West Nile Vaccine: Adverse Effects in Mares?

By Stephanie L. Church
News Editor, The Horse

Allegations that the West Nile virus vaccine might have caused abortions and deformed foals were made in a May 30 Denver Post article. Mare owners calling themselves the "Lost Foals Group" claim the vaccine has caused up to 1,200 abortions and nearly 300 deformed or dummy foals. However, top veterinarians dispute those allegations based on their experiences.

Tom Riddle, DVM, of Rood and Riddle Equine Hospital in Lexington, Ky., said, "I can tell you that my practice has vaccinated thousands of mares with the West Nile virus (WNV) vaccine and we have seen no relationship between the vaccine and abortions. My suspicion with the problem that is reported in the Denver paper is that the vaccine is incidental to the abortions and not related to it."

Jeffrey T. Berk, DVM, a partner in Ocala Equine Hospital in Ocala, Fla. (which has 12 practitioners who treat mainly Thoroughbreds and performance horses), said Florida's horses are at high risk for getting the West Nile virus due to the year-round mosquito-friendly climate of the state.

"We've been using the vaccine ever since the day that it became available--we've used tens of thousands of doses," said Berk (Ocala Equine's pharmacy has sold about 30,000 doses). "In general conversation, there has been no discussion whatsoever of this vaccine causing any problems at all, much less abortions."

Among Berk's clients is Farnsworth Farms, the largest Thoroughbred breeding operation in Florida. "They carry in between 300 and 400 mares (that we've vaccinated). I'd probably know if something was going on," he said.

The equine WNV vaccine manufactured by Fort Dodge Animal Health (FDAH) was given conditional approval by the USDA for use in August, 2001, after showing purity, safety, and reasonable expectation of efficacy through initial studies. During the tests for safety, 649 horses in five states were tested. The vaccine was shown to be 96.28% free of local (stiffness and swelling around the injection site) or systemic reactions. According to Yu-Wei Chang, MS, PhD, of FDAH, in a presentation at the 2002 American College of Veterinary Internal Medicine convention in Dallas, Texas, 32 of the horses in the study were pregnant mares, and had no apparent adverse reactions to the vaccine.

The Post article stated that records from the USDA indicate that five of the pregnant mares in the safety study aborted, but veterinarians doubted the vaccine had any role. However, FDAH representatives have disputed the report by the Post of five abortions, saying that only one mare aborted and the veterinarian did not believe that abortion was caused by the vaccine.

After completing further field studies, the vaccine was granted a full license in early 2003 after efficacy was shown (see articles #4277, #3903). According to FDAH, millions of doses, which are available only through veterinarians, have been given to horses since the initial release. Like many vaccines used year-round in equine medicine (for example, tetanus), it is not labeled for use in pregnant mares.

Rich Koons, a Quarter Horse breeder in Welcome, Minn., and founder of the Lost Foals Group, believes the vaccine is the source of reproductive problems in his herd. He bred three of his mares in April and May of 2002. He said all three were verified to be in foal--two before their initial two WNV vaccines were given, and one immediately after she received the vaccine. All were vaccinated in July and August of 2002. All three came up empty in March of 2003.

Koons said that in nine of 10 years, his herd's pregnancy success rate has been 100%, and that two of the three mares were proven producers, having no prior problems. He asserted that the mares show residual problems after he found they weren't in foal, and he has been unable to rebreed them.

"They don't cycle or they're infected," he said. "And their heat cycles are short."

He started putting notices out on discussion groups on the Internet to see if anyone else had experienced such problems, and he said that he's found "many other people" who have experienced problems in their herds. The Lost Foals group mission is: "To document cases of pregnant mares having been vaccinated with the West Nile vaccine and lost their foals some time after vaccination." He encourages mare owners with similar problems to contact the USDA to report them.

"I want the whole world to know that there's a problem, and I am out there trying to get the word out and that's what our group is about," said Koons.

Although Koons gave general numbers for how many mares he says have been affected (up to 1,500), he was unable to give specifics on health histories for many of the animals, and said that only a few aborted fetuses have been submitted to diagnostic laboratories for autopsies. Of those, the results have been inconclusive. Also, state some veterinarians, the number of reportedly affected mares is difficult to interpret without knowing just how many pregnant mares have been vaccinated against WNV across the country.

According to a recent article in The Horse by Jonathan F. Pycock, BVetMed, PhD, Dipl. ESM, MRCVS, a reproductive specialist and editor of the book Equine Reproduction and Stud Medicine, "A well-managed stud farm should typically achieve pregnancy rates (diagnosed at 15 days of gestation) per estrous cycle of 65%, an end-of-season pregnancy rate of 85%, and a live foal rate in excess of 75%."

The Veterinary Take on Things

Berk added that he considered the Eastern and Western encephalitis vaccines safe, and that he has had no more problems with the WNV vaccine than the other "old standbys."

"We treat it pretty much like the encephalitis vaccines, in the sense that we feel that Florida horses are at risk, and recommend vaccination a minimum of three times per year. In a normal broodmare, we would vaccinate every four months. But we'll try to also time it so that if we have a pregnant mare, we'll vaccinate her within 30 days of foaling. We try to give one of those vaccines then so we impart the passive immunity to the foal through the colostrum."