Equine Vaccines: Diving Deeper

Should I vaccinate my horse against strangles?

There is no consensus on the use of vaccines to prevent strangles, a highly contagious disease for which outbreaks can be difficult to manage.

Intramuscular (IM) and intranasal (IN) vaccines with moderate success rates and efficacy are available in the United States, but protection may be limited and serology testing may not differentiate between infected horses and vaccinated horses. The IM formulation is a purified antigen extract vaccine. Abscess formation at the injection site and purpura hemorrhagica have been noted in some animals. The IN application, an attenuated live vaccine, has been associated with abscess formation in the lower jaw and nasal discharge.

Since this is a live vaccine and accidental contamination of other sites can occur, issues have been reported when administering the IN vaccine at the same time as other IM vaccines. Invasive procedures, such as joint injections, should not be performed concurrently for the same reason.

Vaccination may be recommended if the bacteria is known to be endemic on the premises, or for horses that are otherwise at high risk of exposure. Vaccination during a strangles outbreak is not recommended as it increases the risk of adverse reactions and complications.

Should I vaccinate my horse if there is a disease outbreak at my barn?

Determining whether to vaccinate horses during a disease outbreak depends on the disease, the type(s) of vaccine available, the population, and the local environment. Emergency vaccination has been successful in reducing equine influenza transmission during outbreaks, especially if detected early. Horses that were previously vaccinated may also be revaccinated. In contrast, the benefit of emergency vaccination for EHV-1 and EHV-4 is controversial. Challenge studies are required to provide further direction.

Following proper biosecurity protocols, along with quarantine, isolation, and monitoring are often the most effective approaches to controlling outbreaks.

Are there any new vaccines?

Vaccines that you might not have heard much about include:

Melanoma – This is an extra-label use of a canine melanoma vaccine that targets the protein tyrosinase expressed by melanoma cells. It is safe and effective in horses and anecdotal reports indicate tumor shrinkage and no new melanomas in some cases. However, quantitative data is not yet available and the results are unpredictable. The vaccine is administered in the pectoral muscle of the chest. The initial course is four treatments given two weeks apart, followed by boosters every six months. The vaccine can currently only be obtained by veterinary specialists and the treatments may be costly.

Potomac horse fever (PHF)/equine neorickettsiosis – This is available in the United States as a single agent vaccine or in combination with a rabies vaccine. Reports indicate weak immune responses and inconsistent protection. This may be partly due to the fact that the vaccine contains a single strain of the Neorickettsia risticci bacteria but additional strains exhibiting significant variation have been identified in naturally-infected horses. A recent study found a further weakened response after administration of the combined PHF/ rabies vaccine. Regular revaccination may be recommended for horses in endemic areas.

Pigeon Fever (Corynebacterium pseudotuberculosis) – This vaccine is available under a conditional license from the USDA. No side effects were observed in naïve horses (young horses with no previous exposure or disease) during the initial safety studies. Mild reactions, including localized swelling at the site of administration, inappetance, and lying down have been observed in some older horses with previous exposure to the bacteria. It is recommended to give this vaccine alone, not combined with other vaccines. Appropriate use of the vaccine in combination with good fly control can be used to prevent this disease.

Snake bite – There is currently one conditionally licensed rattlesnake vaccine (Crotalus atrox Toxoid) that may provide protection against venom of varieties of Western rattlesnake, sidewinder, timber rattlesnake, massasauga, and copperhead for horses that are at risk of exposure due to geographic location or travel. Horses six months or older receive a three-dose primary series (one month between each dose), with boosters every six months. It is recommended to ensure peak titers coincide with peak exposure times.