HYPP
What is HYPP? Hyperkalemic Periodic Paralysis Disease (HYPP) is a muscular disease that affects both horses and humans. It is caused by a hereditary genetic defect that disrupts a protein called a sodium ion channel, a tiny gateway in the membrane of muscle cells. The genetic defect disrupts the channel’s normal opening and closing, such that uncontrolled sodium influxes occur. These influxes in turn change the voltage current of muscle cells, causing uncontrolled muscle twitching or profound muscle weakness. High levels of potassium in the blood usually are present when the disruptions in the ion channel occur.
What Are Effects of HYPP? Horses with HYPP can experience unpredictable attacks of paralysis which, in severe cases, can lead to collapse and sudden death. The cause of death usually is cardiac arrest and /or respiratory failure. The disease is characterized by intermittent episodes of muscle tremors manifested by generalized or localized shaking, trembling and weakness. Occasionally, episodes are accompanied by respiratory noises resulting from paralysis of the muscles of the upper airway (larynx and pharynx). In cases of mild attacks, muscle tremors may be so subtle as to be detectable only by an experienced clinician performing EMG testing.
What is the Origin of the Genetic Defect Causing HYPP? The original genetic defect causing HYPP was a natural mutation that occurred as part of the evolutionary process. The majority of such mutations, which are constantly occurring, are not compatible with survival. However, the genetic mutation causing HYPP produced a functional, yet altered, sodium ion channel. This gene mutation is not a product of inbreeding. The gene mutation causing HYPP inadvertently became widespread when breeders sought to produce horses with heavy musculature.
How is HYPP Inherited In Horses? Based upon breeding trials conducted at the Equine Research Laboratory at the University of California at Davis, it was determined that HYPP is inherited as an autosomal dominant trait, which means it can occur in both males and females. The trait is inherited from generation to generation with equal frequency; it does not get “diluted” out. Breeding an affected heterozygous horse (N/H) to a normal horse (N/N) will result in approximately 50% normal offspring, while 50% will carry the defective gene (N/H). Breeding an affected homozygote (H/H) will result in all offspring carrying the gene mutation, regardless of the status of the other parent. Normal (N/N) offspring safely can be bred, without fear of HYPP being inherited. Selective breeding to normal (N/N) horses could entirely eliminate HYPP disease. As HYPP is inherited as a dominant condition, it can and is being spread to other breeds. It is to everyone’s benefit to take the necessary steps to selectively breed HYPP out of existence before it becomes so widespread that this is impossible. For more information on HYPP please click here. Please Breed Responsibly.
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 are 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. For more information on HERDA including graphic pictures, please click here
Overo Lethal White Syndrome
Overo Lethal White Syndrome (OLWS) is a condition that occurs in newborn foals. The condition is genetic, caused by a recessive gene, and both parents must carry a copy of the gene for a foal to be born with this defect. Horses that carry this gene are most commonly overo white patterned horses (frame overos), but there are exceptions. The defective gene has been found in American Paint Horses, American Miniature Horses, Half-Arabians, Thoroughbreds, and horses formerly called cropout Quarter Horses (foals born to registered Quarter Horse parents that have too much white to qualify for registration with the American Quarter Horse-Association. This rule was removed from the AQHA at the 2004 Convention). The mating of two horses carrying the recessive gene will statistically result in a 25% chance of a lethal white foal.
OLWS foals have blue eyes and are completely or almost completely white at birth. These foals initially appear normal except for their unusual coloring. After a varying period of time, signs of colic will emerge due to the foal’s inability to pass feces. The OLWS foal has an underdeveloped, contracted intestine caused by a failure of the embryonic cells that form nerves in the gastrointestinal system. These cells also play a role in determining skin color. There is no treatment for OLWS, and surgery to bypass the intestinal damage has never been successful due to the extensive nature of this type of lesion. Veterinarians advise euthanasia for all OLWS foals because death will inevitably occur from colic caused by fatal constipation.
OLWS is emotionally injurious and often financially devastating for small breeders because it is always fatal. The best way to ensure you will never lose a foal to OLWS is to have your mare(s) and stallion tested prior to breeding and never breed two carriers.
Using clues from other species, researchers at the University of Minnesota investigated the same genes in OLWS foals, and found a mutation. A test for the defective allele (each gene is made of two alleles, one inherited from each parent) was quickly developed. Testing of OLWS foals, their parents, and unrelated horses revealed that all OLWS foals had two copies of the defective gene, each of their parents had one, and unrelated horses had none. Simply put, if carriers are never again bred to each other, there can never be another OLWS foal born.
Horses at greatest risk of carrying the defective allele are overos, particularly of American Paint Horses and American Miniature Horse breeding. A small number of Tobiano and breeding stock horses also carry the defective gene, and a very small number of carrier horses have been detected in other breeds. These other carriers include Pinto horses, which indicates that as other breeds import overo color patterning, they can also import the lethal gene. For more information about OLWS Please click here.
