Vaccination Protocols for Your Horse Discussed at AAEP

Veterinarians agreed that vaccination is a critical component of an equine health maintenance program. A Table Topic on Vaccinations was part of the 2009 American Association of Equine Practitioners (AAEP) Convention in Las Vegas, Nev., in early December.

The attendees noted that veterinarians play a crucial role in client education, risk assessment, and herd evaluation to determine which vaccination program is best suited for an individual horse or herd. The vaccine technologies were briefly reviewed and included inactivated, modified live, chimera, recombinant, and DNA vaccine technologies.

A brief overview of immunization was presented by W. David Wilson, MS, BVMS, MRCVS, of the Department of Medicine and Epidemiology in the School of Veterinary Medicine at the University of California, Davis. Factors that affect vaccine decisions were discussed and included the onset of immunity following vaccination, the duration of immunity, safety, convenience, cost of the product, and challenges with our ability to assess equine immunity.

The AAEP has identified tetanus, Eastern and Western equine encephalomyelitis (EEE, WEE), West Nile virus (WNV), and rabies as core vaccinations for every horse. As a general rule, all adult horses need to be boosted at least annually. Horses living in the southeast need to be boostered more frequently for the encephalomyelitis viruses. The need to vaccinate against EEE twice annually or more frequently was discussed.

Multiple licensed vaccines for West Nile virus, each incorporating a different vaccine technology, are currently available on the market. Use of these varied products provided the basis for a good group discussion. Fortunately, published research indicates excellent safety and efficacy for all of the available West Nile virus vaccines. Additionally, a research abstract presented by Cynthia Gutierrez, DVM, of Intervet/Schering-Plough Animal Health during the convention demonstrated that a single dose of West Nile virus chimeric vaccine to pre-partum broodmares does produce an anamnestic (immunologic memory) response and can be used to boost colostral antibodies. This finding mirrors the results of similar studies with inactivated and recombinant WNV vaccines.

In general, pregnant mares should receive booster vaccinations 4 to 6 weeks prior to expected foaling, and vaccination for equine herpes virus-1 at 5, 7, and 9 months of gestation.

Multiple questions and comments during the table topic focused on general guidelines for vaccination of foals and maternal antibody interference. Maternal antibody interference might occur in young foals that received adequate colostral antibodies, and results in a decreased or absent immune response to vaccination. Foals with residual maternal antibodies typically produce a greater serologic response to vaccination when an initial series of three doses is administered rather than the typical two doses. Most foals should begin receiving immunizations at five to six months of age. If the mare was not vaccinated prior to foaling, then the initial series of immunizations can be started at three months of age. Foals residing in areas with endemic encephalomyelitis viruses (EEE and WEE) and West Nile virus (WNV) will typically be exposed to these viruses before they are six months of age as a result of seasonal mosquito activity. It is, therefore, common practice to begin vaccination for EEE, WEE, and WNV at three months of age, particularly in southeastern states where the mosquito season is prolonged.

Finally, treatment of adverse reactions, pre-treatment, and the number of antigens that a horse should be challenged with at a single appointment provided lots of excellent discussion in the veterinary group. There is no published research available on the recommended number of vaccine antigens to administer at a single occasion. As a general rule of thumb, the discussion facilitators and participants felt that, pending availability of results of research investigating this issue, it is good practice to limit the number of vaccine antigens that are administered at the same time to five or six.

As a reminder, modified live and inactivated vaccines should not be administered at the same site. Two different vaccines should never be mixed in the same syringe prior to administration unless specifically recommended by the manufacturer.

The attendees provided informative feedback and the facilitators agreed that exciting research is likely in the future for equine immunizations.

This table topic was led by W. David Wilson, BVMS, MS, of the University of California, Davis, School of Veterinary Medicine, and Amanda M. House, DVM, Dipl. ACVIM, of the University of Florida.

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