HERDA Research Update: Genetic Testing Is Critical; Disorder Is Not Limited to Connective Tissue of the Skin

It’s a brave new horse world we live in. Just as we once struggled to learn our horses’ pedigrees until we became familiar with bloodlines in a given breed, so we now may have to accept that along with a mile-long pedigree, we will need to analyze a mile-long string of DNA.

And, in the end, it will be that string of DNA much more than the color of a horse’s coat or his size or conformation that will be the deciding factor whether a colt is gelded and whether a filly is destined for the breeding program…or not.

Central to our veterinary colleges now is a department or office of genetic studies. This is an important development that is adding to the knowledge base of new diseases and disorders that have their root in bloodlines. One example of many is the connective tissue disorder known as HERDA. Without genetics testing labs and genetics researchers, we’d be hard put to explain what this disorder is or how to figure out who had it.

Here’s some information on equine genetics testing from the University of California at Davis, split into two parts (with some repetition, sorry!) and about HERDA.

Hereditary equine regional dermal asthenia, known as HERDA, is an genetic disorder of horses that results in fragile skin that tears easily and has limited ability to heal. It is seen predominately in Quarter Horses, with the majority of cases linking to some of the current elite cutting horse bloodlines. This has lead, unfortunately, to the recent increased incidence of HERDA in the United States and other countries in recent years. However, only approximately two percent of all American Quarter Horses are carriers for the condition.

HERDA is often not obvious in newborn foals and may not be discovered until the time comes to break a young horse and problems develop under the saddle. There is a diagnostic test that will determine both if a horse has the condition and if a horse carries the gene for the condition in its bloodlines. A correct diagnosis is very important, since other skin conditions could be confused with HERDA and have a devastating effect on a young horse’s future, a breeder’s reputation and the horse’s resale value.

According to the Veterinary Genetics Laboratory at the University of California at Davis, testing is critical for responsible breeding. Breedings of carrier horses have a 25% chance of producing an affected foal. Breedings between normal and carrier horses will not produce a HERDA foal although 50% of the foals are expected to be carriers. UCDavis’s testing lab facility is available to horse owners all over the United States.

HERDA-affected horses have loose, stretchy folds of skin and are frequently euthanized due to poor wound healing and disfiguring scars. Somewhat similar disorders of connective tissue are found in other species, including humans. One in 5000 humans has Ehlers-Danlos Syndromem (EDS), which affects the skin and other tissues such as the eyes, heart and joints. EDS in humans is often incurable and fatal.

New supplemental information on HERDA has been published in the August 2010 edition of the Journal of the American Veterinary Medical Association. 

Results of ophthalmic and microscopic examinations of the eyes of horses with and without HERDA revealed that the cornea was significantly thinner in horses with HERDA and that horses with HERDA had zones of disorganized, haphazardly arranged collagen fibrils in their corneas. For 28 horses with and 291 horses without HERDA that were part of an equestrian program from 2003 through 2006, the incidence of corneal ulcers was significantly greater for horses with HERDA than for horses without the disease.

Researchers noted that the measurement of the thickness of the cornea did show that it was thinner in the HERDA horses but also that production of tears in horses with HERDA was significantly greater than that of control horses.

Scanning Electron Microscopy (SEM) images accompanying the study reveal zones of disorganized, haphazardly arranged collagen fibrils in corneas of horses with HERDA that were not evident in corneas of control horses.

The study was conducted at Mississippi State University College of Veterinary Medicine; authors are Cathleen A. Mochal, DVM; William W. Miller, DVM, MS, DACVO; A. James Cooley, DVM, DACVP; Robert L. Linford, DVM, PhD, DACVS; Peter L. Ryan, PhD; and Ann M. Rashmir-Raven, DVM, MS, DACVS.

The study was supported by the university, the American Quarter Horse Association, the National Institutes of Health, and private donors.

An abstract detailing the results of the study was presented at the American Association of Equine Practitioners (AAEP) Convention in Las Vegas last year, but this publication presents the full details of the study.

A separate recent study at Mississippi State found that a group of HERDA horses also showed inferior aortic valve insufficiency when undergoing targeted biomechanical testing on the aortic and mitral valves of the heart.

by Fran Jurga | The Jurga Report at Equisearch.comFollow @FranJurga on Twitter.com for more horse health news!




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