Need-based vaccines are those that are specific to a geographic location due to an increased risk of acquiring infection. Your veterinarian is able to counsel you on what those disease risks might be. These vaccines are given in addition to the core vaccines of Eastern and Western equine encephalitis, tetanus, West Nile virus and rabies.
The most common need-based vaccines are the respiratory immunizations against equine influenza virus (EIV), equine herpesvirus (EHV, also called rhinopneumonitis) and equine strangles (Streptococcus equi).
Most areas of the country immunize horses against EIV and herpesvirus twice a year. This is especially important for competition horses and those traveling off the farm. Not only are they at higher risk of exposure, but they can also bring virus back to the farm—even without showing signs of illness—to share with the resident horses.
The equine herpesvirus vaccine for EHV-1 and EHV-4 will not protect against the neurologic form; it only confers immunity against the respiratory and abortigenic forms. Both EIV and EHV immunizations are required to participate in USEF (United States Equestrian Federation) shows while FEI (Federation Equestrian International) events require proof of influenza vaccination.
To achieve the best herd immunity against EIV and herpesvirus, it works best if all horses within a herd are immunized on the same schedule. This concept of herd immunity is best described as “the resistance to the spread of a contagious disease within a population that results if a sufficiently high proportion of individuals are immune to the disease, especially through vaccination.”
In other words, the more horses that develop protective antibodies to a disease, the less chance of horses shedding infective organisms. This lessens the chance of an outbreak.
If immunized horses do contract disease, the seriousness and duration generally are much reduced compared to the circumstances if the horses had not been immunized. In the case of an outbreak, it is recommended to boost horses at high risk with vaccine against that disease. Consult with your veterinarian to ensure what measures you should take during an outbreak.
Strangles vaccine tends to only be used in high-risk situations due to the potential for complications from the modified live virus intranasal vaccine that can cause abscesses in other locations. It is best given only after administration of other injectable vaccines or on a separate occasion.
Other risk-based vaccines include botulism, anthrax, equine viral arteritis (EVA), Potomac Horse fever, leptospirosis, rotavirus diarrhea vaccine and snakebite vaccine. These vaccines are not commonly used and are only administered when the risk of not immunizing is weighed against the benefit of having protection.
Just as core vaccines are given as a primary series of 2-3 injections, most of the risk-based vaccines follow a similar schedule protocols. Boosters are given once or twice a year, depending on the product and the local risk of exposure.