Combined immunodeficiency

From Horse

Severe combined immunodeficiency disease (SCID) of horses is an autosomal, recessive hereditary disease occurring at about 0.2% of Arabian or crossbred Arabian horses[1][2]. The disease is characterized by profound immune incompetence where affected individuals are incapable of generating antigen-specific immune responses[3].

First reported in Arabian foals in 1973[4], and since reported in the USA, Canada, Great Britain and Australia[5], SCID is a genetic defect involving a 5-base pair deletion in the gene encoding the catalytic subunit of the DNA dependant protein kinase (DNA-PKcs)[6].

The disease is invariably fatal in homozygous offspring, with death occurring within the first twelve months of life. Homozygous carriers are often asymptomatic but have a higher incidence of neoplasia such as virally-induced sarcoids[7].

Affected foals often present with variable symptoms such as acute diarrhea, apathy, failure to thrive and pneumonia.

This disease should be differentiated from secondary immunodeficiencies which may present at any age due to failure of passive transfer in neonatal foals[8][9].

Diagnosis is based on clinical symptoms and hematological evidence of anemia and lymphopenia. A definitive diagnosis requires DNA testing.

There is no specific treatment for this condition, and affected foals invariably succumb to bacterial infections.

However, protective effects have been reports from hyperimmune serum transfusions from horses infected with Theileria spp[10].


  1. Bernoco D & Bailey E (1998) Frequency of the SCID gene among Arabian horses in the USA. Anim Genet 29(1):41-42
  2. Larson J et al (2011) Severe combined immunodeficiency in a Caspian filly. J Vet Intern Med 25(4):954-958
  3. Perryman LE (2004) Molecular pathology of severe combined immunodeficiency in mice, horses, and dogs. Vet Pathol 41(2):95-100
  4. McGuire TC & Poppie MJ (1973) Hypogammaglobulinemia and thymic hypoplasia in horses: A primary combined immunodeficiency disorder. Infection and Immunity 8:272-277
  5. Don-van't Slot HP & van der Kolk JH (2000) Severe combined immunodefiency disease (SCID) in the Arabian horse. Tijdschr Diergeneeskd 125(19):577-581
  6. Piro M et al (2008) Frequency of the severe combined immunodeficiency disease gene among horses in Morocco. Equine Vet J 40(6):590-591
  7. Ding Q et al (2002) DNA-PKcs mutations in dogs and horses: allele frequency and association with neoplasia. Gene 283(1-2):263-269
  8. Crisman MV & Scarratt WK (2008) Immunodeficiency disorders in horses. Vet Clin North Am Equine Pract 24(2):299-310
  9. Jelìnek F et al (2006) Severe combined immunodeficiency in a Fell pony foal. J Vet Med A Physiol Pathol Clin Med 53(2):69-73
  10. Mealey RH et al (2012) Protective effects of passively transferred merozoite-specific antibodies against Theileria equi in horses with severe combined immunodeficiency. Clin Vaccine Immunol 19(1):100-104