Determinants of Crimean-Congo haemorrhagic fever virus exposure dynamics in Mediterranean environments

Modelling the small spatial scale questing abundance of Hyalomma lusitanicum Koch, 1844 (Acari: Ixodidae), vector of Crimean-Congo haemorrhagic fever virus

Free-living ticks such as Hyalomma lusitanicum pose a risk to their hosts primarily due to their role as vectors of diseases, with Crimean-Congo haemorrhagic fever (CCHF) being an emerging disease of particular concern in the Iberian Peninsula. This study aims to understand the environmental determinants as well as the host effects that influence the abundance of H. lusitanicum at a small spatial scale. Understanding these factors is essential in identifying areas with a higher risk of tick abundance and, consequently, a greater likelihood of encountering a tick. We conducted 3,840 drags in 32 sampling points in a continental area of the Iberian Peninsula during the peak activity of H. lusitanicum, in May 2021 (1,800 drags in 15 points) and May 2022 (2,040 drags in 17 points). As potential factors influencing H. lusitanicum abundance, we modelled suitability for red deer and we estimated environmental variables that could influence the existence of optimal microhabitats for H. lusitanicum. We modelled H. lusitanicum abundance, performing generalized linear mixed models with a zero-inflated negative binomial distribution, using the abundance per drag as a response variable. We found that a higher local suitability for red deer is the primary determinant of small-scale abundance of H. lusitanicum in our study area. We found other significant factors that affect the abundance of H. lusitanicum which determine the presence of suitable microhabitats, such as adequate vegetation types (suggested by vegetation height), grasslands and mixed vegetation types, and low terrain permeability. This information will significantly aid in the development of more integrated, environmentally friendly, and effective long-term strategies for tick control and the prevention of tick-borne pathogens in regions with climatic and landscape conditions similar to those of our study site.

Spatiotemporal dynamics of Ixodes ricinus abundance in northern Spain

Ixodes ricinus is the most medically relevant tick species in Europe because it transmits the pathogens that cause Lyme borreliosis and tick-borne encephalitis. Northern Spain represents the southernmost margin of its main European range and has the highest rate of Lyme borreliosis hospitalisations in the country. Currently, the environmental determinants of the spatiotemporal patterns of I. ricinus abundance remain unknown in this region and these may differ from drivers in highly favourable areas for the species in Europe. Therefore, our study aimed to understand the main factors modulating questing I. ricinus population dynamics to map abundance patterns in northern Spain. From 2012 to 2014, monthly/fortnightly samplings were conducted at 13 sites in two regions of northern Spain to estimate spatiotemporal variation in I. ricinus questing abundance. Local abundance of I. ricinus was modelled in relation to variation in local biotic and abiotic environmental conditions by constructing generalised linear mixed models with a zero-inflated negative binomial distribution for overdispersed data. The different developmental stages of I. ricinus were most active at different times of the year. Adults and nymphs showed a peak of abundance in spring, while questing larvae were more frequent in summer. The main determinants affecting the spatiotemporal abundance of the different stages were related to humidity and temperature. For adults and larvae, summer seemed to be the most influential period for their abundance, while for nymphs, winter conditions and those of the preceding months seemed to be determining factors. The highest abundances of nymphs and adults were predicted for the regions of northern Spain with the highest rate of Lyme borreliosis hospitalisations. Our models could be the basis on which to build more accurate predictive models to identify the spatiotemporal windows of greatest potential interaction between animals/humans and I. ricinus that may lead to the transmission of I. ricinus-borne pathogens.

Approaching the complexity of Crimean-Congo hemorrhagic fever virus serology: A study in swine

