Impact of Short-Term Transportation Stress on Equine Endocrine and Immune Function

New research shows that veterinarians might need to allow several hours between transportation and testing for conditions such as PPID.
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Transporting horses for one to three hours can elicit a stress response. | iStock

Horses are routinely transported for equestrian events as well as medical care. It is well recognized that long-distance transportation is a risk factor for the development of pleuropneumonia. Results from a nationwide survey showed that horses are most commonly trailered short distances of three hours or less. The Adams Lab at the Gluck Equine Research Center within the University of Kentucky Martin-Gatton College of Agriculture, Food and Environment is currently working to investigate the impact of acute transportation stress on immune function in different groups of horses. Previous research has shown that transporting horses one to three hours can elicit a stress response as seen through increased heart rate. Our lab has previously shown that short-term transportation stress can cause changes in whole blood gene expression of cytokines in aged horses. Following these findings, we wanted to know if similar responses occur in young horses. Additionally, since horses are often transported to veterinary clinics for diagnostic testing, we have investigated the effect of transportation on endocrine responses and testing for insulin dysregulation.

Two studies where horses were transported one and one-half hours (55 miles) round trip were performed. Study #1 focused on evaluating age-differences in stress and immune response following short-term transportation. Horses were grouped by age with six horses in each group. Group one had an average age of two years old and group two an average age of 22-years-old.

Study #2 evaluated differences in endocrine responses to transportation between seven insulin dysregulated (ID) and seven age-matched noninsulin dysregulated (non-ID) horses. For both studies blood, saliva, heart rate and temperature were collected before and after transportation.

Transportation stress led to various physiological changes in both studies. For both studies, heart rate and cortisol increased in response to transportation, with the highest heart rates recorded at loading and both serum and salivary cortisol being highest directly after transport. There were no differences based on age or whether a horse was insulin dysregulated. In Study #1, all horses had increased ACTH following transportation, with three out of five aged horses having ACTH concentrations above the cutoff for pituitary pars intermedia dysfunction (PPID) 15 minutes after transportation. While rectal temperature was not influenced by short-term transportation stress in either study, these studies were conducted in mild weather during the spring and fall.

Study #1 also showed increased gene expression of the pro-inflammatory cytokine IL-6 post-transport in aged horses and increased salivary IL-6 in aged horses compared to young horses. Additionally, aged horses had increased cytokine production from lymphocytes compared with young horses. These results are all indicative of the older horses being in a state of “inflamm-aging” where they have increased inflammatory markers, putting them at higher risk for development of diseases.

Serum insulin was measured in both studies. In Study 1, insulin was increased one to three hours post-transportation in aged horses and followed a similar trend in ID horses in Study #2, although not statistically different from pre-transportation insulin levels. In Study #2, performing an oral sugar test (OST; current standard for diagnosing ID in horses) post-transportation caused insulin concentrations to be above the recommended diagnostic cutoff for insulin dysregulation in non-ID horses and were twofold greater than pre-transportation OST results in some ID horses.

The results of these studies show that there are significant changes in immune and endocrine function following short periods of transportation. Additionally, caution should be used when performing endocrine testing for metabolic diseases post-transportation as ACTH and insulin may be elevated in horses without endocrine disease. It is recommended that ACTH should be collected for PPID testing at least 30 minutes after transportation. Furthermore, performing an OST within the first few hours following transportation could lead to inaccurate diagnostic findings, particularly in non-ID horses. Given the importance of PPID in the aging horse population, we are currently evaluating if short-term transportation stress alters endocrine function and diagnostic results of horses with PPID and plan to report on those findings in the near future.

Editor’s note: This is an excerpt from Equine Disease Quarterly, Vol. 32, Issue 4, funded by underwriters at Lloyd’s, London, brokers, and their Kentucky agents. It was written by Erica Jacquay, PhD Student at the University of Kentucky’s Gluck Equine Research Center, in Lexington.

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