Maureen Long, DVM, PhD, Dipl. ACVIM, assistant professor of large animal veterinary medicine at the University of Florida, has been researching WNV clinical infection since the virus first hit Florida in 2001. She's widely recognized as one of the clinicians most experienced with WNV in the country. "There's definitely more good than bad to vaccinating mares against WNV," said Long. "I think that a reasonable approach needs to be taken towards vaccinating mares during pregnancy. Any mare, whether pregnant or not, can have a reaction to any vaccine, but widespread pregnancy loss just has not been one of them that I can find. In my mind, (these cases haven't) been investigated enough that I'm comfortable even making a comment (on them)."
Long emphasized that it is important to examine complete health histories on these mares--such as abortion histories, what other vaccines were given, and what kind of stresses they had experienced--before making speculation as to what caused an adverse event in the pregnancy.
Dick Bowen, DVM, PhD, associate professor of biology at Colorado State University has been studying WNV since 2000. "I'd be really surprised if that vaccine is inducing abortions, unless it's contaminated with something else, or it's not properly inactivated. In this day and age, I'd be really shocked if that were the case, especially if it were widespread.
"Who knows what happened to coincide with the West Nile vaccine (administration in these horses)?" he added.
Charles McDaniel, DVM, of the USDA's Center for Veterinary Biologics in Ames, Iowa, said that there have been adverse events reports related to reproduction, but only a limited number, and not out of the realm of the numbers of adverse events typically reported with other types of vaccines. The exact number of reports is considered confidential business information, but "these numbers are not large enough to statistically determine that there is a vaccine-associated problem," he said.
"As with any vaccine, we collect adverse event reports. We look at the timing of the event in relationship to the vaccination and severity of the event in determining when and whether we evaluate," said McDaniel. "If we find a problem, we begin an investigation. We actively encourage reporting of any adverse events for all products."
According to Koons, after prodding from the Lost Foals Group, Fort Dodge recently changed their FAQ list on their web site to reflect the concerns that people have been expressing:
"Currently, the West Nile virus vaccine does not carry a claim for vaccination of pregnant mares. However, in pre-release safety trials,  of 649 horses were pregnant mares which were closely monitored following vaccination. There were no ill effects demonstrated in the mares, their pregnancies or their subsequent foals. Since its release, thousands of doses have been administered to pregnant mares. A low number of undiagnosed abortions have been reported from the field following use; these reports appear to be associated with individual animal responses due to stress, which can occur with any vaccine administered to pregnant animals."