West Nile virus (WNV) has been making headlines and striking fear into the hearts of horse owners. Hardly a day goes by when someone doesn't mention WNV. First identified years ago in Africa, WNV was foreign to the United States until the fall of 1999, when birds, horses, and people in New York State developed signs of encephalitis, an infection of the brain (enceph = brain, itis = inflammation). It probably came here in a mosquito, perhaps having hitched a ride on a cargo ship. At first health officials thought the outbreak was one of the encephalitis viruses already known to occur in this country, the most common of which are Eastern Equine Encephalitis (EEE) and Western Equine Encephalitis (WEE), but this virus soon proved to be different and before long was identified as WNV.

Initial hopes were that this virus would not continue to spread, but would die out during the winter of 1999 to 2000. That expectation did not materialize. WNV is able to overwinter in mosquitoes and birds, re-emerging when mosquito season starts again. Since WNV infects migratory birds, it is readily spread from state to state. WNV is therefore not likely to remain geographically contained, as has EEE, WEE, and VEE.

By the fall of 2000 WNV had been confirmed in 12 states: Connecticut, Delaware, Maryland, Massachusetts, New Hampshire, New Jersey, New York, North Carolina, Pennsylvania, Rhode Island, Vermont, Virginia, and Washington, DC.

As of mid August of 2002, it has been confirmed in 37 states and the District of Columbia. Confirmed human cases are those documented to be WNV by the Centers for Disease Control and Prevention (CDC) and confirmed equine cases have been tested at the USDA National Veterinary Services (NVSL) at Ames, IA. Since its first appearance the virus has spread to include more than half the United States, extending as far west as Texas, Colorado, New Mexico, and Wyoming.

Some folks on the West Coast are convinced WNV will never be a problem in their area, but they may well be proven wrong. Surveillance for WNV has been established in the western states, and dead birds and animals with suspicious neurologic diseases are already being tested for WNV.

Beware of Numbers
An awful lot of inaccurate information is being spread about WNV. Some comes from inaccurate interpretation of media reports, but some just seems to crop up like blackberries in Oregon—out of nowhere. The internet has allowed the rapid dissemination of WNV information, some good and some not so good. I have seen "confirmed" reports of entire barns of horses dying or being put down because of WNV. Well, the facts are that not all exposed horses (or people, for that matter) develop the disease. Only between 3% and 14% of exposed horses actually develop disease, and of those only 30% to 40% will be fatal cases. For WNV to infect an entire barn of horses is highly unlikely.

Sometimes the person posting an erroneous message later recognizes and corrects the error, as in the case in which one horse was infected and 12 others on the farm were first said to have died from WNV, later corrected to say 12 others had been vaccinated for WNV. Another recent post, in which a group of horses on a Pennsylvania farm were supposedly buried due to WNV has never been corrected, even though the USDA reports only 2 confirmed equine cases of WNV as of September 2002 in Pennsylvania.

The large numbers published regarding WNV might refer to confirmed WNV in dead birds (3,128 as of August, 2002), to infected horses or people who subsequently recovered, to animals that are being tested for WNV (many of which end up being negative), or to the number of horses that were vaccinated following detection of WNV in the area. Other diseases cause neurologic signs in horses, so a horse with these signs is not necessarily a case of WNV, and every dead bird does not have WNV.

The most accurate and up-to-date WNV information and case numbers are published by the USDA. Although the CDC reports some equine cases, the USDA is specifically tracking animal cases of WNV and has more accurate case numbers. But, even these numbers will be less than actual cases, because the USDA will wait for confirmatory tests before listing a case, and also because veterinarians in areas of WNV outbreaks don't always test every suspicious case. So, it is true that the numbers listed on state and government websites will be less than the actual number of equine cases.

West Nile Virus and Its Signs

West Nile Virus Prevention

Questions and Answers about West Nile Virus

West Nile Virus in Canada

Beth A. Valentine, DVM, PhD, is an equine pathologist at Oregon State University, Corvallis, and co-author of Draft Horses, an Owner's Manual.