Combined Immunodeficiency Disease (CID) in Horses

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.


Something that relieves a condition’s symptoms without curing the condition


The name for the species of horses, donkeys, mules


Has the potential to cause harm to something

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