Hereditary Equine Regional Dermal Asthenia
(Poco Bueno Disease)
"When a horse has HC, there is lack of adhesion within the dermis, the deep layer of the skin, due to a collagen defect. Collagen serves a form of glue that holds the skin layers together. In horses with HERDA, the "glue" is inferior and the skin layers separate."
"In dramatic cases, the skin can split along the back and even roll down the sides, with the horse literally being skinned alive. Generally speaking, she says, the average lifespan for an HC horse is two to four years."
"HERDA is a genetic disease characterized by abnormal skin along the back that tears easily and heals into disfiguring scars. Symptoms typically don't appear until the horse is subjected to pressure or injury on his back, neck or hips. These horses cannot be ridden because the saddle literally tears off the skin, a horrible discovery usually postponed until the horse's initial experiences under saddle."
"Researchers speculate that HERDA results from the matching of two recessive genes, one each from the horse's sire and dam. According to the same researchers, all HERDA horses are related to one another and are the result of selective inbreeding. With very rare exceptions, HERDA is a disease found primarily in quarter horses and of those, cutting-bred horses make up the vast majority."
"According to the researchers, a horse gets HERDA in one of two ways: Both parents are both carriers, each with a dominant (normal) gene and a recessive (HERDA) gene; or one parent is a carrier and the other is an affected horse.
"All of the affected horses are in-bred," Bannasch reiterated. "We haven't seen any evidence that it is a dominant trait, like HYPP, where you only need one line of descent from a horse that has it. In this case, it looks like you have to have it (the HERDA gene) on both sides of the pedigree."
HYPP (hyperkalemic periodic paralysis disease) as been proven to affect the descendents of Impressive, a legendary producer of top halter quarter horses. It was proven that HYPP results from the passing of a dominant gene, meaning that the carrier offspring exhibit symptoms. That is not the case with HERDA, a recessive disease that remains silent in heterozygous horses. Only those HERDA-affected horses that carry double recessive genes outwardly show signs of the disorder.
In most cases, HYPP carriers can be managed through medication and kept normal enough to use, ride and show. Plus, HYPP carriers show signs of the disease while HERDA carriers seem completely normal.
"I think hyperelastosis (HERDA) is more devastating than HYPP," Dr. Poole said. "There is no prevention and there's definitely no cure. You're literally playing Russian roulette because if your horse is affected, that's it, you're done."
"Until DNA testing is able to identify the flawed gene it is the responsibility of all mare and stallion owners to learn about HERDA-affected horses and understand the consequences of breeding affected or carrier horses. Be on the lookout for damaged animals, before they're sold or saddled. Examine the skin along the top line and over the hips. Watch for wounds that do not heal normally."
"Hereditary equine regional dermal asthenia (HERDA) is a genetic skin disease predominantly found in the American Quarter Horse. Within the breed, the disease is prevalent in particular lines of cutting horses. HERDA is characterized by hyperextensible skin, scarring, and severe lesions along the back of affected horses. Affected foals rarely show symptoms at birth. The condition typically occurs by the age of two, most notably when the horse is first being broke to saddle. There is no cure and the majority of diagnosed horses are euthanized because they are unable to be ridden and inappropriate for future breeding. HERDA has an autosomal recessive mode of inheritance and affects stallions and mares in equal proportions. Research carried out in Dr. Danika Bannasch's laboratory at the University of California, Davis has identified the gene and mutation associated with HERDA."
The diagnostic DNA test for HERDA that has been developed allows identification of horses that are affected or that carry the specific mutation. Other skin conditions can mimic the symptoms of HERDA. The DNA test will assist veterinarians to make the correct diagnosis. For horse breeders, identification of carriers is critical for the selection of mating pairs. 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."
"Rashmir and Nena Winand, DVM, PhD, a geneticist and assistant professor in the Department of Molecular Medicine at Cornell University, announced earlier that 100% of the horses they have studied which are afflicted with HC trace through both sire and dam to Poco Bueno or his immediate relatives--Poco Bueno's sire, King, and Poco Bueno's full brother, Old Granddad. Rashmir and Winand are collaborators on HC research."
"The condition has manifested itself more frequently in cutting horses, but other disciplines are not immune, with cases showing up in reiners and pleasure horses which trace back to Poco Bueno."
Because the condition is caused by a recessive gene, it means that both sire and dam must possess the gene before an offspring of the two will be afflicted. Even when both sire and dam possess the gene, the disease will not necessarily be manifested. Under the laws of genetics, if a carrier is bred to a carrier, 25% of the offspring will have HC, 25% will not be afflicted or be carriers (they'll be genetically normal), and 50% will become carriers but not be afflicted."
One of the reasons that HC rears its head more often in cutting horse circles than any other, it is theorized, is that one of the most popular lines traces back to Poco Bueno through Doc O'Lena and Dry Doc and their dam, Poco Lena, a daughter of Poco Bueno. Both Doc O'Lena and Dry Doc are proven carriers of the recessive HC gene. Through the years, cutting horse breeders have tended to utilize the Doc O'Lena and Dry Doc bloodlines on both sides of the pedigree--breeding cousins to cousins. In essence, they have created a gene pool of carriers.
"It isn't the horse's fault," says Rashmir. "The problem revolves around the way we have bred them. Carriers have been bred to carriers. Breeders have a responsibility to avoid doing this if they know that stallion and mare are indeed carriers."
This does not mean, she says, that one should avoid the Poco Bueno bloodline when establishing a breeding program."
The disease is found primarily in the American Quarter Horse, specifically in cutting horse lines. Affected horses have been found to trace to the stallion Poco Bueno, or possibly, farther back to one of his ancestors. Researchers have now named four deceased Quarter Horse stallions that were carriers and produced at least one affacted HERDA foal; they are Dry Doc, Doc O'Lena, Great Pine, and Zippo Pine Bar. These stallions all trace to Poco Bueno through his son and daughter Poco Pine and Poco Lena. Other breeds affected are the American Paint Horse (APHA), and the Appaloosa (ApHC) and any other breed registry that allows outcrossing to AQHA horses.
As of May 9, 2007, Researchers working independently at Cornell University and at the University of California, Davis announced that a DNA test for HERDA has been developed. Over 1,500 horses were tested during the development phase of the test, which is now available to the general public through both institutions.
For more information on HERDA please visit the following websites.
HERDA - The Fatal Flaw