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==Description==

==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.  

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.  

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.

 

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==

==Signalment==

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

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

==Pathogenesis==

==Pathogenesis==

==Diagnosis==

==Diagnosis==

Diagnosis may not be straigtforward as the clinical signs may resemble those of a range of infections occurring in foals. The antemortem diagnosis of SCID is usually based on three criteria; a) a persistent lymphopaenia with less than 1000 lymphocytes per ml, b) a lack of serum IgM in foals over four weeks of age and c) lymphoid hypoplasia. Blood or cheek swabs may be submitted for PCR to identify the mutant allele of the DNA-PK gene in order to obtain definitive diagnosis. 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.

Diagnosis may not be straigtforward 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 with less than 1000 lymphocytes per ml, b) a lack of serum IgM in foals over four weeks of age and c) lymphoid hypoplasia. Blood or cheek swabs may be submitted for PCR to identify the mutant allele of the DNA-PK gene in order to obtain definitive diagnosis. 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==

==Treatment==

Medical treatment of foals with SCID is generally unrewarding as most affected foals die by the age of five months. Treatment is supportive, consisting of antiobiotics to treat secondary infections.  

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

==Prognosis==

==Prognosis==

==Prevention==

==Prevention==

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

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

==References==

==References==

*McClure, J. T. (1997) '''New Developments in Severe Combined Immunodeficiency Disease''' ''Proceedings of the Annual Convention of the AAEP (43) pp61-64''

*McClure, J. T. (1997) '''New Developments in Severe Combined Immunodeficiency Disease''' ''Proceedings of the Annual Convention of the AAEP (43) pp61-64''