Severe combined immunodeficiency

SCID prevents a foal from developing essential defenses against infection

SCID stands for severe combined immunodeficiency. Combined refers to a defect involving both B and T lymphocytes, two major groups of immune cells. Their development is severely impaired, leaving the foal unable to build the defenses needed against many different infections.

The equine form described here involves a small deletion, a missing piece of DNA, in the PRKDC gene. Missing DNA changes the instructions for a protein, producing a shortened version and stopping an enzyme called DNA-PK from working. An enzyme is a protein that helps chemical reactions take place in cells. This enzyme helps build receptors, the structures that immune cells use to recognize a wide range of infectious agents.

References [1] · [3] · [4]

A foal with SCID can look normal at birth

Colostrum, the mare’s first milk, provides antibodies from the mother. These proteins help defend the body against infection and may protect the foal temporarily. Those antibodies cannot repair the inherited defect. As their protection declines, the foal lacks the functioning immune defenses needed to take over.

Problems are often noticed between two and eight weeks of age. They can include fever, breathing difficulties, diarrhea or repeated infections. None of these signs identifies SCID on its own. Exposure to infections affects the course, so a normal-looking newborn cannot be assumed free of this condition.

References [1] · [2]

Equine SCID has mainly been studied in Arabians and Arabian crosses

This form is mainly documented in Arabians and Arabian crosses. An adult carrier may appear completely healthy and may already have produced foals without signs. Neither observation stops that horse from passing on the variant.

Breed membership alone establishes neither presence nor absence in a particular horse. Available findings depend on the populations sampled, their bloodlines and testing practices. When planning a mating, the parents’ own test reports provide more specific information than an assumption based on ancestry.

References [1] · [2]

Two unaffected carriers of the SCID variant can have a foal with SCID

SCID has autosomal recessive inheritance. The gene is not on a sex chromosome, and male and female foals can both be affected. A single copy of the SCID variant does not cause this form of the disease. A foal is affected if it inherits two copies of that variant, one from its dam and one from its sire.

If the mare and stallion each have one copy of the SCID variant, every conception has a 25% chance of an affected foal with two copies, a 50% chance of an unaffected carrier with one copy and a 25% chance of a foal without this variant. These are probabilities for every pregnancy. They do not guarantee one affected foal, two carriers and one non-carrier in four births.

References [2] · [1]

Infection in a foal can have causes other than SCID

A foal with infections does not automatically have SCID. Failure to receive enough protective antibodies through colostrum can also leave a foal poorly protected. That is a different problem from the inherited immune defect discussed here.

A veterinarian considers age, infection history, examination findings and blood tests alongside the DNA result. Abnormal lymphocyte or antibody findings can guide the investigation, while the DNA test establishes whether the foal carries the particular PRKDC variant responsible for this form of SCID. Caring for a sick foal should not be replaced by waiting for a DNA report.

References [1] · [2]

SCID DNA testing distinguishes unaffected carriers from foals with SCID

The DNA test looks for the deletion in PRKDC responsible for this form of SCID. Reports usually distinguish zero, one and two copies of this variant, often written N/N, N/SCID and SCID/SCID. N denotes a version of PRKDC without the tested variant; the gene itself is still present.

A negative result is not a complete assessment of immune function or a screen for every disease. The horse’s identity, submitted sample and test name must match the report being interpreted. Sample requirements should come from the laboratory carrying out the analysis.

References [2]

Treating infections does not restore immune defenses in a foal with SCID

This form of SCID is fatal. University sources describe affected foals as dying within their first months, before six months of age. Treating an infection does not enable B and T lymphocytes to develop normally, and there is no established routine cure for this form.

The veterinarian assesses infection, suffering and the realistic care options for the individual foal. Welfare decisions require that clinical context rather than an online list of signs. The severe outlook for affected foals should not be applied to adults that carry just one copy.

References [2] · [1]

Avoiding matings between two horses carrying the SCID variant

Testing both the mare and stallion allows breeders to avoid mating two SCID variant carriers. If one parent has one copy and a DNA test confirms that the other has none, their foal cannot inherit two copies and will not have this form of SCID. Each conception still has a one-in-two chance of a foal inheriting one copy and becoming an unaffected carrier.

A carrier result therefore does not settle every breeding decision on its own. Mate selection, keeping reliable reports and testing descendants belong within a wider approach to health and genetic diversity. A favorable SCID pairing does not establish that every other aspect of a proposed mating is suitable.

References [2] · [1]

Questions about the condition: Severe combined immunodeficiency

Scientific sources about the condition: Severe combined immunodeficiency

This article brings together the research and the university and laboratory resources listed below. Numbers after each section link to the references used. Sources consulted in November 2026.

1. 1.
   Severe Combined Immunodeficiency (SCID)

   Cornell University — College of Veterinary Medicine · University resource

2. 2.
   Severe Combined Immunodeficiency (SCID): DNA testing and interpretation

   UC Davis — Veterinary Genetics Laboratory · Test documentation

3. 3.
   Shin, Perryman & Meek (1997) — A kinase-negative mutation of DNA-PK(CS) in equine SCID results in defective coding and signal joint formation

   Journal of Immunology — University of Texas Southwestern research · Research

4. 4.
   Wiler et al. (1995) — Equine severe combined immunodeficiency: a defect in V(D)J recombination and DNA-dependent protein kinase activity

   Proceedings of the National Academy of Sciences · Research