A study from Brazil underscores the importance of genetic testing in diagnosing a horse with hereditary equine regional dermal asthenia (HERDA).

HERDA is caused by a genetic mutation that affects the structures of the skin, reducing its tensile strength. HERDA horses have loose and fragile skin, which often develops large sores that are slow to heal. These horses usually cannot be ridden and in severe cases complications of HERDA may lead to euthanatization.

Although it’s possible to test for the genetic mutation that produces HERDA, the condition is usually diagnosed initially based on a horse’s history, clinical signs, skin biopsies and pedigree. Recently, Alexandre Secorun Borges, DVM, served as principal investigator and doctoral candidate Peres Ramos Badial, DVM, supervised a team of researchers at the Universidade Estadual Paulista in Sao Paulo, Brazil, in examining how accurate a diagnosis based on those factors could be.

First, the researchers performed full dermatological examinations on five horses with HERDA confirmed by polymerase chain reaction (PCR) testing of DNA0 and five unaffected horses. In each exam, the researchers measured skin thickness in 25 regions of the horse’s body. “Skin thickness measurement was performed as a part of the dermatological examination in several regions along the body, and the average skin thickness of each evaluated region was thinner in the affected horses in comparison with unaffected horses,” says Borges.

The researchers then collected a total of 60 biopsies at various body points, from horses with HERDA as well as healthy ones. The samples were then analyzed by independent pathologists who did not know each horse’s diagnosis. The data showed that histopathology alone, even when taken from sites with existing dermatological lesions, could produce false negative results.

“Initially, we thought it would be easier to differentiate positive and negative biopsy samples that were taken from the areas where there are evident clinical signs [dorsal areas] in HERDA horses,” says Borges. “However, we did not observe that during the study. Our results indicated that the region where one takes a biopsy sample facilitates the diagnosis of negative samples.”

The researchers say their findings show how genetic testing can complement clinical observations in HERDA diagnosis. “It is important to remember that it might take some time after the horse’s birth for the development of clinical signs that will allow the veterinarians to make the clinical and/or histopathological diagnosis,” says Borges. “Therefore, the molecular characterization of the HERDA mutation is the only way to get the definitive diagnosis, which can be achieved early in life before the development of any clinical signs.”

Reference: “Dermatological and morphological findings in quarter horses with hereditary equine regional dermal asthenia,” Veterinary Dermatology, June 2014

This article first appeared in EQUUS issue #446

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