West Nile Virus

Protection against West Nile virus (WNV) encephalomyelitis, similar to other vector-mediated encephalitides, requires strict attention to control of insects as well as an effective vaccination program. Four vaccines are currently licensed by the U.S. Department of Agriculture for use in horses to aid in protection against WNV: two inactivated vaccines, a nonreplicating canarypox-vectored vaccine, and an inactivated flavivirus chimeric vaccine, are available.

Administration of inactivated WNV vaccines, consistent with label instructions, involves administration of an intramuscular dose 3 to 6 weeks apart, followed by a 12-month revaccination interval. Because recombinant canarypox-vectored vaccine protective antigens are expressed in a canarypox vector that does not replicate in the horse, the vaccine contains an adjuvant. Label recommendations include intramuscular injection with a primary series including a 4- to 6-week interval, followed by a 12-month period before revaccination. Inactivated flavivirus chimeric vaccine with protective antigens expressed in a yellow fever virus vector also contains an adjuvant. Label instructions include a primary series of two vaccinations to be administered with a 3- to 4-week interval and followed by a 12-month period before revaccination.

An adult horse that has previously been vaccinated for WNV should receive an annual vaccine in the spring, before the onset of the mosquito vector season. An adult horse that is unvaccinated for WNV, or is of unknown vaccine status, should receive a primary series of two vaccinations administered with a 3- to 6-week interval, followed by a 12-month interval before the next vaccination. Although there are limited data on the safety and efficacy of WNV vaccination of pregnant broodmares, it is common for veterinarians to vaccinate these animals because of the inherent risk associated with contraction of the disease. A booster vaccination is recommended 4 to 6 weeks before expected foaling to provide WNV-specific immunoglobulin in the colostrum.

Previous reports have demonstrated that foals vaccinated in the face of a significant maternal antibody titer can mount an effective endogenous humoral immune response to WNV. Current recommendations call for the initiation of a three-dose series beginning at 4 to 6 months of age. The initial vaccination should be followed by a booster vaccination in 4 to 6 weeks, with a third vaccination at approximately 10 to 12 months of age, before the onset of vector insect season the following spring. Foals that are delivered by unvaccinated dams (or those with an unknown vaccine status) should receive WNV vaccines at 3 to 4 months of age, with the goal to complete the three-vaccine series before the peak insect season. The duration between the first and second dose should be approximately 30 days, with approximately 60 days passing between the second and third dose. If vaccination is taking place during the mosquito vector season, the entire protocol should be completed closer to an 8-week interval.

Tetanus

Given the ubiquitous nature of the bacterium Clostridium tetani in the environment and in the equine gastrointestinal tract, optimal tetanus immunization protocols should be maintained for all horses. Toxoids are nontoxic derivatives of toxins that are used as antigens. All horses should be vaccinated with an annual booster. A booster vaccination should also be administered at the time of a penetrating wound or invasive procedure, particularly if the previous vaccine was administered longer than 6 months previously. Unvaccinated horses or those with an unknown vaccine status should receive a two-dose series, with a 4- to 6-week interval between vaccines. When vaccinating horses during the first year of life, a three-dose vaccine series should begin at approximately 4 to 6 months of age, with the initial two vaccines administered approximately 4 to 6 weeks apart and followed by a third booster vaccination at 10 to 12 months of age. If an injury occurs and vaccine status is not known, the vaccine series should be initiated with a priming vaccine. In addition, and at a distant muscular site, tetanus antitoxin should also be administered. A second booster vaccination should be administered approximately 4 to 6 weeks later to complete the vaccination series. Because of the risk for Theiler’s disease developing after the administration of tetanus antitoxin, the risks should be carefully considered.

Pregnant broodmares should be properly vaccinated against tetanus. If they have not been previously vaccinated, they should receive a two-dose series with a 4- to 6-week interval. All mares should receive a booster tetanus vaccine approximately 4 to 6 weeks before expected foaling. This will ensure protection for the mare from tetanus secondary to a foaling injury, retained placenta, or endometritis. In addition, this will ensure adequate colostral antibody protection for the neonatal foal. Foals delivered to unvaccinated mares should receive their initial tetanus vaccine at 1 to 4 months of age. The initial series should include three vaccinations to ensure optimal immune response. The initial two vaccines are administered with a 4- to 6-week interval, followed by the third vaccination 4 to 6 weeks after the second. Annual booster vaccination is recommended for all horses. If the foal is born to an unvaccinated mare and is at high risk for the development of tetanus, tetanus antitoxin should be administered as previously described.

Rabies

Three inactivated vaccines are currently licensed for the protection of horses from rabies. Rabies virus is an excellent antigen that induces a pronounced serologic response following administration of a single vaccine. All horses maintained in areas where rabies is endemic should be vaccinated. Veterinarians should carefully read the label of rabies vaccines before administration.

All adult horses should receive an annual booster as a component of a routine preventive health program. Pregnant broodmares may receive a booster vaccine 4 to 6 weeks before expected foaling date to provide adequate colostral immunoglobulin concentrations. Alternatively, because rabies is such a pronounced immunogen and serologic responses are maintained for extended periods of time, it may be preferable to vaccinate mares before breeding and not administer a booster before foaling. This practice will aid in reduction of the number of vaccines that late-term pregnant broodmares receive before foaling. Label instructions do not include approval for use in pregnant mares, but in endemic or high-risk situations, it is appropriate to vaccinate at-risk individuals. When rabies vaccine is administered to a horse in its first year of life and the vaccine status of the dam is current, the initial two-dose series should begin at approximately 6 months of age, with a booster vaccine administered in 4 to 6 weeks. An annual booster vaccination is recommended in all horses. Foals born to mares that were not vaccinated or those with an unknown vaccine status should be initially vaccinated at 3 to 4 months of age, followed by a booster vaccine in 4 to 6 weeks.