By Tracy Gantz
California is bracing for an onslaught of West Nile virus in horses this year. Dr. N. James MacLachlan of the University of California at Davis said that he and his colleagues expect to see the first dead birds infected with West Nile in April, followed by the first horse and human cases in June and July. To help California horsemen prepare, the U.C. Davis Center for Equine Health (CEH) coordinated a West Nile seminar on March 9 at Santa Anita.
At the seminar, which was sponsored by the three major Southern California racetracks and horsemen's groups such as the Thoroughbred Owners of California, U.C. Davis veterinarians distributed a recommended vaccination schedule for California horses. Dr. Gregory L. Ferraro, director of the CEH, moderated the seminar and also brought in experts from Florida and South Africa.
"West Nile virus is destined to become an endemic disease throughout the U.S.," Ferraro said. "California will undoubtedly become affected this summer, so horsemen need to heed the lessons learned by the experience of other states and properly prepare themselves now."
Last year, one isolated, unexplained case of a woman infected with West Nile occurred in California. Seminar speaker Dr. Pamela Hullinger of the California Department of Food and Agriculture (CDFA), said that they also discovered one equine case, but that the horse had come from Nebraska, where it apparently had contracted the disease.
Hullinger said that California's surveillance plan includes a sentinel chicken program. Dr. Sharon Hietela of U.C. Davis noted that West Nile testing was added to the sentinal chicken program in 2000. The state also conducts mosquito testing, encephalitis case surveillance, and dead bird testing.
Dr. John Madigan of U.C. Davis said that Davis recommends that "when the first West Nile cases are confirmed in California, booster all horses that haven't been vaccinated within six to eight weeks." Madigan said that while the vaccine manufacturer recommends annual boosters once a horse has undergone the initial two-dose primary series, he and his colleagues are recommending boosters every six months based on how the disease has progressed.
Drs. Robert J. MacKay and Maureen T. Long of the University of Florida spoke on their experiences in Florida, one of the states hardest hit by West Nile. MacKay showed a video of horses afflicted with West Nile so that horse owners could learn what symptoms to expect. He noted that some horses relapse after about eight months and prognosis for survival in those cases is usually poor.
Long said that the disease is showing "increased activity over time and hasn't quieted down as we'd hoped." She said that the season of infection has widened, beginning sooner in the year and lasting longer.
Dr. Alan Guthrie of the University of Pretoria in South Africa discussed a study conducted in that country on the progression of a West Nile strain that does not cause disease in horses. The study showed an alarming rate of exposure to the condition -- 15% of foals were sero-positive and 74% of mares were sero-positive. Guthrie said that by age 12, all horses were sero-positive.
Dr. W. David Wilson of U.C. Davis spoke about the vaccination repercussions in mares and foals. His vaccination recommendations include giving foals born to unvaccinated mares the first shot at three months. Foals born to a mare who was booster vaccinated within two months of foaling should receive their first shots at four to six months. Birth date should influence when to vaccinate because of the disease's cyclical nature.
Wilson also recommends giving foals three doses instead of the two suggested by the manufacturer. This is based on a small study that showed four of five foals developing good immunity after two doses and the fifth developing good immunity after the third dose.
Further information on California's plans for combating West Nile can be found on the CEH website (www.vetmed.ucdavis.edu/ceh/wnv_info.html).