Serosurvey in two rural areas evidences recent and previously undetected WNV and SLEV circulation in Costa Rica

West Nile virus (WNV) and Saint Louis encephalitis virus (SLEV) share similar virus transmission cycles that involve birds as amplifiers and mosquitoes as vectors. Mammals, including humans, are dead-end-hosts that may be asymptomatic or develop more severe symptoms. Costa Rica is a hyperendemic country for several flaviviruses such as Dengue (DENV) and Zika (ZIKV) and previous research showed limited and restricted SLEV and WNV circulation in horses, sloths, and monkeys. Nevertheless, actual seroprevalence and high transmission areas are not yet identified. To determine putative WNV and SLEV circulation, we sampled peri-domestic and domestic animals, humans, wild birds, and mosquitoes in rural households located in two DENV and ZIKV hyperendemic regions during the rainy and dry seasons of 2017-2018 and conducted PRNT assays for serology and RT-PCR for virus detection. At Cuajiniquil, serological evidence of WNV and SLEV was found in equines, humans, chickens, and wild birds. Also, 5 seroconversion events were recorded for WNV (2 equine), SLEV (1 human), and DENV-1 (2 humans). At Talamanca, a lack of WNV circulation was found, but evidence of SLEV circulation was recorded in equines, humans, and wild birds. No evidence of active viral infection was found in any mosquitoes or wild bird samples. This seroconversion evidence supports the active and recent circulation of SLEV and WNV in these two regions. This study provides clear-cut evidence of WNV and SLEV circulation and should be considered by the health and epidemiology authorities for future prevention and differential diagnostics. Author summaryMosquitoes serve as vectors for the transmission of infectious diseases such as WNV and SLEV. The natural virus cycle of these viruses is maintained between birds and mosquitoes. Yet, humans and horses are dead end-hosts and can develop severe diseases such as encephalitis. We aimed to elucidate if WNV or SLEV were silently circulating in two regions of the country that historically report numerous cases of other arboviruses such as Dengue and Zika. Eight households were sampled at each region twice during the rainy (high number of arbovirus related infections are reported) and dry season (lower number of infections reported) to record seroconversion events. Serum samples from different species were analyzed using serology and virus presence was detected through molecular methods for wild bird and mosquito pools samples. We found serological evidence of WNV and SLEV infection in horses, humans, wild birds, and chicken samples, but did not detect actual virus in any tissue or mosquito sample. Taken together, our result shows the active but silent circulation of those viruses at both sampling sites. Action to include these arboviruses into diagnostics and public health measures must be taken.