P24 - The status of La Crosse virus in La Crosse, Wisconsin

Abstract: La Crosse virus (LACV) can cause encephalitis and has a lethal history in La Crosse, Wisconsin, with children being most at risk. The first human case of LACV was discovered in 1964 in La Crosse. Surveillance of La Crosse encephalitis and mosquito control began in 1978 by the La Crosse County Health Department and ended in 2017 because diagnosed  cases of the disease have declined dramatically in La Crosse County since the late 1970’s. We postulated that the decline in cases could be related to successful control of the vector species in the area or a change in the virus associated with more asymptomatic infections. In 2020 and 2021, I returned to historical case sites to survey for the primary vector species, Aedes (Ochlerotatus) triseriatus. I collected mosquito eggs via oviposition traps fitted with seed germination papers during the summer in 2020 and 2021. Mosquitoes were reared to adulthood, identified to species and then tested for LACV via RT-PCR. None of the mosquitoes collected in 2020 tested positive for LACV and the results for 2021 are forthcoming. Our results show that Ae. triseriatus continues to occupy the habitat provided by deciduous forests around the city of La Crosse, and that this mosquito is now competing for habitat with Aedes japonicus.