Hereditary equine regional dermal asthenia
Encyclopedia
Hereditary equine regional dermal asthenia (HERDA) aka hyperelastosis cutis (HC) is an inherited autosomal recessive
Dominance relationship
Dominance in genetics is a relationship between two variant forms of a single gene, in which one allele masks the effect of the other in influencing some trait. In the simplest case, if a gene exists in two allelic forms , three combinations of alleles are possible: AA, AB, and BB...

 connective tissue
Connective tissue
"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...

 disorder. It develops from a homozygous recessive mutation that weakens collagen fibers that allow the skin of the animal to stay connected to the rest of the animal. Affected horses have extremely fragile skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...

 that tears easily and exhibits impaired healing. In horses with HC, the skin separates between the deep and superficial dermis
Dermis
The dermis is a layer of skin between the epidermis and subcutaneous tissues, and is composed of two layers, the papillary and reticular dermis...

. There is no cure. Most individuals receive an injury they cannot heal, and are euthanized
Animal euthanasia
Animal euthanasia is the act of putting to death painlessly or allowing to die, as by withholding extreme medical measures, an animal suffering from an incurable, especially a painful, disease or condition. Euthanasia methods are designed to cause minimal pain and distress...

. Managed breeding strategy is currently the only option for reducing the incidence of the disease.

The disease is found primarily in the American Quarter Horse
American Quarter Horse
The American Quarter Horse is an American breed of horse that excels at sprinting short distances. Its name came from its ability to outdistance other breeds of horses in races of a quarter mile or less; some individuals have been clocked at speeds up to 55 mph...

, specifically in cutting
Cutting (sport)
Cutting is an equestrian event in the western riding style where a horse and rider are judged on their ability to separate a single animal away from a cattle herd and keep it away for a short period of time.-Description:...

 horse lines. Affected horses have been found to trace to the stallion Poco Bueno
Poco Bueno
Poco Bueno a brown quarter horse stallion foaled April 10, 1944. He was sired by the great Quarter horse stallion King P-234 and out of the mare Miss Taylor who was by Old Poco Bueno. Poco Bueno was named for his maternal grandsire, and the name means pretty good in Spanish...

, 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
Doc O'Lena
Doc O'Lena was a Quarter Horse stallion and an outstanding cutting horse and sire of cutting horses.-Life:Doc O'Lena was foaled in 1967, the offspring of Doc Bar and Poco Lena. He is one of only two horses in the American Quarter Horse Hall of Fame with both parents also inductees. He was a bay...

, Great Pine, and Zippo Pine Bar
Zippo Pine Bar
Zippo Pine Bar is the leading Western Pleasure sire of Quarter Horses.-Life:Zippo Pine Bar was a 1969 sorrel son of Zippo Pat Bars out of Dollie Pine, a daughter of Poco Pine. Poco Pine was a son of Poco Bueno...

. These stallions all trace to Poco Bueno
Poco Bueno
Poco Bueno a brown quarter horse stallion foaled April 10, 1944. He was sired by the great Quarter horse stallion King P-234 and out of the mare Miss Taylor who was by Old Poco Bueno. Poco Bueno was named for his maternal grandsire, and the name means pretty good in Spanish...

 through his son and daughter Poco Pine and Poco Lena
Poco Lena
Poco Lena was an outstanding cutting mare, and dam of two famous Quarter horse cutting horses and stallions: Doc O'Lena and Dry Doc.-Life:Poco Lena was foaled in 1949, the daughter of Poco Bueno out of a daughter of Pretty Boy named Sheilwin...

. Other breeds affected are the American Paint Horse
American Paint Horse
The American Paint Horse is a breed of horse that combines both the conformational characteristics of a western stock horse with a pinto spotting pattern of white and dark coat colors. Developed from a base of spotted horses with Quarter Horse and Thoroughbred bloodlines, the American Paint Horse...

 (APHA), and the Appaloosa
Appaloosa
The Appaloosa is a horse breed best known for its colorful leopard-spotted coat pattern. There is a wide range of body types within the breed, stemming from the influence of multiple breeds of horses throughout its history. Each horse's color pattern is genetically the result of various spotting...

 (ApHC) and any other breed registry that allows outcrossing to AQHA horses.

