Epizootic Hemorrhagic Disease (EHD) is a serious viral infection primarily affecting white-tailed deer populations across North America. It is caused by the Epizootic Hemorrhagic Disease virus (EHDV), an Orbivirus that targets the lining of the deer’s blood vessels. This damage causes extensive internal hemorrhaging, leading to high fatality rates. EHD outbreaks cause rapid, significant mortality, making understanding the virus’s spread crucial for wildlife management efforts.
The Biting Midge: EHD’s Primary Vector
The spread of EHD hinges on a tiny insect known as the biting midge, or “no-see-um,” belonging to the genus Culicoides. These small, blood-feeding flies are the obligate biological vector for the EHD virus. This means the virus cannot move from one deer to another without the midge acting as an intermediary. Transmission is strictly indirect; EHD is not spread through direct contact between deer, such as shared food or water. A deer only contracts the disease after being bitten by a midge that is actively carrying the virus.
The Virus Transmission Cycle
The cycle begins when a midge feeds on a deer that has the EHD virus circulating in its bloodstream. The midge ingests the virus particles and becomes infected. The virus must then replicate within the midge’s body, a necessary developmental period known as extrinsic incubation. Once the virus has multiplied, the midge becomes infectious and transmits the pathogen during its next feeding by injecting the virus into a susceptible deer through its saliva. Following infection, the deer generally develops symptoms within five to ten days, and acute cases often lead to death within 8 to 36 hours.
Environmental Drivers of Outbreaks
The timing and severity of EHD outbreaks are heavily influenced by environmental conditions that favor the midge’s life cycle. Warm temperatures accelerate both the breeding cycle of the Culicoides midge and the rate at which the EHD virus replicates inside the insect. This increased efficiency allows the midge population to explode and the virus to become transmissible more quickly.
The combination of warm weather and drought conditions facilitates widespread disease transmission. As shallow streams and ponds recede, they expose muddy edges and wet soil, which are the preferred breeding grounds for midge larvae. Simultaneously, the lack of widespread water sources forces deer to congregate densely around the few remaining water holes. This concentration of susceptible deer and virus-carrying midges leads to an increased rate of biting and infection.
EHD outbreaks are highly seasonal, occurring predominantly in the late summer and early fall months when midge populations are at their peak. The spread is halted with the change of seasons. The first hard frost effectively kills the adult midge population, interrupting the virus transmission cycle until the following warm season.