Getting to the Bottom of Chronic Idiopathic Anhidrosis
Nonsweater. It’s the term we often give horses with anhidrosis, a condition characterized by the inability to sweat. Because horses are one of only three species that use sweat as their main thermoregulatory method, not being able to sweat is a big deal.

During the University of Florida Veterinary Extension’s 2020 Healthy Horses Conference, Laura das Neves Patterson Rosa, DVM, PhD, described this condition and what researchers are learning about it.

What Is CIA?

Chronic idiopathic anhidrosis (CIA) is a lifelong (sometimes seasonal, sometimes year-round) inability to sweat, with no known cause. Common signs include:

  • A dry coat;
  • Rapid breathing;
  • Decreased appetite and water intake;
  • Alopecia (hair loss); and
  • Depression.

“The risk is that animals with this condition can overheat, which can lead to heat shock, convulsions, and even death,” Patterson Rosa said.

Horses with a family history of anhidrosis appear to be more likely to have CIA. While some people consider it a tropical climate condition, she said it can happen to horses living anywhere.

Studying Genetic Risks

Patterson Rosa and her colleagues recently conducted a study to determine whether genetic variants contribute to the onset and severity of CIA in stock-type horses. They recruited samples from local veterinarians and sent out an online survey about horses’ health histories. Based on the 385 survey and 22 vet-submitted samples received, they ultimately selected 100 stock-type horses (Quarter Horses, Paint Horses, Appaloosas, etc.) and Thoroughbreds for genotyping. They then performed whole genome sequencing and variant analysis on one case horse and one control.

“Survey results showed that after stock types, Thoroughbreds, Warmbloods, and ponies were most frequently reported,” Patterson Rosa said. “Seasonally affected horses are usually younger, and the year-round affected are usually older.”

Other notable results included:

  • 1% (167/172) of study horses had a lack of sweating on their hindquarters;
  • 5% (154) had shortness of breath/rapid breathing;
  • 89% (153) lack of sweating on the neck;
  • 86% (50) had elevated heart rate (tachycardia);
  • 2% (107) had lack of sweating under saddle or on the back;
  • 3% (71) were lethargic or reluctant to work or move
  • 8% (53) had a body temperature above 104 F;
  • 9% (48) had increased or excessive water consumption (polydipsia);
  • 8% (34) had hair thinning or loss;
  • 3% (28) had dry, flaky skin;
  • 5% (25) had decreased water consumption; and
  • 1% (14) had poor appetite (anorexia).

They also found that CIA is, indeed, highly heritable.

“The most interesting finding of this study is that we have a candidate gene,” said Patterson Rosa. “The marker most significantly correlated to anhidrosis explains about 70% of case/control differences. Within the region that’s covered by this marker, there’s also a gene responsible for an ion channel (a type of protein) that has some interesting variance.”

In horses with CIA, malfunctioning ion channels likely cause irreversible cell damage and even epithelial cell loss, she hypothesized, adding that the next step is to run studies on the ion channel.

“If CIA is proven to be an ion channel problem, there are a few ways to target this condition. One is gene therapy medications, following the example of the ones used in humans with very good results,” she added. “Those may be an option in the future for horses suffering from CIA, aside from selection against it in proven carriers.”

Take-Home Message

In the meantime, what can you do to manage the horse with CIA? Patterson Rosa recommended the following steps:

  • Cold water hosing, being sure to scrape the excess water off the horse’s body;
  • Spraying the horse’s body with alcohol; and
  • Placing fans or even air conditioning units in stalls.

She said that moving to a cooler climate, however, may not be a viable option, because veterinarians are seeing CIA in horses all over the United States and even the world.