Vaccine Strategies for the Whole Herd
One of the critical aspects of equine preventive care and herd health is vaccination. Regular immunizations with core and risk-based vaccines help reduce a horse’s chances of contracting disease, but not every type and age of horse requires the same vaccination regimen.

At the 2014 American Association of Equine Practitioners Convention, held Dec. 6-10 in Salt Lake City, Utah, Elizabeth Davis, DVM, PhD, Dipl. ACVIM, professor and section head of equine medicine and surgery at Kansas State University’s College of Veterinary Medicine, described the fundamental objectives of a successful vaccine program. “The goal is to safely induce immunity that provides antigen-specific protection that is efficacious and long-lasting,” she said.

Davis first presented vaccination protocols for young foals during their first year of life. Foals that ingest quality colostrum (first milk) from their vaccinated dams typically develop adequate immunity for the first four to six months of life. Further, maternal antibodies in colostrum would actually block the foal’s immune response to vaccines, particularly against influenza and tetanus, so veterinarians don’t generally begin vaccinating foals until four to nine months, depending on the vaccine. Core vaccines (Eastern and Western equine encephalomyelitis, rabies, tetanus, and West Nile virus) are given earlier than the risk-based respiratory virus vaccines, which aren’t begun until seven to nine months of age. In high-risk situations, as might be seen with encephalitis or West Nile virus outbreaks, however, immunization against these viruses might need to begin earlier than normal recommendations.

She recommended starting a foal on a three-dose series. Timing depends on whether the mare was immunized prior to foaling. If the mare was immunized, then the foal receives a first dose of most core vaccines (Eastern and Western encephalitis, tetanus, West Nile virus) at 4 to 6 months of age, the second dose 4 to 6 weeks later, and the third dose at 10 to 12 months of age. Foals born to unvaccinated mares receive dose one at 3 to 4 months of age, dose two 3 to 4 weeks later, and dose three 6 to 8 weeks after the second dose. Another core vaccine, rabies, is given as a 2-dose primary series, starting at six months followed by a second dose 4-6 weeks later.

The mature horse that has not been immunized regularly or ever should receive the primary series of any vaccine in two doses given 4 to 6 weeks apart. Then the veterinarian can boost these annually or, in the case of vaccines against respiratory viruses, administer every six months. Before immunizing, it is important to evaluate current stresses in the horse’s life. Stress influences a horse’s ability to mount an immune response and can be caused by travel, rigorous training, herd dynamics, or disease. Immunosuppression (reduced disease-fighting ability) also occurs in response to certain medications. If a horse is exposed to any of these stress situations, it might be necessary to immunize them with a primary series of three, and to booster more frequently than usual following the primary series.

“Also, it is important to consider differences in antibody responses to monovalent versus multivalent vaccine products,” Davis said. A monovalent vaccine is one with only one antigen (substance capable of inducing an immune response) in it, such as influenza or West Nile virus (WNV). A multivalent vaccine contains a multitude of antigens in a single injection, such as the five-way Eastern/Western equine encephalitis, tetanus, WNV, influenza, and rhinopneumonitis vaccine.

In one study researchers reported that when they administered the monovalent WNV vaccine, titers (antibody concentrations in the blood) increased significantly more by Day 28 than they did with multivalent vaccines that contain WNV.

“While the response to multivalent vaccines is sufficient, in a high-risk situation monovalent WNV vaccine will induce a superior antibody response,” Davis said.

Horses older than 20 years of age experience an attenuated response to vaccines due to immunosenescence and inflamm-aging, said Davis, which are age-associated immune system changes that result in reduced lymphocyte (important type of white blood cell) numbers and chronic, low-grade systemic inflammation. Aged horses with reduced immune responses, therefore, might need more regular and frequent vaccination to maintain their immunity.

Vaccination is an essential approach for optimizing herd immunity, and owners and veterinarians should design protocols to maximize each horse’s innate and adaptive responses to disease challenges. And when you combine it with other crucial strategies, from smart biosecurity practices to decrease the risk of pathogens entering to infection control techniques to help limit pathogen spread, overall herd health benefits.