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SCID in Foals

Severe combined immunodeficiency (SCID) is an autosomal (not linked to the chromosomes for sex) recessive genetic disease affecting Arabian foals. These foals cannot produce B and T lymphocytes, which are special types of white blood cells that are vital for a healthy immune system. Without B and T lymphocytes, the immune system cannot fight antigens properly.

SCID-affected foals seem normal at birth, but after the first few months of life, they begin to suffer from a variety of infections. This time period coincides with the loss of the protective maternal antibodies they ingested from their mother’s milk at birth. 

One of the most common causes of infection in SCID foals is adenovirus, which causes a severe bronchopneumonia.  Other types of infections can involve bacteria, fungal, and protozoal infections. Testing for the SCID gene is now commercially available.

Symptoms and Types

Symptoms will vary according to the type of infection the foal succumbs to, including:

• Pneumonia: nasal discharge, coughing, trouble breathing (dyspnea)
• Diarrhea
• Fever
• Weight loss
• Stunted growth
• Persistently low lymphocytes on blood work

Causes

This genetic defect results in a deletion of a gene that prevents production of an enzyme necessary for the maturation of B and T lymphocytes (a type of white blood cell), which are needed to fight off infections. 

Diagnosis

In the past, testing was done by observing for clinical signs and checking complete blood count results for a consistent dearth of lymphocytes. To support a diagnosis, a lab-run radial immunodiffusion test was performed. If it showed no serum IgM (a type of antibody) in presuckling blood, then the foal was diagnosed with SCID.

Currently, a genetic test for SCID is available. This commercial test requires a sample of whole blood or cheek swabs and is dependent on polymerase chain reaction (PCR) techniques to amplify the foal’s DNA present in the sample. This test can also reveal carriers of the gene so as to allow horse owners to prevent two carriers from breeding, which increase the chances of producing an SCID foal.