Since West Nile virus (WNV) was first identified in North America in 1999, researchers have worked hard to determine how it affects horses. Now a study of two 2002 outbreaks of WNV-related disease offers intriguing clues to which horses are most susceptible to the worst effects of the mosquito-borne virus.

WNV infection usually causes little or no illness, but in rare cases it triggers a life-threatening swelling of the brain. Since 1999, more than 21,000 horses have become ill as a result of WNV infection, and an estimated 7,000 have died. However, incidence of WNV disease has declined dramatically since a vaccine, West Nile Innovator, was introduced in 2001. A second WNV vaccine, Recombitek, became available in 2004.

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For their study, researchers at Colorado State University worked with state department of agriculture officials in conducting telephone interviews with the owners of horses in Nebraska and Colorado who had confirmed cases of WNV-related illness. The owners were asked to describe their horses' clinical signs and vaccination status, as well as their general farm-management practices, such as mosquito-control measures.

Information was gathered on 484 horses, 137 of which (28.4 percent) died or were euthanatized as a result of WNV infection.

The resulting data revealed that an infected horse's prognosis may depend on several factors:

• Initial signs: Horses who were so weakened by their illness that they were unable to rise were 78 times more likely to die than were animals who never lost the ability to stand.
• Gender: Mares were nearly three times more likely to succumb than were geldings or stallions.
• Vaccination status: Unvaccinated horses were twice as likely to die than were animals that had received at least one WNV inoculation before becoming ill.
• Age: Older horses were at a greater risk of mortality than were younger animals.

These findings leave many questions unanswered, says Tricia Salazar, DVM, who worked on the project, but they demonstrate the potential severity of WNV-related illness.

"For that reason, protective measures are extremely important," Salazar says. "We recommend vaccination against WNV, as well as mosquito mitigation practices."

This article originally appeared in the November 2004 issue of EQUUS magazine.