Crimean-Congo hemorrhagic fever virus (CCHFV) is a tick-borne zoonotic orthonairovirus of public health concern and widespread geographic distribution. Several animal species are known to seroconvert after infection with CCHFV without showing clinical symptoms. The commercial availability of a multi-species ELISA has led to an increase in recent serosurveillance studies as well as in the range of species reported to be exposed to CCHFV in the field, including wild boar (Sus scrofa). However, development and validation of confirmatory serological tests for swine based on different CCHFV antigens or test principles are hampered by the lack of defined control sera from infected and non-infected animals. For the detection of anti-CCHFV antibodies in swine, we established a swine-specific in-house ELISA using a panel of swine sera from CCHFV-free regions and regions with reported CCHFV circulation. We initially screened more than 700 serum samples from wild boar and domestic pigs and observed a correlation of ≃67% between the commercial and the in-house test. From these sera, we selected a panel of 60 samples that were further analyzed in a newly established indirect immunofluorescence assay (iIFA) and virus neutralization test. ELISA-non-reactive samples tested negative. Interestingly, only a subset of samples reactive in both ELISA and iIFA displayed CCHFV-neutralizing antibodies. The observed partial discrepancy between the tests may be explained by different test sensitivities, antibody cross-reactivities or suggests that the immune response to CCHFV in swine is not necessarily associated with eliciting neutralizing antibodies. Overall, this study highlights that meaningful CCHFV serology in swine, and possibly other species, should involve the performance of multiple tests and careful interpretation of the results.

Epidemiologic Survey of Crimean-Congo Hemorrhagic Fever Virus in Suids, Spain

We conducted a cross-sectional study in wild boar and extensively managed Iberian pig populations in a hotspot area of Crimean-Congo hemorrhagic fever virus (CCHFV) in Spain. We tested for antibodies against CCHFV by using 2 ELISAs in parallel. We assessed the presence of CCHFV RNA by means of reverse transcription quantitative PCR protocol, which detects all genotypes. A total of 113 (21.8%) of 518 suids sampled showed antibodies against CCHFV by ELISA. By species, 106 (39.7%) of 267 wild boars and 7 (2.8%) of 251 Iberian pigs analyzed were seropositive. Of the 231 Iberian pigs and 231 wild boars analyzed, none tested positive for CCHFV RNA. These findings indicate high CCHFV exposure in wild boar populations in endemic areas and confirm the susceptibility of extensively reared pigs to CCHFV, even though they may only play a limited role in the enzootic cycle.

Animal Exposure Model for Mapping Crimean-Congo Hemorrhagic Fever Virus Emergence Risk

To estimate the determinants of spatial variation in Crimean-Congo hemorrhagic fever virus (CCHFV) transmission and to create a risk map as a preventive public health tool, we designed a survey of small domestic ruminants in Andalusia, Spain. To assess CCHFV exposure spatial distribution, we analyzed serum from 2,440 sheep and goats by using a double-antigen ELISA and modeled exposure probability with environmental predictors by using generalized linear mixed models. CCHFV antibodies detected in 84 samples confirmed low CCHFV prevalence in small domestic ruminants in the region. The best-fitted statistical model indicated that the most significant predictors of virus exposure risk were cattle/horse density and the normalized difference vegetation index. Model validation showed 99.7% specificity and 10.2% sensitivity for identifying CCHFV circulation areas. To map CCHFV exposure risk, we projected the model at a 1 × 1-km spatial resolution. Our study provides insight into CCHFV ecology that is useful for preventing virus transmission.

Mapping the risk of exposure to Crimean-Congo haemorrhagic fever virus in the Iberian Peninsula using Eurasian wild boar (Sus scrofa) as a model

Crimean-Congo haemorrhagic fever (CCHF) virus (CCHFV) is a tick-borne zoonotic pathogen that can cause a lethal haemorrhagic disease in humans. Although the virus appears to be endemically established in the Iberian Peninsula, CCHF is an emerging disease in Spain. Clinical signs of CCHFV infection are mainly manifested in humans, but the virus replicates in several animal species. Understanding the determinants of CCHFV exposure risk from animal models is essential to predicting high-risk exposure hotspots for public health action. With this objective in mind, we designed a cross-sectional study of Eurasian wild boar (Sus scrofa) in Spain and Portugal. The study analysed 5,291 sera collected between 2006 and 2022 from 90 wild boar populations with a specific double-antigen ELISA to estimate CCHFV serum prevalence and identify the main determinants of exposure probability. To do so, we statistically modelled exposure risk with host- and environment-related predictors and spatially projected it at a 10 × 10 km square resolution at the scale of the Iberian Peninsula to map foci of infection risk. Fifty-seven (63.3 %) of the 90 populations had at least one seropositive animal, with seroprevalence ranging from 0.0 to 88.2 %. Anti-CCHFV antibodies were found in 1,026 of 5,291 wild boar (19.4 %; 95 % confidence interval: 18.3-20.5 %), with highest exposure rates in southwestern Iberia. The most relevant predictors of virus exposure risk were wild boar abundance, local rainfall regime, shrub cover, winter air temperature and soil temperature variation. The spatial projection of the best-fit model identified high-risk foci as occurring in most of western and southwestern Iberia and identified recently confirmed risk foci in eastern Spain. The results of the study demonstrate that serological surveys of CCHFV vector hosts are a powerful, robust and highly informative tool for public health authorities to take action to prevent human cases of CCHF in enzootic and emergency settings.

