LACK OF EVIDENCE OF HERD IMMUNITY TO HEMORRHAGIC DISEASE IN WHITE-TAILED DEER (ODOCOILEUS VIRGINIANUS) IN WEST VIRGINIA, USA

Although outbreaks of hemorrhagic disease (HD) in white-tailed deer (Odocoileus virginianus) have been historically rare in the northeastern and northcentral USA, the frequency, intensity, and scale of regional outbreaks have increased over the past two decades. It has been suggested that the impact of these outbreaks will lessen over time as a result of increasing herd immunity, but this phenomenon has not been supported by regional data to date. To assess this possibility, we tested 770 deer from 17 counties in West Virginia, USA, for antibodies against epizootic hemorrhagic disease virus and bluetongue virus over a period (1981–2023) of increasing HD reporting. Despite a significant increase in the mean number of counties annually reporting HD during 2002–23 (x̄=8.45, SE=2.41) versus 1981–2001 (x̄=0.91, SE=0.38), we found no concurrent evidence for the development of significant herd immunity. We also assessed seroprevalence following individual large-scale HD outbreaks that occurred from 1981–2023. We defined an outbreak year as a year when HD was detected in West Virginia during a large-scale regional outbreak or when HD was reported in greater than five West Virginia counties. We examined seroprevalence temporally, centered on outbreak years, in the 3 yr preceding and following each large-scale event. Although seroprevalence peaked in the year following an outbreak of HD, it returned to near zero within 3 yr following the outbreak, thus yielding no evidence for the development of significant herd immunity at a statewide scale. Additional serologic data is needed over time to assess the potential establishment of herd immunity to HD in the northeastern and northcentral regions of the USA; however, our case study in West Virginia indicates no apparent increase in antibody prevalence despite 20 yr of significantly increased HD activity.