West Nile Virus: Washington Reports Highest Number of Cases

West Nile virus cases are popping up across the country, with Washington state reporting the highest number of equine cases.

As of Aug. 19, Washington animal health authorities had reported 12 equine cases. Other states reporting cases included Montana (4), California (3), Kentucky (2), and Louisiana (2). Mississippi, Texas, Utah, and West Virginia each reported one infected horse.

West Nile virus started its westward march across the United States in 1999. Western states now lead in the number of equine cases of the mosquito-borne virus, with Washington topping the 2008 tally with 41, followed by California with 32.

In calendar year 2008, there were 179 equine cases of West Nile virus reported from 30 states and Puerto Rico.

West Nile virus (WNV) causes a potentially fatal encephalomyelitis (inflammation of the brain and spinal cord) in a variety of mammals such as birds, horses, and humans.

WNV is maintained in the wild bird population and is spread between birds by biting mosquitoes. Humans and horses become infected after being bitten by mosquitoes that became infected with the WNV by feeding on infected birds. The virus enters the horse's bloodstream and spreads to the spinal cord and brain causing a widespread inflammation. Clinical signs of disease typically present within three to 15 days of the animal being bitten by an infected mosquito.

Horses and humans are considered to be dead-end hosts of WNV and, therefore, do not contribute to the transmission cycle. The virus is not directly transmitted from horse to horse or horse to human. Similarly, indirect transmission via mosquitoes from infected horses is highly unlikely because horses do not experience a significant viremia (i.e., they have negligible amounts of virus circulating in their blood).

Watch our free Webinar, "West Nile Virus: Reality Check!," presented by Steve Reed, DVM, Dipl. ACVIM, of Rood & Riddle Equine Hospital in Lexington, Ky.

Veterinarians recommend that horses be vaccinated to protect against mosquito-borne diseases such as West Nile virus and Eastern and Western equine encephalitis as part of their routine health care program. There is no human vaccine.

The Centers for Disease Control (CDC) provides weekly reports of arbovirus case information from its ArboNET reporting system. Access these reports through the USDA Animal Health Monitoring and Surveillance Web site.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.