A study suggests that special care may be required when planning the vaccination schedule of horses with pituitary pars intermedia dysfunction (PPID, also known as Cushing’s syndrome).

A dysfunction of the pituitary gland, PPID is common in horses over the age of 20. “Horses with PPID are more likely to be predisposed to life-threatening complications such as laminitis and secondary infections,” says Amanda Adams, PhD, assistant professor at the Gluck Equine Research Center at the University of Kentucky. “Changes in immune function may contribute to this increased susceptibility in PPID horses.”

To determine how impaired immune function in PPID horses may affect vaccination response, Gluck researchers tested the response of 33 horses to a combination vaccine that protects against influenza, herpesvirus (EHV) types 1 and 4, West Nile virus (WNV), Eastern and Western equine encephalomyelitis0 and tetanus.

The study horses were first tested for PPID using a dexamethasone suppression test and a thyrotropin-releasing hormone stimulation test. They were then divided into one of the following treatment groups: Those without PPID given the vaccine; those that were positive for PPID on one test (single-positive) and given the vaccine; those that were positive on both tests (double positive) for PPID and given the vaccine. Finally, horses that were PPID negative or tested positive on one or both tests were not vaccinated to serve as controls.

Using two diagnostic tests gave the researchers some indication of how advanced a particular horse’s PPID might be, says Adams. “Based on previous research, those horses that test positive using the dexamethasone suppression test may have later stage disease. Single-positive PPID horses in this study had only significantly elevated basal ACTH levels and elevated ACTH levels following the TRH stimulation test. So it is likely that the double-positive PPID horses are in a later stage of disease than single-positive PPID horses.”

Researchers drew blood from each study horse prior to the initial vaccination and again two and four weeks later. A vaccine booster was administered four weeks into the study and more blood samples were collected on weeks six and eight in order to measure immune responses to the vaccine.

Blood analysis revealed all the horses had higher titers (increased antibodies) against the target diseases shortly after vaccination. However, in double-positive PPID horses, antibody titer levels to the West Nile virus component of the vaccine dropped off more quickly than in the single-positive horses and those without PPID.

“Unfortunately, we do not have certain ‘cut-off’ titers to assume a horse is protected or not protected if faced with West Nile virus,” says Adams. “However, these levels significantly dropped so that they were not different compared to pre-vaccination titers, which is not a good thing.” The study data showed a similar pattern with EHV-1 titers, with double-positive horses having less of a titer response to vaccination than single-positive horses and horses without PPID.

Adams says this means it is possible that horses with PPID may need to be vaccinated on a more frequent schedule than other horses. “We are in the process of further investigating this question. It is important to make sure that geriatric horses are maintained on a regular vaccination program, instead of just forgotten about,” she says. “Make sure they are always up-to-date on core vaccines. For horses at higher risk, such as older show jumpers or 4-H horses who travel and co-mingle, your veterinarian may want to consider a booster every six months for risk-based vaccines such as EIV/EHV and WNV.”

Reference: “Does equine pituitary pars intermedia dysfunction (PPID) affect immune response to vaccination?” AAEP 60th Annual Convention Proceedings, December 2014

This article first appeared in EQUUS issue #452

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