With the help of great medical science, horse owners and equine practitioners are able to provide horses with improved quality of life into advancing years. For horses with pituitary pars intermedia dysfunction (PPID), pergolide has changed the paradigm of how well these individuals do in the face of multi-systemic maladies associated with such endocrine disruption.
While many problems created by PPID are well resolved with pergolide treatment, questions still arise about how it might affect the immune system. A recent study has delved into that very issue [Miller, A.B.; Loynachan, A.T.; Bush, H.M.; et al. Effect of pituitary pars intermedia dysfunction and Prascend (pergolide tablets) treatment on endocrine and immune function in horses. Domestic Animal Endocrinology July 2020, vol. 74; https://doi.org/10.1016/j.domaniend.2020.106531].
The 15-month study (from the beginning of one fall season until the end of the second fall season) used 28 horses owned by the University of Georgia’s College of Veterinary Medicine—10 horses were non-PPID, 9 were PPID positive controls, and 9 were PPID positive that were treated. Due to circumstances unrelated to the study, six of the 28 horses were humanely euthanized resulting in a final study size of 9 non-PPID horses, 7 PPID controls, and 6 PPID-treated animals.
Pergolide is known for its action as a dopamine agonist that decreases plasma adrenocorticotropin hormone (ACTH) concentrations. To determine the endocrine status of horses within the stud, thyrotropin-releasing hormone (TRH) stimulation tests were performed followed a week later by administering the oral sugar test (OST) using Karo light corn syrup. Body weight, neck crest scores, hypertrichosis scores, body temperature, heart and respiratory rates were assessed throughout the study.
Blood samples evaluated a number of parameters: ACTH, insulin, total and free cortisol, CBC, plasma myeloperoxidase, cytokine/receptor gene expression for interferon, tumor necrosis factor and other toll-like receptors, in vitro response of Rhodococcus equi and E. coli to whole blood, as well as peripheral blood mononuclear cells. All horses had free-choice pasture and/or hay relative to seasonal availability of pasture.
The PPID horses treated in the study received 2 mcg/kg BW for the first four weeks. The dose was increased weekly until resting ACTH levels approximated levels in the non-PPID horses or until the maximum recommended dose of 4 mcg/kg BW was reached to the nearest half of a 1 mg tablet. The non-PPID horses and the pergolide-treated PPID horses reached similar plasma ACTH concentrations within six weeks.
The results yielded some useful information:
- Pergolide was able to control the typical autumnal ACTH rise in PPID horses to bring the levels within normal expected limits. That said, the response to the TRH stimulation testing at non-fall time points did not reduce ACTH concentrations to the level of non-PPID horses. The authors recommend that monitoring of responses to pergolide treatment is best assessed with resting plasma ACTH rather than with TRH stimulation testing.
- PPID horses tended to lower body temperature and higher respiration rates compared to non-PPID horses, possibly due to thermoregulation issues.
- Pergolide did not impact insulin, total or free cortisol or immune function.
All the horses in the study were managed on the same farm, with similar treatment, seasonal and environmental conditions. While the sample size of this study posed some limitations, the authors concluded: “Horses with PPID are likely at increased risk for opportunistic infection.”
At this time, more studies are necessary to investigate the causes of immune dysfunction in PPID-horses. In the meantime, it is recommended that diligent biosecurity precautions should be followed with care of PPID horses, especially those in high-risk situations such as transport, events and competitions. Early recognition through endocrine testing for PPID is important to identify horses at higher risk of contracting infectious disease so preventive measures and treatment can be implemented.