La Crosse virus spread within the mosquito population in Knox County, TN

Current Status of La Crosse Virus in North America and Potential for Future Spread

La Crosse virus (LACV) is an arthropod-borne RNA virus with substantial potential for future spread in North America. La Crosse virus is responsible for La Crosse encephalitis, a leading cause of arboviral encephalitis in children in the United States. Primarily transmitted by Aedes triseriatus (Eastern treehole) mosquitos and amplified by small mammal hosts, LACV has caused infections throughout the upper Midwest and, more recently, the mid-Atlantic and southeastern United States. Notably, in recent years, infections have also been identified increasingly in the Appalachian region. Anthropogenic and environmental factors have likely contributed to recent LACV spread, including the introduction of invasive vector species (especially Ae. albopictus), biotic interactions between and among vector and host species, land-use change, habitat disturbance, increased human travel and transport, and rising global temperatures. Prevention and control strategies, such as increased surveillance of vector and host populations, increased awareness among populations at risk for infection, and increased awareness among physicians are needed to limit future spread. Continued climate change with increases in global temperatures and erratic weather patterns may result in the expansion of competent mosquito vector species and thus could facilitate the geographic spread of LACV.

Do Aedes triseriatus Respect State Boundaries?: A Paucity of La Crosse Virus in the South Carolina Appalachian Mountains

Background: La Crosse virus is an important endemic public health concern in the North Carolina Appalachian Mountains; however, human incidence is not commonly noted in this region on the South Carolina side of the mountain range border. No relevant studies have been performed in South Carolina evaluating mosquito vector populations for La Crosse virus (LACV) infection; thus, a pilot mosquito surveillance study was executed in summer 2020. Material and Methods: Vector surveillance occurred at five South Carolina state parks bordering neighboring state endemic counties from May to August 2020. Collections were approved by the state park authority, as noted in Permit No. N-8-20. Results: All three competent mosquito vectors were collected during the study duration; however, these vectors were collected in low abundance: Aedes triseriatus (4.5% of all collected mosquitos); Aedes albopictus (2.0%); Aedes japonicus (1.4%). Principal mosquito vector specimens, Ae. triseriatus, were sent to Centers for Disease Control and Prevention for testing of LACV by real-time reverse transcription PCR-all were negative. Discussion: While entomologic evidence suggests low transmission risk for this arbovirus in the South Carolina Appalachian Mountain region, further eco-epidemiologic investigations are warranted to understand this endemicity variance within a relatively small geographic area.

Geographically persistent clusters of La Crosse virus disease in the Appalachian region of the United States from 2003 to 2021

La Crosse virus (LACV) is a mosquito-borne pathogen that causes more pediatric neuroinvasive disease than any other arbovirus in the United States. The geographic focus of reported LACV neuroinvasive disease (LACV-ND) expanded from the Midwest into Appalachia in the 1990s, and most cases have been reported from a few high-risk foci since then. Here, we used publicly available human disease data to investigate changes in the distribution of geographic LACV-ND clusters between 2003 and 2021 and to investigate socioeconomic and demographic predictors of county-level disease risk in states with persistent clusters. We used spatial scan statistics to identify high-risk clusters from 2003-2021 and a generalized linear mixed model to identify socioeconomic and demographic predictors of disease risk. The distribution of LACV-ND clusters was consistent during the study period, with an intermittent cluster in the upper Midwest and three persistent clusters in Appalachia that included counties in east Tennessee / western North Carolina, West Virginia, and Ohio. In those states, county-level cumulative incidence was higher when more of the population was white and when median household income was lower. Public health officials should target efforts to reduce LACV-ND incidence in areas with consistent high risks.