West Nile Virus--An Evolving Epizootic

"West Nile virus (WNV) is coming to a state near you if it hasn't already arrived," said Eileen Ostlund, DVM, PhD, head of the equine/ovine viruses section at the Diagnostic Virology Laboratory, National Veterinary Services Laboratories in Ames, Iowa, during the Western Veterinary Conference held Feb. 15-19 in Las Vegas, Nev. In a comprehensive overview of WNV's activity in the United States since its arrival in 1999, she discussed its transmission methods, hosts, U.S. history, clinical signs, diagnosis, vaccination options, impact on the U.S. horse
population, duration of immunity, and supportive care.

In four years, the disease has spread all the way across the country and infected thousands of horses. "Now it's pretty much coast to coast and obviously not going away," she said. "The greatest number of cases tend to occur in September and October."

Birds and Mosquitoes

Ostlund noted that WNV has been found in more than 130 bird species to date. "It's not a picky virus in terms of its hosts," she said. "No state lacks birds that can harbor WNV."

Corvids (birds including crows, bluejays, and magpies) have a high mortality rate when infected with WNV, she said. However, she added that sparrows, finches, grackles, and some other species have a low mortality rate, but high viremia (a high level of virus in the bloodstream). Thus, even though you don't see them dropping dead everywhere, they are significant reservoirs of the virus.

Mosquitoes that pick up WNV from birds fall into two categories, she explained--amplification cycle vectors and bridge vectors. The former spread the virus between birds and mosquitoes (mosquitoes bite infected birds, then transmit the virus to non-infected birds). These include many species of the Culex genus. The bridge vectors bite more than one host, "bridging" infection between species (they bite infected birds, then humans, horses, etc.). Candidate (possible) bridge vectors include some species from the Culex, Aedes, and Ochlerotatus genuses.


Ostlund discussed various laboratory tests for the virus in detail, noting that horses are much less viremic than birds, and viremia occurs well before clinical signs. "Tests designed for birds can miss half of the equine cases," she said. "Check with the lab--if they're only adapting bird tests, they may not be sensitive enough for testing horse tissues."

Ostlund also discussed prevention of WNV disease with the two currently available vaccines--the killed vaccine West Nile-Innovator (from Fort Dodge) and the recombinant canarypox vectored vaccine (from Merial). Both are labeled for a two-dose initial series followed by a single annual booster dose. For more on using the two vaccines, see article #5065 .

Additional Notes

"We're still learning about WNV in the United States," Ostlund stated. In the five years the disease has been here, she said, researchers and practitioners have learned several additional facts about the disease and its spread. For one, she said that there is vertical transmission (from parent to offspring) in some mosquito species. "We don't know yet how important that is," she said. "The virus has been found in some (mosquito) egg floats." She also opined that for every horse which expresses clinical signs of WNV, 10 others are exposed and develop antibodies with no clinical signs. "Over a two-year period, 100% of our lab horses seroconverted (began producing antibodies to the virus, indicating exposure to the disease) without ever having been vaccinated, and they didn't get sick," she said.

For horses which did get sick, she noted that head protectors were helpful, since ataxia and weakness are common to nearly every WNV case. "Slings are helpful for really severe cases," she said. "Many horses harm themselves thrashing around, so nursing care is extremely critical.

"It seemed like the horses that recovered did so fully," she went on. "But as we see more cases, we realize that not all recover fully quickly; recovery may take many months."

She added that mules and donkeys are not precluded from WNV infection.

WNV in 2004

Ostlund described the following expectations regarding future WNV activity and horse owner response:

  • More horses will be vaccinated and/or have natural exposure;
  • More facilities for WNV testing will influence the number of horses (and other species) tested;
  • Testing facilities will expand and data collection will decrease (handled differently by different states); and
  • More animal species will be identified as susceptible to rare clinical WNV illness.

More information: See article #5065.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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