mosquito pooled samples. The model fitted well the Cx. pipiens catching results (ρ = 0.94, P = 0.017). The spatial distribution of its abundance was well represented when using local rainfall and flooding measures (ρ = 1, P = 0.083). The global population dynamics were mainly influenced by temperature, but both rainfall and flooding presented a significant influence. The best and worst suitable periods for mosquito abundance were around March to May and June to October, respectively.

Conclusions

Our study provides the first available data on the presence of potential RVF vectors that could contribute to the maintenance and dissemination of RVF virus in the Okavango Delta. Our model allowed us to understand the dynamics of Cx. pipiens, the most abundant vector identified in this area. Potential predictions of peaks in abundance of this vector could allow the identification of the most suitable periods for disease occurrence and provide recommendations for vectorial and disease surveillance and control strategies.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1712-1) contains supplementary material, which is available to authorized users.

Plant resting site preferences and parity rates among the vectors of Rift Valley Fever in northeastern Kenya

BACKGROUND

Mosquito lifespan can influence the circulation of disease causing pathogens because it affects the time available for infection and transmission. The life-cycle of mosquitoes is determined by intrinsic and environmental factors, which can include the availability of hosts and suitable resting environments that shelter mosquitoes from extreme temperature and desiccating conditions. This study determined the parity rates (an indirect measure of survival) and plant resting preference of vectors of Rift Valley fever (RVF) in northeastern Kenya.

METHODS

Resting mosquitoes were trapped during the rainy and the dry season using a Prokopack aspirator from vegetation, whereas general adult populations were trapped using CDC light traps. At each site, sampling was conducted within a 1 km(2) area, subdivided into 500 × 500 m quadrants and four 250 × 250 m sub-quadrants from which two were randomly selected as sampling units. In each sampling unit, plants were randomly selected for aspiration of mosquitoes. Only Aedes mcintoshi and Ae. ochraceus were dissected to determine parity rates while all mosquito species were used to assess plant resting preference.

RESULTS

Overall, 1124 (79 %, 95 % CI = 76.8-81.1 %) mosquitoes were parous. There was no significant difference in the number of parous Ae. mcintoshi and Ae. ochraceus. Parity was higher in the rainy season than in the dry season. Daily survival rate was estimated to be 0.93 and 0.92 among Ae. ochraceus and Ae. mcintoshi, respectively. Duosperma kilimandscharicum was the most preferred plant species with the highest average capture of primary (3.64) and secondary (5.83) vectors per plant, while Gisekia africana was least preferred.

CONCLUSION

Survival rate of each of the two primary vectors of RVF reported in this study may provide an indication that these mosquitoes can potentially play important roles in the circulation of diseases in northern Kenya. Resting preference of the mosquitoes in vegetation may influence their physiology and enhance longevity. Thus, areas with such vegetation may be associated with an increased risk of transmission of arboviruses to livestock and humans.

Vector competence of Aedes vexans (Meigen), Culex poicilipes (Theobald) and Cx. quinquefasciatus Say from Senegal for West and East African lineages of Rift Valley fever virus

BACKGROUND

Rift Valley fever virus (RVFV; Phlebovirus, Bunyaviridae) is a mosquito-borne, zoonotic pathogen. In Senegal, RVFV was first isolated in 1974 from Aedes dalzieli (Theobald) and thereafter from Ae. fowleri (de Charmoy), Ae. ochraceus Theobald, Ae. vexans (Meigen), Culex poicilipes (Theobald), Mansonia africana (Theobald) and Ma. uniformis (Theobald). However, the vector competence of these local species has never been demonstrated making hypothetical the transmission cycle proposed for West Africa based on serological data and mosquito isolates.

METHODS

Aedes vexans and Cx. poicilipes, two common mosquito species most frequently associated with RVFV in Senegal, and Cx. quinquefasciatus, the most common domestic species, were assessed after oral feeding with three RVFV strains of the West and East/central African lineages. Fully engorged mosquitoes (420 Ae. vexans, 563 Cx. quinquefasciatus and 380 Cx. poicilipes) were maintained at 27 ± 1 °C and 70-80% relative humidity. The saliva, legs/wings and bodies were tested individually for the RVFV genome using real-time RT-PCR at 5, 10, 15 and 20 days post exposure (dpe) to estimate the infection, dissemination, and transmission rates. Genotypic characterisation of the 3 strains used were performed to identify factors underlying the different patterns of transmission.

RESULTS

The infection rates varied between 30.0-85.0% for Ae. vexans, 3.3-27% for Cx. quinquefasciatus and 8.3-46.7% for Cx. poicilipes, and the dissemination rates varied between 10.5-37% for Ae. vexans, 9.5-28.6% for Cx. quinquefasciatus and 3.0-40.9% for Cx. poicilipes. However only the East African lineage was transmitted, with transmission rates varying between 13.3-33.3% in Ae. vexans, 50% in Cx. quinquefasciatus and 11.1% in Cx. poicilipes. Culex mosquitoes were less susceptible to infection than Ae. vexans. Compared to other strains, amino acid variation in the NSs M segment proteins of the East African RVFV lineage human-derived strain SH172805, might explain the differences in transmission potential.

CONCLUSION

Our findings revealed that all the species tested were competent for RVFV with a significant more important role of Ae. vexans compared to Culex species and a highest potential of the East African lineage to be transmitted.