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==Description==

==Introduction==

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. The most common opportunistic infections in affected foals are pneumonia caused by viral (Adenovirus), bacterial (Rhodococcus equi), fungal (Pneumocystis carinii) or protozoal (Cryptosporidium) organisms.

 

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 ([[Equine Adenovirus|Adenovirus]]), bacterial ([[Rhodococcus equi|''Rhodococcus equi'']]), fungal ([[Pneumocystis carinii|''Pneumocystis carinii'']] or protozoal (''[[Cryptosporidium]]'') organisms.

==Signalment==

==Signalment==

==Clinical signs==

==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.  

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 reflect 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==

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.  

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 - Introduction|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.  

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 [[Type IV Hypersensitivity|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.

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==

==Treatment==

==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==

[[Category:Primary Adaptive Immunity Deficiencies]]

[[Category:Primary Adaptive Immunity Deficiencies]]

[[Category:To Do - SophieIgnarski]]

[[Category:To Do - SophieIgnarski]]

[[Category:To Do - Review]]

[[Category:Expert Review]]

[[Category:Horse]]

[[Category:Horse]]