General Description

Also known as “non-sweater,” Anhidrosis is the medical term describing a compromised or complete inability to sweat in response to increased body temperatures, due to exercise or environmental high temperatures. This condition is dangerous (sometimes life-threatening) for horses. Horses regulate their body temperature (thermoregulate) primarily through sweating, which is responsible for 65-70% of cooling (AAEP, Understanding Anhidrosis). Although cases of Anhidrosis are more observed in the hot and humid regions of the southeastern United States, they are also reported in cooler climate regions like the North US, Canada and even Ireland. It can affect several breeds of horses, although previous studies have demonstrated an increased incidence in Thoroughbreds and Warmbloods.

The condition’s onset can be gradual or acute. Increased respiratory rate and lack of sweat after exercise are the most common initial findings. In situations that should cause copious sweating, affected animals produce minimal sweat – perhaps a small amount under a saddle pad – when all other horses are completely lathered. Severely affected horses might not even be able to tolerate turnout on hot days; they will stand outside in distress with an extremely high respiratory rate and body temperatures higher than 104 degrees Fahrenheit.

Horses suffering from acute cases may have elevated respiratory rate and an elevated temperature that requires an extended period of time to return to normal, yet most of these go back to sweating normally after a season or two. Acute cases can be caused by several reasons: stress, as a side effect of some medications, or even due to environmental causes. Yet, horses with chronic cases of Anhidrosis - called “Chronic Idiopathic Anhidrosis” or CIA - have a history of complete lack of sweat response, sometimes even all year long. For these, even moving to a cooler climate region may not help them recover the ability to sweat. These were the cases targeted by Dr. Patterson Rosa’s study[1}, as their severe inability to thermoregulate puts them in danger of overheating, collapsing and even dying. You can learn more about chronic Anhidrosis here.

Epidemiological studies conducted in Florida, have demonstrated that horses with a family history of Anhidrosis have over 21x the odds of being affected as well [2]. Recent studies conducted by Dr. Patterson Rosa and Dr. Samantha Brooks focused on the chronic form of the condition, and implicated a mutation in a potassium channel as the possible cause for CIA [1]. Hypothesized to be a potassium channel failure, even chronic cases are dependent on the exhaustion of the sweat response, thus, of the respective environmental triggers that lead to this effect: overworking, excessive sweating to control body temperature, extreme heat conditions, etc. Thus, it is possible that horses carrying two copies of the mutation do not develop chronic anhidrosis if properly managed, and that is something Etalon will look further into!

References

[1]Patterson Rosa L, Walker N, Mallicote M, MacKay RJ, Brooks SA. Genomic Association of Chronic Idiopathic Anhidrosis to a Potassium Channel Subunit in a Large Animal Model. J Invest Dermatol. 2021 May 31:S0022-202X(21)01307-5. doi: 10.1016/j.jid.2021.05.014. Epub ahead of print. PMID: 34081968.

[2]Johnson, E. B., R. J. MacKay, and J. A. Hernandez. 2010. "An Epidemiologic Study of Anhidrosis in Horses in Florida." Journal of the American Veterinary Medical Association 236: 1091–1097. https://doi.org/10.2460/javma.236.10.1091