Vaccinating your horse for every disease you can is costly and unnecessary. Several factors enter into the decision about which vaccinations to perform. These include:
• Severity of the disease. Is it life-threatening (like tetanus), or more of a nuisance (Rhinopneumonitis)?
• Risk of exposure.
• Effectiveness of available vaccines.
• Vaccine-associated dangers.
Severity Of Disease
When considering severity of disease, the program is built around the most dangerous ones. "Core" vaccines are ones that address life-threatening diseases and therefore shouldn't be skipped in most cases. Tetanus tops this list. Next are the encephalitis viruses: Eastern, Western, Venezuelan and West Nile. EEE, WEE and VEE have higher mortality rates than WNV, but even 50% mortality is unacceptable if you can prevent it. Last on the list of core vaccines is rabies, definitely fatal, but also relatively rare.
Exposure, Effectiveness
The risk of exposure comes into play for both core vaccines and others. All horses are at equal risk of tetanus. The organism is everywhere. Horses on the East Coast don't need WEE protection, and vice versa. West Nile is in all the states now, although it's much more active in some areas than in others.
Since timing vaccination to occur shortly before your horse's high-risk exposure season is wise, you may choose to keep track of mosquito-surveillance data for your state (available online) to note activity in mosquitoes and birds before vaccinating. However, this does put your horse at some risk of being infected and you should discuss the timing thoroughly with your veterinarian.
Of all the potentially fatal diseases, rabies poses the lowest risk. Equine cases do occur, but they're infrequent. The coyote strain of this disease has been eliminated in the United States as of 2007. Focus is now turning to the raccoon and skunk strains in hopes of also eliminating those from Canada and U.S. by a similar program of oral vaccine-loaded baits for wild animals.
Most rabies cases are in wild animals, and they are clustered heavily along the Eastern states. Although there are approximately 10 million horses in the United States, typically less than 10 per year are infected with rabies. For details on rabies prevalence in your state, you can contact your state's health department.
Botulism is another disease with a high fatality rate, but it's not included in the core vaccines because it's not common. Sporadic botulism, usually from contamination of hay or grain with dead animals, can occur anywhere, but outbreaks in pastured horses occur in some areas. Your veterinarian is your best source of information as to the relative risk of botulism in your area.
Exposure risk is a big factor in vaccinating against the respiratory diseases. Influenza is much more severe than the Rhinopneumonitis virus.
Furthermore, an estimated minimum of 60% of horses are chronically infected with the Rhinopneumonitis virus, which has come to a truce with the immune system.
Rhinopneumonitis vaccines don't have a very good track record for preventing respiratory disease or abortions either, but they do reduce shed virus so are a social service in a group environment.
The modified live vaccine, Rhinomune, may provide some protection against the neurological form, but this is from limited experimental data. Influenza is primarily an issue for horses on high-traffic farms or traveling extensively.
Potomac horse fever causes a serious disease, but it's not a problem in all areas. In any case, it's probably a moot point since the vaccine performs poorly under field conditions and most vets agree it's not very useful.
An EPM vaccine exists, but it's never been granted FDA approval and again is of questionable value.
Another vaccine with poor efficacy is the intramuscular Strangles vaccine, which also has a high rate of serious injection site abscesses. If you need to vaccine for Strangles, the intranasal vaccine is best. We also like the intranasal influenza vaccine.
