Prevalence and risk factors associated with West Nile virus infection in horses and wild birds in Madagascar

Objective: West Nile virus (WNV) is considered as the most prevalent arbovirus in Madagascar, yet few studies have focused on WNV circulation in wild birds and horses. The aims of this study are (i) to provide recent estimates of WNV seroprevalence and incidence in horses, (ii) to assess which wild bird species are exposed to WNV and (iii) to identify risk factors associated with exposure in horses and wild birds. Materials and methods: Horses from 4 regions of Madagascar were sampled before and after the 2015-2016 rainy season to estimate prevalence and incidence. Stable and horse characteristics were collected through a questionnaire. A wide range of wild birds species from the same regions were sampled in 2016-2017. Antibodies against WNV in horses and birds were tested using a competition ELISA test; results of a subset of samples were further confirmed using either a specific microsphere immunoassay for horses or a viral neutralization assay for birds. Logistic models were developed to identify risk factors. Results: Overall seroprevalence in horses was 33.5% (n=254) and incidence during the rainy season was 8.8% (n=147). Age, presence of ponds, use of insecticides and combined presence of rice fields and ruminants in the vicinity of stables were identified as risk factors. Overall, 352 birds (belonging to 41 species) were tested and 12.5% (belonging to 19 species) were seropositive. Birds caught outside wetlands, in Alaotra Mangoro region, belonging to the Passeriformes order, and species with an area of distribution in the Indian Ocean islands were significantly more exposed to WNV. Conclusion: This study confirms that WNV is endemic with high levels of circulation in horses, although no clinical cases were recorded. For the first time in Madagascar, 19 species of wild birds (among which 12 of the 21 Passeriformes species) were shown to be exposed to WNV.