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Mitigating the Risk of Mosquitos: EEE, WEE, and WNV Diagnostic Guidance

By Mallory Pfeifer
Each summer, the Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) confirms several cases of Eastern Equine Encephalitis (EEE) and West Nile Virus (WNV) in horse populations across the state. Mosquitos play a vital role in the transmission of both diseases. It is important for horse owners to work with their veterinarian to determine the proper vaccination program for their horse and ways to mitigate the risk of infection.
Thus far, TVMDL has confirmed EEE in 3 horses from the southeast Texas area.
EEE Clinical Signs and Veterinary Guidance
EEE is a viral disease that typically cycles between wild birds and mosquitoes. As more birds are infected with the virus, the likelihood of it being transmitted to a mosquito, that later bites horses or humans, increases.
Horses and humans are considered “dead-end” hosts which means, if infected, they cannot transmit the virus back to feeding mosquitoes. Additionally, EEE cannot be transmitted from horse to horse, or horse to human. Once transmitted, the virus abruptly attacks the central nervous system.
As a reportable zoonotic disease, the Texas Department of State Health Services (DSHS) and the Texas Animal Health Commission (TAHC) were made aware of the positive test result.
Eastern Equine Encephalitis has a higher mortality rate than Western Equine Encephalitis (WEE) or WNV in horses.
Clinical signs of EEE include flaccid (droopy) lips, dullness, muscle fasciculation, ataxia and head pressing.  Signs for various neurologic diseases can present similarly so diagnostic testing is recommended for a definitive diagnosis.
WNV Clinical Signs and Veterinary Guidance
WNV was first detected in the United States in 1999 and is now considered endemic across most of North America.
Similar to EEE, WNV is maintained in nature by cycling between wild birds and mosquitos; birds are used as a reservoir host while mosquitos are used as vectors, moving the virus to other hosts. It is possible for mosquitos to transmit the virus to horses and humans, therefore validating the importance of mosquito control around homes and barns. Currently, there is no evidence to suggest that horse-to-horse or horse-to-human transmission is possible.
Clinical signs of WNV typically include neurological issues such as depression, ataxia (stumbling, staggering, or wobbly gait), leg weakness, lameness, partial paralysis, muscle twitching, recumbency, or the inability to stand. Horses may also exhibit an altered mental state, a reduced appetite, grinding of teeth, blindness, or a fever.
Clinical signs cannot be the sole basis for diagnosis. Other diseases may cause similar signs in horses and therefore diagnostic testing is necessary for an accurate diagnosis.
Diagnostic Testing
TVMDL offers diagnostic testing for EEE and WNV, in addition to Western Equine Encephalitis (WEE) and other diseases affecting equine.
Highly effective vaccines are available for EEE, WNV and WNV. These equine neurologic diseases are preventable with proper vaccination. Owners should consult with their veterinarian to determine vaccination needs.