Horse breeding: genetic diseases

Although the subject of genetic diseases has sometimes been neglected by breeders and stallion owners, it is now emerging and many of them are deciding to test both mares and stallions to limit the risks. Depending on the breed and type of horse, different genetic diseases are to be taken into account and can nowadays be detected by means of tests.  It is therefore strongly recommended that breeders test their breeding stock for possible diseases.

Summary

I. Sport horses: beware of WFFS and PSSM.

II. Sport ponies: congenital myotonia and HWSD.

III. Arabian and Arabian crossbred horses: CA, SCID, LFS and OAAM.

__

I. Sport horses: beware of WFFS and PSSM

WFFS (Warmblood Foal Fragility Syndrome) causes extreme skin fragility in foals due to poor collagen in the epidermis. Foals with WFFS also have difficulty standing due to hyperlaxity of the joints. There is no known treatment and the life span of the affected foal will usually not exceed 8 days. The disease is lethal in the homozygous state, a heterozygous horse will be a healthy carrier. It is advisable never to cross two healthy carriers, to avoid the risk of a homozygous foal that is not viable.

PSSM1 (polysaccharide storage myopathy) is a genetic disease that occurs in both sport horses and also affects leisure and working horses. A horse affected by PSSM1 (even if heterozygous for the gene concerned) has abnormalities in the storage of glycogen in the muscle cells: the clinical signs are difficulties in moving and severe muscle pain (cramps, spasms) after exercise. In extreme cases, PSSM can lead to death. A horse with the disease can live, provided that its work and living conditions are adapted. Very moderate physical exercise, a suitable diet and a life outdoors are strongly recommended. The IFCE advises breeders, in terms of reproduction, to ensure that no homozygous individuals are born.

Unfortunately, only PSSM1 is testable. There are also other PSSM genes (including "PSSM2") that activate the disease, but which are not yet testable.

More information on the IFCE website.

Whether you own mares or stallions, if you plan to breed them, it is therefore strongly recommended that they be tested for these diseases.

II. Sport ponies: congenital myotonia and HWSD

If these terms do not ring a bell, then read on:

Myotonia congenita is a genetic muscle tone disease of the familial periodic paralysis family. It mainly affects New Forest ponies (and crosses with this breed). In animals with myotonia, there is an imbalance between muscle contraction and relaxation. When over-stimulated, the muscles contract rapidly but do not relax as quickly, resulting in stiff limbs and an inability to move or even fall. The disease is only expressed in the homozygous state, an animal heterozygous on the gene concerned will be a healthy carrier. The production of homozygous foals should therefore be avoided in breeding, and breeding stock should be tested. 

More information on the RESPE website.

HWSD (Hoof Wall Separation Disease) is the "hoof wall separation disease". It affects, as its name suggests, the pony's foot, and has so far been identified in Connemara ponies. It appears before weaning and affects all 4 feet of the pony. It consists of the separation of the dorsal wall of the hoof. If the disease is detected early, proper care can prevent the disease from progressing. As with myotonia congenita, the disease is only expressed in the homozygous state. Genetic testing of breeding stock, both stallions and mares, remains the best option to avoid the birth of a homozygous and therefore affected individual.

More information on the IFCE website.

III. Arabian and Arabian crossbred horses: CA, SCID, LFS and OAAM

CA (Cerebellar Abiotrophy) is a genetic neurological disease that occurs in Arabian horses and horses of Arabian crosses. Symptoms of the disease include ataxia or lack of balance, head tremor, hyper-reactivity, lack of reflexes, a stiff gait with exaggerated foreleg action and poor perception of space and distance. The first symptoms are usually visible from 4 months of age.

SCID (Severe combined immunodeficiency disease) is a genetic disease that causes a decrease in the immune system. An affected foal will have a deficiency of B and T lymphocytes and will be prone to the development of infectious diseases. The immune system of a foal with SCID does not allow it to produce its own antibodies, which favours the development of disease. In the majority of cases, the foal will be unable to survive more than 45 days after the onset of the first symptoms.

Lavender Foal Syndrome (LFS) is also a genetic disease that affects the nervous system of horses, particularly Arabians and their offspring from crossbreeding. Foals affected by this disease show symptoms such as a thinning of the coat, an inability to stand or turn around, and uncontrolled limb movements. Unfortunately, these foals are often euthanised within the first two days of life.

Even rarer, Atlantoaxial Occipital Malformation (AOM) is a developmental defect that causes compression of the upper spinal cord, resulting in neurological damage. Symptoms range from lack of coordination and weakness of the limbs to paralysis of the fore and hind limbs. Most often the disease severely affects the quality of life of the foal and leads to euthanasia.

These 4 diseases are expressed in the homozygous state of the gene concerned, so animals carrying a single mutated allele will be healthy carriers. In order to control the problem and to crossbreed without risk, it is therefore essential to test the breeding stock.

More information on the World Arabian Horse Organization website.

 __

To remember:

It is therefore important to understand that individuals, stallions or mares, can be in excellent shape and totally asymptomatic, and yet carry a gene carrying a serious genetic disease. So even if 2 or 3 generations of horses showed no signs of disease, the gene may be present. It is only relatively recently that science has identified genetic disease genes in horses and developed tests to detect them. It is important that breeders take advantage of these advances and test their breeding stock.

Note: Some other genetic diseases are also present in draft horses and American horses. It is therefore also recommended that breeders using breeding stock from these breeds carry out the necessary tests.

__