A University of California-Davis study shows that vaccination against West Nile virus (WNV) encephalitis not only works but is 45 times less expensive than treating a horse with the disease.

A flavivirus transmitted by mosquitoes, WNV causes potentially fatal neurologic illness in approximately 5 percent of infected horses. About 25,000 horses have become clinically ill and 8,000 have died as a result of WNV infection since the organism was first identified in North America in 1999. The first equine vaccine against WNV was introduced in 2001; there are now three on the market.

The study, designed to compare WNV incidence in vaccinated and unvaccinated horses, was based on 192 horses. Of these, 155 were vaccinated before the start of mosquito season in 2004, with either a killed or live virus vaccine, and the remaining 37 were not vaccinated.

Noting that WNV was not present in northern California until the summer of 2004, David Wilson, DVM, says, “During the following year, none of the vaccinated horses developed clinical signs of WNV, while two of the nonvaccinated horses became ill and exhibited neurological signs.”

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In addition, 68 percent of the unvaccinated horses developed antibodies to WNV, indicating exposure to the organism, even though they remained healthy. Both ill horses were referred to the UC-Davis Veterinary Medical Teaching Hospital and required intensive supportive care, including slings, intravenous fluids, Banamine and hyperimmune plasma. One horse became so ill he was euthanized; the other recovered.

When the researchers averaged the treatment cost of the two horses, they found it to be nearly $3,000. The annual expense for vaccinating a horse against West Nile virus is between $60 and $80, depending on the cost of a farm call to administer the injection. “Of course, treatment costs will vary with the severity of the illness,” says Wilson, “but even a mild case of WNV is still going to be much more expensive than vaccination.” The researchers conclude that WNV vaccination is a cost-effective management measure.

Reference: “Incidence and effects of West Nile virus infection in vaccinated and unvaccinated horses in California,” Veterinary Research, Jan. 2007

This article originally appeared in EQUUS magazine.

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