On July 1, 2016, an unvaccinated five (5) year-old grey thoroughbred (TB) mare in San Diego County displaying neurologic signs including ataxia, depression, stiffness in neck and spine an severe lip twitching, was confirmed positive for West Nile Virus (WNV).  The onset of clinical signs began on July 24th.  The mare is alive and recovering slowly.


West Nile Virus (WNV) is a mosquito-borne virus first detected in the United States (U.S.) in the New York City area in 1999. Since 1999, the virus has spread throughout the U.S. and Canada, infecting birds, humans, horses, and other animals.

As of 2015, more than 27,000 horses in the U.S. have been infected since the disease was first identified. The virus is maintained in the wild bird population and is spread between birds by mosquitos. Birds are considered the natural reservoir for WNV since high levels of virus circulate in their bloodstream. Mosquitos acquire WNV in blood meals from infected birds and pass it on to other birds, animals, and people. Mosquitos that feed on an infected horse or human have not demonstrated the ability to ingest enough of the virus to transmit it to other animals or humans; therefore, horses and humans are considered “dead end hosts.”

West Nile Virus may cause a wide range of clinical illness ranging from mild” flu-like” signs to encephalitis (inflammation of the brain) that may be fatal to both humans and horses. While horses are susceptible to WNV infection, many infected horses do not develop clinical illness and recover uneventfully.