Variant (Genetic) Health Panel Testing Information

USAHR Mission Statement establishes the need to educate the Membership, people in general, on these genetic tests and why it is so important to protect the welfare and integrity of the Purebred Arabian breed. The Foundation which will be aligned with USAHR has as part of its work to aid where ever and however possible, the research and identification of these genetic tests. USAHR continues the Foundation work in the Rules and Regulations.

Understanding genetic testing and why it is so important as a safeguard for the Purebred Arabian breed requires delving into the different gene tests. The following is a simplified explanation: Genes comes in pairs; each parent contributes one copy to the offspring. With respect to the lethal Arabian genetic diseases listed below (SCID and LFS), only one defective gene is necessary to express the genetic disease in question. This gene can be inherited from either of the parents or from both of the parents. If from both of the parents, the resulting foal dies. Horses with only one copy of the defective gene are considered “carriers”.

REMEMBER: To continue doing a CA list [or any list, database or public notification or certificate registration data, on any of the genetic diseases], there must be proper test results submitted which removes any liability for the person recording it in a database, NOT an email claiming clear or carrier status.

A laboratories must be an Institutional member of the International Society for Animal Genetics (ISAG institutional member number 105247) and participate in ISAG comparison tests and report DNA types according to standardized nomenclature. DNA genotyping DNA type (aka DNA profile, aka DNA fingerprint) DNA type shows the markers used (microsatellites – VHL20; HTG10 and etc.) with allele sizes for the markers coded in letters (VHL20 – PR; HTG10 – LR; AHT5 – KN and etc.). DNA types are reported according to standardized nomenclature of ISAG. If one of the parents or both parents are available, parentage verification will be performed and it will reflect on the report (sire/dam qualification/exclusion). This is not meant to limit any type of DNA testing; haploid and further Chromosome Y testing is also to be included where ever this testing may be performed.

Each individual has two microsatellite numbers. One comes from the sire and the other comes from the dam. This is how to verify parents. The numbers (Ex: 202/208) represent the molecular weight of the DNA fragments and the markers (INRA23) typically represent the laboratory who founded the markers. In equine ancestry, instead of numbers the microsatellites are represented in letter. The basic principles are exactly the same, only the letter is a part of a code which represents the particular fragment of DNA.

SCID is Severe combined immunodeficiency (SCID), a fatal condition of both B (humoral) and T (cellular) cell dysfunction. This immunodeficiency may occur in Arabian foals (or breeds carrying Arab bloodlines), and manifests clinically by susceptibility to viral, bacterial, fungal, and protozoal organisms (e.g. adenovirus, coronavirus, Rhodococcus equi, Pneumocystis carinii, and/or Cryptosporidium parvum). Foals are normal at birth but soon develop fatal infections, particularly when circulating colostrum-derived antibody concentrations become low. The mode of inheritance of the genetic defect is an autosomal recessive trait. [Cornell University research and education departments]

LFS is Lavender Foal Syndrome (LFS), a genetic disorder with neurological features thought to be caused by a brain lesion. This disorder is usually associated with Arabian horses, although it has been reported in other breeds. In addition to a difficult birth, affected foals often have a diluted coat color that can appear pale lavender. As affected foals are usually euthanized within days of birth, this technology offers for the first time, the ability detect the mutation associated with LFS to diagnose both the condition and to guide breeding practices so as to prevent its continuation in bloodlines via carriers. [Cornell University]

CA has two lists; one for carriers and one for affected. It is not lethal.
CA is Cerebellar Abiotrophy, an inherited neurological condition found primarily in Arabian horses, and is characterized by neurological defects in foals including head tremors and ataxia. Affected horses may show exaggerated action of the forelegs, a wide-based stance, and be unable to rise from a reclining position. They tend to startle easily and often fall due to ataxia. Some foals show very severe signs, including the exaggerated gaits and dramatic lack of balance. Others have little more than a head tremor, which may only manifest itself during goal-directed movement.
Horses with N/N genotype will not have cerebellar abiotrophy and cannot transmit this cerebellar abiotrophy variant to their offspring.
Horses with N/CA genotype will not be affected by cerebellar abiotrophy, but are carriers. They may transmit this cerebellar abiotrophy variant to 50% of their offspring. Matings between two carriers result in a 25% chance of producing a cerebellar abiotrophy-affected foal. Horses with CA/CA genotype will have cerebellar abiotrophy.
[University of California, UC of Davis]

OAAM1 is the Arabian strain of Occipitoatlantoaxial Malformation. OAAM is malformation of the occipital bone of the skull resulting in a neurologic disorder caused when the first two cervical vertebrae (the atlas and axis), fuse to the base of the skull. Because the foal with this condition cannot move its head properly, the foal will flip head over heels. THIS TEST IS NOT REQUIRED BY USAHR FOR REGISTRATION.

PSSM1 Type 1 Polysaccharide Storage Myopathy (PSSM1) have a muscle disease characterized by accumulation of abnormal complex sugars (glycogen) in skeletal muscles. The accumulation of abnormal sugars can cause breakdown of muscle fibers (rhabdomyolosis) which leads to muscle pain, weakness, skin twitching, sweating, and reluctance to move.

  • Horses with N/N genotype will not have type 1 Polysaccharide Storage Myopathy and cannot transmit the PSSM1 variant to their offspring.
  • Horses with N/PSSM1 genotype will have the PSSM1 variant and may show signs of type 1 disease. Horses with this genotype may transmit the PSSM1 variant to 50% of their offspring.
  • Horses with PSSM1/PSSM1 genotype are homozygous for the PSSM1 variant and may be more severely affected than N/PSSM1 horses. Horses with this genotype will transmit the PSSM1 variant to all of their offspring.
  • There is a separate webpage for the genetic disorder PSSM2 in the Member Section which defines and explains the various types including muscle wasting.