Equine Severe Combined Immune Deficiency

((Equine SCID)

Description

Severe combined immunodeficiency disease (SCID) of horses is an autosomal, recessive hereditary disease occurring in 2-3% of Arab or part-bred Arab foals. About 16-25% of Arabian horses are carriers of the disease and are phenotypically normal but pass on the defective gene to 50% of their offspring. Affected foals result from 25% of matings of two carrier horses. SCID is characterised by a complete absence of functional B and T lymphocytes and affected (homozygous) foals fail to produce antigen-specific immune responses. The disease results in the absence of a functional immune system and affected foals are unable to resist or recover from infections. The most common opportunistic infections in affected foals are pneumonia caused by viral (Adenovirus), bacterial (Rhodococcus equi), fungal (Pneumocystis carinii) or protozoal (Cryptosporidium) organisms.

Signalment

Only Arab or cross-bred Arabs foals are affected. No sex predilection has been reported.

Pathogenesis

SCID occurs due to a mutation in the allele encoding for DNA-dependent protein kinase (DNA-PK) that is involved in V(D)J lymphocyte recombination. V(D)J recombination is required for antigenic receptor expression on B and T lymphocytes. Without these receptors, differentiation of B and T lymphocytes does not occur and lymphoid tissue fails to develop. The result of this mutation is a complete is a severe immunodeficiency.

Clinical signs

Affected foals are clinically normal at birth and usually develop infections at around two to three months of age due to declining maternal antibodies in the colostrum. The most common clinical signs relate to infections of the respiratory tract such as nasal discharge, cough, dyspnoea or increased respiratory sounds. Other clinical signs may relect further sites of infection including intermittent fever, pneumonia, colic, weight loss and diarrhoea. Infection of the pancreas may result in loss of functional endocrine tissue leading to stunted growth and weight loss.

Diagnosis

Diagnosis may not be straightforward as the clinical signs may resemble those of many other infections occurring in foals. The antemortem diagnosis of SCID is usually based on three criteria; a) a persistent lymphopaenia (occurring over 1-2 weeks) with less than 1000 lymphocytes per ml, b) a lack of serum IgM in foals over four weeks of age and c) lymphoid hypoplasia. Affected foals may also develop anaemia late in the course of the disease.

The intradermal phytohemagglutinin (PHA) test may be used as a test for immunocompetence and assesses T lymphocyte function. It can be performed in foals of all ages as the test is not affected by maternal antibodies. Intradermal PHA causes a delayed hypersensitivity reaction, resulting in a skin swelling. A foal suffering from SCID fails to respond to intradermal PHA.

In order to obtain a definitive diagnosis, blood or cheek swabs may be submitted for PCR to identify the mutant allele of the DNA-PK gene The test also identifies carriers of the disease which is important for screening prior to breeding. Additionally, post mortem findings of a small thymus and/or absent lymph nodes combined with the presence of opportunistic infections support a diagnosis of SCID. Histologically, lymph node follicles and germinal centres are absent with severe cellular hypoplasia of the thymus and lymph nodes.

Treatment

Medical treatment of foals with SCID is generally unrewarding. Treatment is supportive, consisting of antiobiotics to treat secondary infections.

Prognosis

The prognosis for affected foals is grave even with intensive treatment and most foals die within five months of birth.

Prevention

SCID may be prevented in foals by DNA testing of the mare and stallion and only breeding non-carrier animals.

References

• Lavoie, J. P., Hinchcliff, K. W. (2009) Blackwell's Five-Minute Veterinary Consult: Equine John Wiley and Sons
• McClure, J. T. (1997) New Developments in Severe Combined Immunodeficiency Disease Proceedings of the Annual Convention of the AAEP (43) pp61-64
• Swinburne, J., Lockhart, L., Scott, M., Binns, M. M. (1999) Estimation of the prevalence of Severe Combined Immunodeficiency Disease in UK Arab horses as determined by a DNA-based test The Veterinary Record 145 22-23