APHIS: West Nile virus vaccine safe for use

Prompted by many phone calls from anxious horse owners, veterinarians, and others involved with horses, the Department of Agriculture's Animal and Plant Health Inspection Service has looked into alleged problems in pregnant, vaccinated mares and concluded there is no association with the West Nile virus vaccine.

In a July 8 press release, APHIS acknowledged recent stories suggesting that Fort Dodge Animal Health's Innovator vaccine may cause pregnant mares to abort or give birth to deformed foals. The agency is concerned that the "misleading information" in those articles may lead horse owners to elect not to use the vaccine.

"Horse owners should be assured that the vaccine is safe, and it should be used as protection against West Nile virus," APHIS asserts. "Millions of doses of the vaccine have been used since USDA's Center for Veterinary Biologics approved its use in 2001."

Allegations implicating the vaccine in mare abortions and deformed foals have spread via news articles, Web sites, Internet rumors, and e-mails. One of the news reports was a May 30 Denver Post article. Fort Dodge countered the allegations in a June 12 press release, saying, in part, "The circumstances of the reported abortions are unclear and lack veterinary involvement and/or confirmation."

Ed Curlett, APHIS public affairs specialist, characterized the number of adverse event reports involving reproductive anomalies or failures that have been sent to the Center for Veterinary Biologics and Fort Dodge as being very small. Many of the reported problems were purely anecdotal, Curlett told JAVMA, but the agency looks into adverse event reports on a continuing basis.

Curlett added that there is a baseline of foal deaths that occur, unrelated to vaccinations, and that the WNV vaccine is a killed-virus product, reducing the likelihood it caused the problems.

"Our biggest concern is that people will stop using the vaccine. We've licensed it, we believe the vaccine is safe, and it should be used," Curlett said.

Dr. Rocky Bigbie, director of field veterinary services for Fort Dodge Animal Health, said, "The number of pregnant mares vaccinated is a very small percentage of the number of doses we sell. We've looked at the number of (adverse event) reports very carefully, compared to the number of doses given. We don't believe, nor does the USDA, there is any concern with the vaccine whatsoever."

When suspicions about the vaccine started surfacing, Dr. Bigbie called several renowned equine reproduction veterinarians, each of whom had administered thousands of doses to pregnant mares. None of them perceived any danger from the vaccine.

Veterinarians and researchers at the Colorado State University Veterinary Teaching Hospital and the College of Veterinary Medicine and Biomedical Sciences issued a statement June 18 refuting "incorrect accounts widely reported" in mid-June that they filed adverse reaction reports linking the West Nile Innovator vaccine to mare abortions and deformed foals. Equine veterinarians at CSU have observed no abortions attributable to the vaccine and recommend that clients have their horses vaccinated.

Spokespersons from the University of Wyoming and CSU veterinary diagnostic laboratories and the Wyoming and Colorado state veterinary offices also call the claims unfounded, saying none of the horses in question has been scientifically tested. Dr. Donal O'Toole, head of the Department of Veterinary Sciences and the Wyoming State Veterinary Laboratory, said, "None of the veterinary diagnostic laboratories in the U.S. are seeing this syndrome or associating it with West Nile virus."

Like most vaccines, West Nile Innovator is not labeled for use in pregnant mares, but many veterinarians decide the benefits of vaccinating them outweigh the risks. An adverse reaction could threaten the mare's life, and in that case, the fetus or embryo may be at risk, Dr. Bigbie said. But the incidence of such episodes is rare.

In any given year, the number of pregnancies diagnosed in mares is 10 percent to 12 percent higher than the number of mares that actually have a foal, he added. The diagnosis of pregnancy is not reliable in some cases, and in others, the embryo or fetus is aborted and sometimes resorbed.

Applying epidemiologic principles reveals no trends implicating the vaccine, and no increase in abortions. "As a matter of fact," he said, "nationwide, there are decreased numbers of abortions during the time that the vaccine has been on the market." As an analogy, Dr. Bigbie said that there is no more of a causal link between the decrease in abortions and use of the WNV vaccine than there is between mare abortions and the vaccine.

No vaccine is 100 percent effective, but a vaccinated horse has a better survival rate if it becomes infected with WNV. A recent collaborative study by Colorado State University, the Colorado and Nebraska state veterinarians, and the Veterinary Diagnostic Center in Nebraska found that even animals that hadn't received both prescribed doses of WNV vaccine were more likely to survive than unvaccinated animals were. In the study, Colorado reported 378 laboratory-confirmed WNV cases in horses. Only 13 of the animals were fully vaccinated, ie, had two vaccinations three to six weeks apart, with adequate time for the vaccine to take effect before the vector season. All 12 animals whose survival status is known, lived. The researchers noted it is also important to employ additional strategies for prevention of the disease, including mosquito mitigation. (To access the study online, go to www.ag.state.co.us/animals/WNV/WNVreport.pdf.)

The manufacturer recommends that horses receive two doses, three to six weeks apart, the first year, with the first dose to be given preceding the mosquito season or as soon as the attending veterinarian believes it is appropriate. That first year, a third dose may be given, at the discretion of the veterinarian, in areas with a year-round mosquito season. In subsequent years, the manufacturer recommends one dose, but the attending veterinarian may advise a second dose. In almost all parts of the country, Dr. Bigbie said, the peak incidence rate for West Nile virus is August and September.

The number of horses affected by WNV has grown steadily since the virus was first detected in the United States in 1999. In 2002 alone, the CVB had reports of 14,717 WNV cases in horses, with about a third of infected, unvaccinated horses dying. Dr. Bigbie hears estimates that three to four times that many equids may be infected. For one thing, veterinarians who see multiple cases may dispense with the paperwork involved in reporting subsequent cases.

The Center for Veterinary Biologics and Fort Dodge will continue to collect, monitor, and track the performance of this vaccine. Veterinarians and other vaccine consumers are encouraged to report problems with vaccines to the manufacturer and the CVB. The agency maintains a toll-free hotline, (800) 752-6255, and a mailbox on its Web site, www.aphis.usda.gov/vs/cvb.

Dr. Bigbie urges professional discretion about when to use and not use the vaccine. "Certainly, veterinarians have to do a risk-benefit analysis. But it would be a shame if thousands of horses did not get vaccinated because of an irrational fear, and then those horses succumbed to the virus. The danger is the virus; the danger is not the vaccine."