Inherited Diseases of Horses
Arabians, like some other horse breeds such as the Quarter horse and Belgian Draught, are at risk of genetic disorders. Considering the many conditions that can be inherited by humans, the surface has only been scratched when considering inherited diseases in animals.
One inherited condition which affects Arabians is Severe Combined Immunodeficiency Disorder (SCID). This occurs in other mammals, including humans, but in different forms. SCID was first reported in Australia in 1973. In 1980, after extensive and expensive research in the USA, it was established that SCID in Arabians was inherited as an autosomal recessive condition. 'Autosomal recessive' means that the flawed gene is not sex-linked. Thus, unless the recessive gene is passed on by both parents, offspring will be free of the disease.
This condition is similar to 'bubble baby' conditions in humans. The affected foal is born with no immune system and will generally die before the age of five or six months. Opportunistic infections such as pneumonia usually attack an infected animal. Once diagnosed as SCID, it is kinder to have the animal euthanized as it is only a matter of time before an infection will strike.
There is now a DNA test to detect carriers. Testing of all breeding horses will eliminate the possibility of a foal being born with the affliction as the disease will not appear unless both parents are carriers.
Lavender Foal Syndrome (LFS) is also known as Coat Colour Dilution Lethal (CCDL). This is another recessive disorder which is fatal when a flawed gene is inherited from both parents. The name comes from the dilution of the coat colour. The tips or sometimes the entire hair shaft is light-coloured. The coat is a dull pinkish-grey and the individual hairs may be somewhat silvery. Some foals are a very unusual iridescent silver varying to pale lavender blue. Foals may be misdiagnosed as having neonatal maladjustment as many of these foals have a difficult delivery and many have a strong suckle reflex. However the dilute coat should alert one to the possibility of LFS.
Foals with this condition are unable to stand when born and will often have seizures. If not euthanized, they will die within a few days. Again, there is a DNA test to detect carriers of LFS.
Cerebellar abiotrophy (CA) (also known as cerebellar cortical abiotrophy (CCA), like SCID and LFS, is another recessive disorder. It is a neurological disease and is found in breeds other than Arabians. Foals may be born unaffected but will develop extreme unco-ordination and a wide-legged stance. A palsy-like tremor may affect the head and is most noticeable when the foal is trying to focus on something. A normal blink response is absent. 'Hypermetric action' may be evident in the forelegs. This awkward and highly exaggerated form of action is like a goose-step action with the leg being thrown out from the elbow at both the walk and the trot. The heel of the hoof may hit the ground first and with a thump.
The cause is only in the brain. Affected animals do not lose weight, there is no loss of muscle tone or increasing weakness. They are not in pain and they are not lethargic. The spinal column is not involved. Because of the difficulty the horse has in balancing itself and judging distance, it may appear panicky and flighty. When maintained in a constant environment, it may eventually adapt quite well. However change the parameters and the horse is as bad as ever.
The disease causes the death of neurons known as purkinje cells in the cerebellum of the brain. The cerebellum plays an important role in motor control. Damage to the cerebellum (or Purkinje cells) results in loss of coordination with subsequent involuntary movements. Balance, equilibrium and muscle tone are also affected. The animal is unable to judge space and distance. Generally the cells begin to die off shortly after birth and symptoms are noticeable before the foal reaches six months of age. Often a foal will run into a fence or wall or may fall over backwards. A head injury may then be misdiagnosed as the cause of the accident.
The severity of the condition varies but most are affected to the point that they are not safe to ride or handle. There is now a genetic test to detect carriers and those animals affected with the disease.
Occipital Atlanto-Axial Malformation (OAAM) occurs when the cervical vertebrae fuse in the neck and at the base of the skull, causing compression of the spinal cord. There may be mild incoordination or full paralysis of all four legs. Some foals will be unable to stand to nurse while others will not show symptoms for a few weeks. Diagnosis is by radiograph. The hereditary component is not well researched yet nor is there a genetic test.
Foals affected with Equine Juvenile Epilepsy or Juvenile Idiopathic Epilepsy (sometimes referred to as 'benign' epilepsy) will show signs of the disease anywhere from two days to six months from birth. Between seizures, foals appear normal and the animal may 'grow out' of the disease between 12 and 18 months when seizures will stop appearing. Anti-seizure medication may reduce the severity of the attacks.
The way in which the disease is inherited is not clear. There is some thought that epilepsy may be linked to Lavender Foal syndrome as some horses have produced foals with both conditions.
Guttural Pouch Tympany (GPT) affects young horses from birth to twelve months old. It is less common in colts than in fillies. The pharyngeal opening of the Eustachian tube is defective and acts like a one-way valve. When air enters and cannot escape, the guttural pouch distends forming a characteristic non-painful swelling. If the animal is severely affected, breathing will be noisy and swallowing may be difficult. Clinical signs and radiographic examination of the skull will confirm a diagnosis of guttural pouch tympany. Surgical procedures consist of providing a pathway for air to be expelled into the pharynx. Once treated, an affected animal can have a normal, useful life.