Can Vaccinated Horses Still Get Sick?

Q. This fall I saw reports about horses getting infected with diseases like Eastern equine encephalitis, even though they were up to date on their vaccinations. Sometimes the horses even died from their infections. Why would vaccinated horses still get sick?

Carol, via e-mail

A. When questioning vaccine efficacy, consider several factors:

Vaccine handling

When following the American Association of Equine Practitioners’ recommendations for vaccine handling, vaccines should maintain their ability to protect against pathogens. Specifically, they should be handled according to manufacturer recommendations. Storage and handling instructions might be product-specific. For this reason, vaccines must be stored at the recommended temperature with little to no light exposure and be mixed properly (via shaking) to ensure the suspension is uniform prior to administration. A break in the cold chain or any other component of proper vaccine handling and storage might impact vaccine efficacy, leading to failure. 

Immune function

Factors that can affect host immune response might include horse’s age, previous vaccine history, and whether he has concurrent issues that could negatively affect immune activation. Possible causes of impaired immunity that could negatively impact a horse’s vaccination response include excessive stress (e.g., strenuous exercise, competition, long-distance transport) or immunosuppressive medications such as high-dose or long-term corticosteroid therapy. Elderly horses with concurrent diseases such as pituitary pars intermedia dysfunction (PPID) might also have reduced immune responses. It’s important that host immunity is optimal at the time of vaccination to ensure the vaccine can induce a protective immune response.

Level of pathogen challenge

In a horse that was vaccinated previously but developed subsequent disease, one causative factor could be the level of pathogen challenge. For instance, perhaps the duration of time from vaccine administration to pathogen exposure was at or near the length of protection provided by the vaccine. In this scenario, if the pathogen exposure was excessive, it could overwhelm immune responsiveness, resulting in disease. This is a good example of why it is important to maintain accurate medical records for all vaccines; this helps ensure you have them administered at the proper intervals to maintain maximal host immunity. 

Vaccination history

It is also important to be informed about a horse’s previous vaccine administration. For instance, if you purchase a horse with an unknown vaccine history and assume he was vaccinated previously, when in actuality he was not, then the horse might not respond as expected to an individual booster vaccine. In this case you should have your veterinarian administer the priming dose series (adult horse protocol) of two vaccines at a three- to four-week interval, followed by booster vaccination at the appropriate times. Without this initial priming series, the vaccination likely won’t induce a memory immune response, thereby leaving the horse susceptible to pathogen challenge.

Incorrect diagnosis

In some instances, a horse might develop disease with clinical signs suggestive of a certain condition but, without confirmatory testing, the final diagnosis remains unknown. For example, say a horse properly vaccinated against Eastern equine encephalitis (EEE)/Western equine encephalitis (WEE)/West Nile virus (WNV) develops acute neurologic disease believed to be a result of EEE/WEE or WNV. Diagnosing the horse based on clinical signs alone could be misleading, as the horse could have another disease, such as equine protozoal myeloencephalitis (EPM), that is not a result of vaccine failure.

Vaccines aren’t perfect; they’re merely one component of a healthy horse program. Collectively, it is important to have horses living on the same property well-vaccinated to maintain herd immunity against relevant pathogens. Additional steps to maintain optimal health include a strong biosecurity program to minimize the chance of disease transmission from new arrivals. This is particularly true for horses with unknown vaccine backgrounds; don’t allow these horses to commingle with resident horses until they have been screened and monitored for health status and are vaccinated against the same diseases as the resident horses.