Severe combined immunodeficiency (SCID) is a lethal disease of the Arabian foal characterised by failure of B- and T-lymphocyte development. Affected foals are born apparently healthy thanks to passive immunity from colostrum, but from the first weeks they suffer severe recurrent infections (diarrhoea, pneumonia) that do not respond to treatment and die within the first months. The DNA test identifies carriers, which are clinically normal, and prevents risky matings.

Restricted to the Arabian horse and its crosses. Tarr et al. (2014) described in South African Arabians a carrier prevalence that fell from 6.4% (2004/5 season) to 3.4% (2009/10) after the introduction of genetic testing. Systematic screening keeps the frequency at low levels today.

- Severe recurrent infections from 2-8 weeks of age
- Persistent diarrhoea and progressive pneumonia (adenovirus and Pneumocystis common)
- Fever, weakness and failure to grow despite treatment
- Marked lymphopenia and absence of endogenous immunoglobulin M (IgM)
- Inevitable death within the first months despite intensive support

Arabian foal SCID was described in the early 1970s in the United States as a primary immunodeficiency with lymphoid hypoplasia. In the 1990s it was shown that the defect lies in DNA repair and V(D)J recombination, essential for building lymphocyte receptors, and the responsible gene was identified: the catalytic subunit of DNA-PK (PRKDC gene). The mutation, a frameshift deletion, spread through breeding lines worldwide from the American population. The carrier test, available since the late 1990s, has markedly reduced cases in the breed.

- Test Arabian breeding animals, prioritising lines with a history of foals with recurrent infections
- Never mate carrier with carrier: 25% risk of SCID foals
- A carrier may be mated with a clear animal, keeping only clear offspring for breeding if you seek to eliminate the allele
- Death from infections in a young foal does not prove SCID: confirm it with the test before labelling the parents
- Record the results with the breeders' association to keep the carrier map up to date

The differential diagnosis of the foal with recurrent infections includes failure of passive transfer of immunity (insufficient colostrum), ruled out by measuring serum IgG in the first 24 hours, and other primary immunodeficiencies. SCID is confirmed by a blood count (marked lymphopenia), absence of IgM, lymphoid hypoplasia at necropsy and, above all, genetic testing of the foal and its parents. There is no practical treatment; bone marrow transplantation has been only experimental. Finding a case makes it necessary to study the entire line.

1. McGuire TC, Poppie MJ (1973). Hypogammaglobulinemia and thymic hypoplasia in horses: a primary combined immunodeficiency disorder. Infect Immun 8:272-277. PMID: 4199158
2. Wiler R et al. (1995). Equine severe combined immunodeficiency: a defect in V(D)J recombination and DNA-dependent protein kinase activity. Proc Natl Acad Sci U S A 92:11485-11489. PMID: 8524788
3. Shin EK et al. (1997). A kinase-negative mutation of DNA-PK(CS) in equine SCID results in defective coding and signal joint formation. J Immunol 158:3565-3569. PMID: 9103416
4. Tarr CJ et al. (2014). The carrier prevalence of severe combined immunodeficiency, lavender foal syndrome and cerebellar ataxia in Arabian horses in South Africa. Equine Vet J 46:512-514. PMID: 24033554
5. OMIA:000220-9796. Severe combined immunodeficiency disease, autosomal, PRKDC-related in Equus caballus (domestic horse).