The Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) confirmed West Nile virus (WNV) in a horse located in south Texas. This is not the first case of WNV in Texas in 2015; the first recorded positive was in a Houston County horse on July 2. However, the most recent positive comes from a horse in Atascosa County, south of San Antonio.
The Atascosa County horse exhibited mild lameness when ridden and when taken to the veterinarian, presented at a low-grade fever but no other classic WNV clinical signs. The horse’s veterinarian requested a variety of blood tests to determine the cause: West Nile virus, Eastern Equine Encephalitis, Western Equine Encephalitis and Venezuelan Equine Encephalitis. WNV was the only positive result from this animal.
WNV is a viral disease that normally cycles between wild birds and mosquitoes. As the virus infection rate increases in birds it is more likely to be transmitted by an infected mosquito that bites horses and humans. The virus abruptly attacks the central nervous system. As a reportable zoonotic disease, the Texas Animal Health Commission (TAHC) was made aware of the positive test results. Human infection with WNV is known to occur in the United States. However, horse to human transmission is not a concern.
No horse, regardless of age, is immune to WNV infection. In the US, clinical signs for WNV develop in only 10-39 percent of infected horses. The death rate among U.S. horses ranges from 30 to 40 percent for West Nile disease. Of horses that recover from the disease, up to 40 percent may exhibit neurological signs for six months or more after the initial diagnosis. Symptoms for neurologic diseases can present similarly; a serologic sample was sent to the lab for neurologic testing. Clinical signs usually include depression, ataxia, lameness, partial paralysis, muscle twitching or recumbency. The horse may also exhibit an altered mental state, a reduced appetite, grinding of teeth, blindness or a fever.
“The recurring rain earlier this year will potentially have an affect on the number of West Nile cases seen this summer and fall,” said Terry S. Hensley, MS, DVM, TVMDL assistant agency director and Texas A&M AgriLife Extension Service veterinarian. “Vaccination is the first step to protecting your horse. Mosquito control around the barn is also important to reduce the risk of West Nile exposure. It is impossible to keep mosquitoes away from you or your animal but you can be proactive by vaccinating against this potentially deadly disease.”
TVMDL, TAHC and the Texas Department of State Health Services have information related to West Nile virus and mosquito control available for free download.
Additional Texas horses have tested positive for Eastern Equine Encephalitis (EEE). The first case was reported on July 13; the total positive EEE cases now number five.
Often horses infected by EEE present with sudden onset of severe ataxia and fever. The Jasper County horse exhibited classic clinical signs of neurological disease. The second case, in Jefferson County, exhibited impaired vision, flaccid lips and could not rise from its downed position. It was confirmed that neither horse was vaccinated against EEE. As with previous cases, the veterinarians requested several neurologic tests, of which, only EEE was positive.
EEE is a viral disease that normally cycles between wild birds and mosquitoes. As the virus infection rate increases in birds it is more likely to be transmitted by an infected mosquito that bites horses and humans. The virus abruptly attacks the central nervous system. EEE cannot be transmitted from horse to horse, or from a horse to a human. Horses and humans are considered “dead-end” hosts which means if infected they cannot transmit the virus back to feeding mosquitoes.
As with the three previous EEE cases, the horses were located in the eastern area of the state. TVMDL will continue to update our website if any additional cases are confirmed positive. Information on previous updates can be found here at http://wp.me/p62HEK-33G.
For more information on TVMDL’s equine neurologic disease testing, visittvmdl.tamu.edu, or contact the agency headquarters at 1.888.646.5623.