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WNV in Mares and Foals-When to Vaccinate?

As we begin our fifth year in dealing with the dreaded WNV, over 14,000 cases in the United STates as of 2002 have been reported. The question was raised about a link between the WNV and abortions in equines.A retrospective study by the University of Kentucky's Livestock Disease and Diagnostic Center, from July of 2002 through early 2003, looked at 400 equine abortions for evidence of WNV. Their findings were suprising. Of the 400 horses examined, 35 had evidence of WNV. Although this is only approximately 8 percent, it does require more research to see if there is a connection between the WNV and aborted fetuses. At this time there is no evidence that the WNV caused the abortions; only that there was evidence of the virus in the aborted fetuses. Further testing and research is ongoing to determine the relationship between WNV and abortion.

*Vaccinating Mares and Foals

Renowned veterinarian Rob Holland, DVM, PhD, a private practitioner in Kentucky and a technical services veterinarian for the Intervet pharmaceutical company, explained the protocol for vaccinating broodmares.

He recommends you vaccinate your mares four to six weeks BEFORE foaling, what you're doing is bolstering their IgG (a type of antibody) and all their immunological parameters. In the case of the mare and the (unborn) foal, there's a six-layer placenta that does a very good job of protecting the foal against potential disease that affects
the mare, and doesn't allow any antibodies to cross it.

*Maternal vs. Foal Antibodies

W. David Wilson, MS, BVMS, MRCVS, of the Department of Medicine and Epidemiology in the School of Veterinary Medicine at the University of California, is recommending, based on information gathered from studies with other vaccines, that if the mares are NOT vaccinated against WNV or they haven't been exposed (which is now the situation for only horses in the far western states) that foals can be vaccinated starting at two to three months of age.

IMPORTANT NOTE: Dr.Wilson would have serious concerns about vaccinating foals at such a young age if their dams WERE vaccinated or had been previously EXPOSED to WNV. Studies with influenza, EEE, WEE, tetanus, rabies, and EHV have shown that maternal antibody interference extends up to to six months and beyond. Therefore, many foals vaccinated at less than six months of age fail to mount a protective immune response to the standard two-dose primary vaccination series.

To avoid this problem, Wilson has recommended that veterinarians delay vaccination of foals from mares which were vaccinated or exposed to WNV until the foal is about six months of age. Wilson recommends the following series:

FIRST vaccination at six months or older.

SECOND vaccination three to four weeks later.

THIRD vaccination six to eight weeks after the second dose of vaccine.

What Dr.Wilson and others have found with other vaccines is that many (foals) don't respond optimally after two doses of vaccine even when vaccination is started after maternal antibodies have waned. A third dose gives a little more assurance that the ones that haven't responded to two doses will respond to the third dose.

Please check with your personal veterinarian for more information on if and when to vaccinate your mares or foals for WNV.

By Michele Anderson

Publisher/editor of Horse Tales and Clips an informative FREE newsletter on the care and business of horses. If you would like to sign up for our FREE newsletter please use the link provided:

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