Dynamics of Crimean-Congo hemorrhagic fever virus in two wild ungulate hosts during a disease-induced population collapse

Identifying the role that host species play in pathogen transmission and maintenance is crucial for disease control, but it is a difficult task, in particular for vector-borne and multi-host pathogens, and especially when wildlife species are involved. This is the case for a Crimean-Congo hemorrhagic fever virus (CCHFV) hotspot in north-eastern Spain, where Iberian ibex (Capra pyrenaica) and wild boar (Sus scrofa) are involved, but their roles in disease transmission are unclear. In this context, we studied the dynamics of CCHFV transmission in these two species during the collapse of an Iberian ibex population due to a sarcoptic mange outbreak. We carried out a repeated cross-sectional study measuring the trends of CCHFV seroprevalence in Iberian ibex and wild boar and their abundances. In addition, we identified the tick species present in this area on the vegetation and on wild boars, and evaluated relevant meteorological factors. Results show that while the trends in CCHFV seroprevalence in Iberian Ibex and density of wild boars remained constant (p = 1.0 and p = 0.8, respectively), both the trends in Iberian ibex census and CCHFV seroprevalence in wild boars decreased significantly (p = 0.003 and p = 0.0001, respectively), and were correlated (Spearman's rank, 0.02 < p-adjusted<0.05). The correlation between the patterns of reduction of Iberian ibex abundance and the decrease of seroprevalence in wild boars suggests some sort of shared transmission cycle between the two species. Data from tick species in the area suggest a possible role of Rhipicephalus bursa in CCHFV transmission. The dynamics of CCHFV were unlikely caused by changes in meteorological variables such as temperature or water vapor pressure deficit. Further studies will be needed to confirm these hypotheses.

Insights into the spatiotemporal dynamics of West Nile virus transmission in emerging scenarios

The incidence of West Nile fever (WNF) is highly variable in emerging areas, making it difficult to identify risk periods. Using clinical case records has important biases in understanding the transmission dynamics of West Nile virus (WNV) because asymptomatic infections are frequent. However, estimating virus exposure in sentinel species could help achieve this goal at varying spatiotemporal scales. To identify the determinants of inter-annual variation in WNV transmission rates, we designed a 15-year longitudinal seroepidemiological study (2005-2020) in five environmentally diverse areas of southwestern Spain. We modeled individual annual area-dependent exposure risk based on potential environmental and host predictors using generalized linear mixed models. Further, we analyzed the weight of predictors on exposure probability by variance partitioning of the model components. The analysis of 2418 wild ungulate sera (1168 red deer - Cervus elaphus - and 1250 Eurasian wild boar - Sus scrofa) with a highly sensitive commercial blocking ELISA identified an average seroprevalence of 24.9% (95% confidence interval (CI): 23.2-26.7%). Antibody prevalence was slightly higher in wild boar (27.5%; CI: 25.1-30.1%) than in deer (22.2%; CI: 19.8-24.7%). We observed a spatial trend in exposure, with higher frequency in the southernmost areas and a slight, although area-dependent, increasing temporal trend. Host-related predictors were important drivers of exposure risk. The environmental predictor with the highest weight was annual cumulative precipitation, while temperature variations were also relevant but with less weight. We observed a coincidence of spatiotemporal changes in exposure with the notification of WNF outbreaks in horses and humans. That indicates the usefulness of wild ungulates as sentinels for WNV transmission and as models to understand its spatiotemporal dynamics. These results will allow the development of more accurate predictive models of spatiotemporal variations in transmission risk that can inform health authorities to take appropriate action.