A report published in the Journal of Wildlife Diseases, from the Wildlife Disease Laboratory of the state of Michigan, describes an outbreak of neurological disease in wild deer. Seven captured deer with neurological symptoms were confirmed to have either positive antibody staining in the nervous system or positive isolation of the Eastern equine dncephalitis (EEE) virus. One of the deer was also infected with West Nile virus (WNV).
This is the first report of an actual outbreak of EEE clinical infection in deer, although an isolated case was reported in the past (from Georgia) and positive blood antibodies indicating exposure have been found in many different species.
The EEE virus is transmitted by mosquitoes. The virus circulates between horses and wild birds via the mosquito. While other species of animals have been found to have antibodies, it’s always been felt this only represented exposure to the virus, not actual infection or multiplication of the virus in those other animals.
We need to know more. If further research shows this to be more of a problem for deer than was expected, it will be important to determine if the level of circulating virus in deer is high enough to make them a source of infection for horses after a mosquito feeds on an infected deer.
If so, roaming deer could be a bridge between areas of marshland with high concentration of infected birds and mosquitos, into drier areas where most horses are kept and mosquitoes could transmit the virus from the infected deer to the horses.
Don’t neglect your encephalitis vaccines, even on horses that ”only stay at home.” The potential for infection may be as close as the nearest wooded area.