During the past several decades, research has helped us recognize and understand a number of genetic diseases in horses. Some of these appear more commonly in certain breeds, since they originated from mutations in recent years. If the mutation occurred in a popular animal, it got passed along to numerous descendents. Mutations are common in humans and animals, but generally don’t cause many problems because they are greatly diluted in a very large gene pool. But if the mutation appeared in an animal that has hundreds or thousands of descendents – the defect may eventually appear in a large number of animals.
A perfect example is a serious skin defect in Quarter Horses, Paints, Appaloosas and any other breeds that have utilized certain Quarter Horse bloodlines. This skin abnormality was originally termed hyperelastosis cutis and is also called HERDA (hereditary equine regional dermal asthenia), a term that simply mean dermal weakness in certain areas of the body. It is an inherited defect in connective tissue, characterized by abnormal skin that tears easily, and separates readily from the underlying tissues. Any trauma or pressure can literally pull the skin apart. The affected foal may suffer more nicks and scrapes than usual, but the problem is often not discovered or recognized until the young horse is started in training – and then wearing a saddle creates massive injury to the skin.
This devastating problem was first documented in 1960, but no one knew what caused it. Later, research at Cornell University led by Dr. Nena Winand, and studies at UC-Davis, and at Mississippi State University where Dr. Ann Rashmir kept a group of affected horses, showed that the defective gene is a recessive trait that must be inherited from both parents before it can show up in the offspring.
In recent years, HERDA has been found in some of the most popular bloodlines in cutting and reining horses, and in many Quarter Horse pedigrees and a few other breeds as well. The researchers traced the defect back to Poco Bueno (a famous and popular sire) and his immediate ancestors.
For a while, skin biopsies were used to diagnose the problem, since the abnormal tissue could be identified in the lab. Then a DNA test was created, and now horse breeders can readily determine whether any horse in their herd is a carrier.
All affected horses are related, and all of them are the result of inbreeding or line breeding links that have doubled up this recessive gene. The defective gene did not cause a problem in the offspring of the first mutant animal because those offspring only possessed half of the equation. The trait must be doubled up, with one defective gene from each parent, in order to be expressed. Thus the defect was carried forward in a certain percent of the offspring of that first mutated animal, without the skin disease showing up, until some of the descendents were bred to each other.
In recent years, with the inbreeding and line breeding so prevalent in producing top athletes in various equine sports, we’ve seen this skin problem popping up with more frequency. Any horse that traces back to Poco Bueno has a chance of being a carrier, but many are not. During the past several decades, breeders had to find out the hard way if they had this defect in their herd – producing foals that suffered devastating skin injuries, or that could not be ridden when they grew up, eventually having to be euthanized. Now they can check an animal before breeding, using the DNA test.
“Poco Bueno was an important foundation sire in the Quarter Horse breed, and this bloodline is fundamental in the performance, pleasure and ranch horse industry,” says Dr. Nena Winand, a veterinary microbiologist at Cornell University who has worked on this genetic research. “A lot of horses have Poco Bueno in their background. Some people say they have linebred these bloodlines for years and not had any problems, but other foundation linebreeders have produced foals with HERDA,” she says.
All animals have some undesirable recessive traits, but these don’t show up unless doubled. “Some of the horses that are carriers of HERDA are spectacular animals and very athletic, and if you don’t breed them to a related horse you will never double up the recessive trait. You’ll never see that genetic defect. It’s not the carrier that’s the problem. It’s the way we breed them,” says Winand. If horse breeders can make informed decisions on how they breed these horses, by testing them first, they can safely continue to use them in their breeding program, by breeding them to non-carriers.