HERDA is characterized by abnormal skin along the back that tears or rips easily and heals into disfiguring scars. The skin is loose, and hyper-elastic in affected horses. Symptoms typically don’t appear until the horse is subjected to pressure or injury on their back, neck or hips, usually around two yrs of age. However foals can show signs when injured, while other horses mature and only show signs in the joints. The expression of HERDA is variable, and the phenotypic
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

 range of expression is still being determined.

After being tested, the results either show "normal", "carrier" or "affected" and can be determined by DNA testing of either a blood or hair sample.
  • N/N - Normal: does not have the HERDA gene
  • N/HRD- Carrier: carries one copy of the gene
  • HRD/HRD- Affected: has two copies of the gene


The expected lifespan of an affected horse is 2–4 years. There is currently no cure for this disease. To prevent it from occurring, the only solution is not to breed horses who both carry the HERDA allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...

.

History

The disease first was recognized in 1971. Research of affected animals indicated that 95% of the identified HERDA horses have traced back to Poco Bueno on both sides of the pedigree
Pedigree chart
A pedigree chart is a diagram that shows the occurrence and appearance or phenotypes of a particular gene or organism and its ancestors from one generation to the next, most commonly humans, show dogs, and race horses....

. Some also trace to his full brother, Old Grand Dad. The remaining 5% trace to King, the sire of both horses. This may suggest that Poco Bueno could have inherited the HC mutation form his sire, with the possibility that the condition goes even further back. It is difficult to trace the condition further because no pedigrees to date completely distinguish the lines of King.

A disease consistent with HERDA/HC was described in the scientific literature in 1988 documenting two related affected Quarter Horses. Researchers at Mississippi State University described a separation of the upper and lower dermis associated with a loose packing of collagen fibrils in a one year old filly with hyperelastosis cutis. In 2004, clinical veterinarians at the University of California, Davis first coined the term hereditary equine regional dermal asthenia (HERDA) after examining 50 horses with stereotypical presentation of the disease. Shortened and thinned collagen fibrils in the deep dermis was the significant characteristic shared among the affected skin of diseased horses.

Throughout the years, closely breeding back, or inbreeding
Inbreeding
Inbreeding is the reproduction from the mating of two genetically related parents. Inbreeding results in increased homozygosity, which can increase the chances of offspring being affected by recessive or deleterious traits. This generally leads to a decreased fitness of a population, which is...

, to the lines of Poco Bueno increased the frequency of homozygousity in the population, thus increasing the number of affected animals. The 2004 Quarter Horse News Stallion Register revealed that of the top 100 cutting horse stallions, 14 are known HERDA carriers. Today, approximately 28% of all horses of cutting horse bloodlines are carriers of HERDA.

Using DNA from the UC Davis clinical collection of HERDA samples, collaborating UC Davis geneticists searched for discreet areas of DNA that were homozygous, or identical by descent, uniquely in affected horses. UC Davis first reported successful preliminary mapping in January 2004. Additional work verified the disease was inherited, rather than caused solely by environmental insult, and strongly indicated an autosomal recessive mode of inheritance. In January 2007, UC Davis presented their findings at the Plant and Animal Genome Conference Equine Workshop indicating the ability to identify horses that carry HERDA. A full accounting of this work, detailing the mapping and identification of a mutation in cyclophilin B, or peptidyl-prolyl isomerase B (PPIB), was published on May 11, 2007. Concurrent with publication of these results, the Veterinary Genetics Laboratory at UC Davis began offering a genetic test to identify carriers of the disease allele.

In Spring 2007, researchers working independently at Cornell University
Cornell University
Cornell University is an Ivy League university located in Ithaca, New York, United States. It is a private land-grant university, receiving annual funding from the State of New York for certain educational missions...

 and at the University of California, Davis
University of California, Davis
The University of California, Davis is a public teaching and research university established in 1905 and located in Davis, California, USA. Spanning over , the campus is the largest within the University of California system and third largest by enrollment...

 announced that a DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

test for HERDA had been developed. Over 1,500 horses were tested during the development phase of the test, which was initially available to the general public through both institutions. A United States patent for the HERDA test was issued to UC Davis on October 27, 2009.
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