Genetic diseases

There are four known genetic conditions in Arabian horses which usually result in euthanasia of the affected animal. All four are thought to be autosomal recessive conditions, which means that the flawed gene is not sex-linked and has to come from both parents for an affected foal to be born. There are two other genetic conditions that are not inevitably fatal, but can be disabling or fatal if not treated.

Arabians are not the only breed of horse to have problems with inherited diseases; fatal or disabling genetic conditions also exist in many other breeds.

The "genetic lethal" conditions (so called, though two of the four are not invariably lethal) in Arabian horses are:

• Cerebellar abiotrophy (CA). An affected foal is usually born without symptoms, but at some point, usually after six weeks of age, develops severe incoordination, a head tremor, wide-legged stance and other symptoms related to the death of the purkinje cells in the cerebellum. Such foals are frequently diagnosed only after they have crashed into a fence or fallen over backwards, and sometimes their symptoms are misdiagnosed as a head injury caused by the accident. The only way to confirm a diagnosis of CA is to examine the brain after euthanasia. The degree of severity varies, with some foals having fast onset of severe coordination problems, others showing milder symptoms. In theory, mildly affected horses could live a full lifespan, but in practice most are euthanized before adulthood because they are so accident-prone as to be a danger to themselves and others. There is currently no genetic test for CA.
• Guttural Pouch Tympany (GPT) occurs in horses ranging from birth to 1 yr of age and is more common in fillies than in colts. It is thought to be genetic in Arabians, possibly polygenic in inheritance, but more study is needed. Foals are born with a defect that causes the pharyngeal opening of the Eustachian tube to act like a one-way valve. Air can get in, but it cannot get out. The affected guttural pouch is distended with air and forms a characteristic nonpainful swelling. Breathing is noisy in severely affected animals. Diagnosis is based on clinical signs and radiographic examination of the skull. Medical management with NSAID and antimicrobial therapy can treat upper respiratory tract inflammation. Surgical intervention is needed to correct the malformation of the guttural pouch opening to provides a route for air in the abnormal guttural pouch to pass to the normal side and be expelled into the pharynx. Foals that are successfully treated may grow up to have fully useful lives.
• Juvenile Epilepsy Syndrome (JES), sometimes referred to as "benign" epilepsy or "idiopathic" epilepsy, is not usually fatal. Foals are born normal and appear normal between epileptic seizures, usually outgrowing the condition between 12 and 18 months. Affected foals may show signs of epilepsy anywhere from two days to six months from birth. Symptoms of the condition can be treated with traditional anti-seizure medications, which may reduce the severity of symptoms. Though the condition has been studied since 1985 at the University of California, Davis, the genetic mode of inheritance is unclear, though the cases studied were all of one general bloodline group. Some researchers have suggested that epilepsy may be linked in some fashion to Lavender Foal Syndrome due to the fact that it occurs in similar bloodlines and some horses have produced foals with both conditions.
• Lavender Foal Syndrome (LFS), also called Coat Color Dilution Lethal (CCDL). The condition gets it name because most affected foals are born with a coat color dilution that lightens the tips of the coat hairs, or even the entire hair shaft. Foals with LFS are unable to stand at birth, often have seizures, and are usually euthanized within a few days of birth. There is currently no genetic test for LFS.
• Occipital Atlanto-Axial Malformation (OAAM). This is a condition where the cervical vertebrae fuse together in the neck and at the base of the skull. Symptoms range from mild incoordination to the paralysis of both front and rear legs. Some affected foals cannot stand to nurse, in others the symptoms may not be seen for several weeks. This is the only cervical spinal cord disease seen in horses less than 1 month of age, and a radiograph can diagnose the condition. There is no genetic test for OAAM, and the hereditary component of this condition is not well researched at present.
• Severe Combined Immunodeficiency (SCID). Similar to the "bubble boy" condition in humans, an affected foal is born with no immune system, and thus generally dies of an opportunistic infection, usually before the age of five months. There is a DNA test that can detect healthy horses who are carriers of the gene causing SCID, thus testing and careful, planned matings can now eliminate the possibility of an affected foal ever being born.

The Arabian Horse Association in the United States has created a foundation that supports research efforts to uncover the roots of genetic diseases. The organization F.O.A.L. (Fight Off Arabian Lethals) is a clearinghouse for information on these conditions. Additional information is available from the World Arabian Horse Association (WAHO).