Can discomfort in horses affect the results of hormone tests for Cushing’s disease?

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Cushing's disease in horses commonly causes excessive hair growth, muscle wasting along the top line, and abnormal fat distribution. Photo: AbujoyCC BY 3.0 via Wikimedia Commons
Cushing’s disease in horses commonly causes excessive hair growth, muscle wasting along the top line, and abnormal fat distribution. Photo: Abujoy CC BY 3.0 via Wikimedia Commons

Can test results for equine Cushing’s disease be affected if the horse is in pain?

The question was raised by researchers in Germany, who were concerned that pain in horses can elevate the same hormone that veterinarians check when determining whether a horse has Cushing’s, more formally known as pituitary pars intermedia dysfunction (PPID).

PPID is an endocrine disorder affecting the pituitary gland of horses. It is most common in older animals and is typically linked with the development of a long, wavy coat and chronic laminitis.

It involves an enlargement of part of the pituitary gland known as the pars intermedia, leading to increased secretion of hormones, including adrenocorticotropic hormone (ACTH).

The main tests for a diagnosis of PPID are the measurement of base levels of ACTH and the thyrotropin-releasing hormone (TRH) stimulation test, in which TRH stimulates the secretion of ACTH.

“Since pain can also lead to elevated concentrations of ACTH, it is unclear whether horses with pain can be tested for PPID correctly,” Heidrun Gehlen and her colleagues at the Free University of Berlin noted in the open-access journal Animals.

The four-strong study team set out to determine whether pain caused a marked increase of ACTH, which could lead to a false positive result in the diagnosis of PPID.

They examined 15 horses treated for different pain conditions. The animals served as their own controls as soon as they were pain-free again.

The horses were divided into three groups: those with colic, those with laminitis, and those with orthopedic problems. A composite pain scale was used to evaluate the intensity of pain for each animal.

The ACTH level and cortisol (to evaluate the influence of pain and stress) were measured before and after the TRH stimulation test.

The researchers found no significant difference in the ACTH concentration between horses with pain and the controls, nor between different pain intensities or between disease groups.

“Thus, measuring the basal ACTH concentration and performing the TRH stimulation test for the diagnosis of PPID seems to be possible in horses with a treated low to moderate pain condition,” they concluded.

The authors noted that the three horses with laminitis in the study were found to have had a lower ACTH value during the painful period than when serving as the pain-free control.

“It is possible that continued stimulation of the hypothalamic-pituitary-adrenal axis may have caused system exhaustion, resulting in a decrease in stress hormones,” they said.

Horses with colic showed the highest cortisol levels in the study, followed by those with laminitis and orthopedic problems.

“The results suggest that the origin of the pain may influence the secretion of cortisol.”

The authors concluded that the measurement of ACTH and the performance of the TRH stimulation test for PPID diagnostics can be performed on horses in discomfort as long as they are not suffering from massive pain or showing a significantly disturbed general condition.

“It might, however, be possible that pain induces a diminished response to TRH stimulation in individual animals,” they added.

“Since a uniform gold standard for assessing pain intensity has not yet been developed, it would first be necessary to develop a generally applicable pain assessment scale for different types of pain in horses to be able to differentiate clearly between different pain intensities.

“Further studies on a greater number of horses with uniform pain patterns are also necessary to determine the factors influencing the stress hormones even more precisely.”

Studies on the effects of pain and the influence of medication on the TRH stimulation test are still lacking, they said.

The full study team comprised Gehlen, Nina Jaburg, Roswitha Merle and Judith Winter.

Gehlen, H.; Jaburg, N.; Merle, R.; Winter, J. Can Endocrine Dysfunction Be Reliably Tested in Aged Horses That Are Experiencing Pain? Animals 2020, 10, 1426.

The study, published under a Creative Commons License, can be read here

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