A newborn Arabian foal is possibly one of the most beguiling creatures on Earth. Elegant of feature and blessed with beauty, he’s full of promise for the future–and often descended of royal blood and worth many thousands of dollars. Imagine how crushing it is for a breeder to discover that a seemingly healthy foal has inexplicably died from what, for most young horses, would be a fairly routine and minor respiratory infection…or even from a wound with a bit of dirt in it. What went wrong?

In the case of the Arabian, there’s a good possibility that what went wrong was CID–combined immunodeficiency. First described in 1973 by Travis McGuire, DVM, and Marinel Poppy, DVM, of Washington State University, CID is a genetic disorder that results in a defective immune system–specifically, the inability of the horse to produce functional T- and B-lymphocytes (specific types of white blood cells). Without the defense against disease that these lymphocytes provide, a foal is completely vulnerable to infection from the millions upon millions of disease-causing organisms he encounters every day in the soil, water, and air.

A Foal With No Defenses

A CID foal appears perfectly normal at birth, and might remain symptom-free for weeks or months. The reason for this is that he absorbs a temporary passive immunity from his dam’s colostrum. But once this passive immunity has worn off, the foal has no chance of survival against the everyday onslaught of infectious agents in his environment. Respiratory disease is the most common killer, but practically any sort of infection eventually will be fatal–usually at an age ranging from 15 days to about four months (with three to four weeks being most common). Sometimes CID foals respond to antibiotic treatment at first, but eventually they succumb to an infection for which antibiotics are ineffective. Because CID foals are unable to launch a normal immune re