West Nile Virus Spreads: Horses in at Least 15 States Affected

West Nile virus (WNV) is steadily spreading across the country, having been detected in at least 33 states so far this year*. Horses in 15 states have been infected with the virus in 2003; the most recent announcements of equine WNV cases were from Kansas and New Mexico.

Health officials in Kansas announced July 9 that they had logged the state's first equine WNV case for this year. Results on the Wichita County horse were received from Kansas State University. KFOX News of Las Cruces, NM, reported that a horse in Sierra County was infected with WNV and euthanized.

Already this year, the Centers for Disease Control (CDC) has confirmed human WNV cases in South Carolina and Texas. Nebraska officials announced the state's first presumptive positive human case of WNV for 2003.

During a press conference in Atlanta, Ga., on June 15, federal officials said that the virus has spread more quickly this year than in other years (the virus made its first appearance in 1999 in New York City). By this time last year, WNV had been detected in only 20 states. Dr. Julie Gerberding, director of the CDC, said, "It is too soon to predict the shape of the epidemic, but the signs all indicate that there is reason to anticipate a problem."

So where is the virus going to rear its head next?

"If you remember at the very beginning of West Nile, it started right there in Manhattan, and then it moved down the Eastern seaboard the next year," said Gerberding. "Last year it moved across the United States, and it's very early to predict what we're going to see on the West Coast. So obviously, the surveillance system to look for mosquitoes and infected birds and animals is very active in the Western states right now, because we won't be at all surprised to see that it does emerge there, and it may emerge very quickly."

Gerberding said the way the virus spreads to previously unaffected areas would depend on bird migration patterns, weather, and mosquito abatement procedures.

Equine Infection in Retrospect

By the end of 2002, 40 states had logged a total of total of 14,717 equine cases of WNV. Officials reported 738 equine cases in 2001, 60 in 2000, and 25 in 1999. An official national equine case count has not been published this year, but reports on prior years can be viewed at http://www.aphis.usda.gov/lpa/issues/wnv/wnv.html.

Remembering the four previous WNV seasons, officials anticipate an upswing in disease activity toward the end of July and August, so "we have to stay tuned to really appreciate what the full scope and magnitude of the outbreaks will be this year," said Gerberding.

States with WNV equine infection thus far in 2003 include Alabama, Arkansas, Florida, Georgia, Kansas, Kentucky, Minnesota, Missouri, New Mexico, North Dakota, Oklahoma, South Dakota, Texas, Wisconsin, and Wyoming.

To learn more about WNV and to view maps showing its spread, visit www.TheHorse.com/wnv.

*The Centers for Disease Control (CDC) has reported that the virus has been found in 32 states total, but that count does not include Massachusetts. Public health officials from that state report an avian case of WNV on their web site.