Combined Immunodeficiency Disease (CID) in Horses

By PetMD Editorial on Nov. 2, 2011

Combined immunodeficiency disease, or equine CID, as it is commonly called, is a deficiency of the immune system, a known genetic disorder that is found in young Arabian foals. It may also be found in horses that have been crossbred with Arabians.

In most cases, foals that are born with this genetic disorder appear and behave normally at birth. Their immune systems function normally for about six to eight weeks, but around the second month of life, the symptoms of CID begin to become apparent. The horse may begin to develop illnesses that are not curable through normal treatment methods.

CID is almost always fatal. Although the affliction in itself does not kill, the inability of the immune system to fight infections -- infections that would normally be trivial to a healthy foal -- has a deleterious effect on its health, causing its health condition to spiral downward.

Equine adenovirus and related respiratory infections are the most common cause of death in Arabian foals with CID.


Foals are often normal at birth and then, around the age of two months, it contracts seemingly incurable respiratory illnesses. Also, other illnesses that would normally be easily treated are persistent, leading to suspicion of weak immune system


  • Genetic disorder
  • Improper development of immune system
  • Lack of antibodies provided during nursing
  • Inability to combat normal foal-related infections


In most cases of CID the young horse is first diagnosed with a respiratory condition.  When the condition proves incurable through normal methods, CID may be looked into.

If your horse is either descended from the Arabian breed, or is known to have mixed blood from an Arabian, your veterinarian may advise you to have your foal's DNA tested for the genetic strain for CID. There may also be other ways for your veterinarian to determine if the cause of your horse’s persistent illness is related to equine CID.


Unfortunately, there are no effective methods for treating equine CID. The infections and illnesses that occur as the result of this condition can be treated, at least in the beginning, but if the horse's immune system is not building antibodies to fight further infections, your horse will continue to be sick until finally its body can no longer withstand the onslaught of infections.

If you do choose to have your horse treated, the medical choices will be mainly of a palliative nature, given to make your horse as comfortable as possible while it lives. Infections may be treated by antibiotic, and other illnesses will be treated accordingly, and if possible, analgesics may be given to relieve symptoms of pain. However, as the disease progresses, secondary conditions usually will not respond to treatment as the immune system will rapidly go downhill.

Living and Management

Horses that suffer from combined immunodeficiency disease do not often live to adulthood. Those that do live any length of time will have difficult, sickly lives until they die.


There is no vaccine available for CID, and no way to treat or prevent it. For this reason, if you have an Arabian horse or an Arab crossbreeds, it is recommended that you look into having your horse tested for the genetic strain that is associated with equine CID to either confirm or rule out that your horse is a carrier of the CID gene.

Those horses that are shown to be carrying the gene should be sterilized to prevent them from reproducing and passing the gene along, as it can be inherited from horses that carry it as a recessive gene, but which never themselves show symptoms of a disorder. This is the only way to control CID.

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