The genetics of anhidrosis

Anhidrosis, the complete or partial inability to sweat, occurs more frequently in some breeds and among horses with a family history of the condition, which suggests a heritable component.
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Question: Can a horse inherit a susceptibility to anhidrosis? I’m considering breeding my 9-year-old Quarter Horse mare, who is lovely in every respect but has a very limited capacity to sweat. She was actually relocated up here from down south because living in a hot climate was intolerable and unsafe for her.

Anhidrosis, the  complete or partial inability to sweat, has such a substantial impact on affected horses that it is a welfare concern.

Anhidrosis, the complete or partial inability to sweat, occurs more frequently in some breeds and among horses with a family history of the condition, which suggests a heritable component.

My mare sweats a bit now, but not nearly close to normal. Even though our summers here are fairly temperate, I have to keep a close eye on her. I don’t ride her when the temperature is above 80 degrees or it’s humid. She is otherwise healthy, has great gaits and is highly trainable so I would love to have a foal from her. But I don’t want to breed another horse that can’t sweat normally.

If anhidrosis can be inherited, would selecting a stallion who doesn’t have the problem “cancel out” my mare’s DNA contribution?

Megan Simmons
Menomonie, Wisconsin

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Answer: I am so pleased you asked this question, as it’s clear you are thinking hard about whether all your mare’s traits are worth passing on to the next generation instead of just focusing on the “good ones.” Anhidrosis has a substantial impact on affected horses, so it really is a welfare concern that is to be avoided as much as possible. Anhidrosis, the complete or partial inability to sweat, occurs more frequently in some breeds and among horses with a family history of the condition, which suggests a heritable component. A research team led by Samantha Brooks, PhD, and Laura Patterson Rosa, DVM, PhD, of the University of Florida, has been investigating the genetics of anhidrosis and attempting to identify a defect associated with the syndrome (full disclosure: I am also a small contributor to this research).

Analysis using the Genome-Wide Association Study (GWAS) technique to broadly look for genomic associations with anhidrosis yielded additional information that was recently published by Brooks and Patterson. A case-control GWAS, using state-matched cases and controls (collected from across the United States) totaling 200 individual horses, showed a strongly supported candidate region containing the KCNE4 gene. Sequence analysis revealed a single nucleotide polymorphism (SNP)---a variation in a base pair of genes within a DNA sequence---likely to alter KCNE4 protein function. This gene encodes a portion of a potassium channel protein with a possible function in sweat gland outflow.

The upshot of this genomic detail: The analysis points to a novel genetic factor for anhidrosis, suggesting some degree of heritability. This work is far from complete---much more research needs to be done to validate and confirm these findings. But as an equine veterinarian advising a horse owner about how to use this information right away, these findings suggest that anhidrosis is heritable. Unfortunately, we don’t yet have enough information or tests available to help select the ideal stallion to “cancel out” your mare’s anhidrosis. The best recommendation I can give at this point is to avoid breeding her.

Martha Mallicote, DVM,
MBA, DACVIM
Clinical Associate Professor
University of Florida Large
Animal Hospital
Gainesville, Florida

Martha Mallicote, DVM, MBA, DACVIM, completed her undergraduate work at the College of Charleston, a Master’s of Business Administration at University of Florida, and received her veterinary degree from the University of Tennessee College of Veterinary Medicine in 2006. Since then, she has worked in both ambulatory and referral hospital settings. In 2012, Mallicote completed her residency in large animal internal medicine at the University of Florida and joined the faculty.

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