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Basics ⬇

Outline

BASICS

Overview!!navigator!!

  • Inheritable lethal disease of foals characterized by complete absence of functional B and T lymphocytes and lack of adapted immune system
  • autosomal recessive trait; affected (homozygous) foals have no antigen-specific immune responses, heterozygous foals show no abnormalities
  • ~8–15% of Arabian horses carriers; 0.18–4% of Arabian foals affected; decreasing numbers owing to genetic testing for breeding

Signalment!!navigator!!

  • Arabian or Arabian-cross foals
  • Clinical signs develop by 1–2 months of age; most foals die by 5 months

Signs!!navigator!!

  • Affected foals physically normal at birth
  • When circulating, colostrum-derived antibodies drop below protective levels (1–2 months), SCID foals become susceptible to infectious agents
  • Clinical signs include cough, nasal discharge (pneumonia), recurrent pyrexia (bacteremia), diarrhea (enteritis), lameness (arthritis), colic (peritonitis), impaired growth, and weight loss
  • Equine adenovirus most common pathogen isolated from affected foals; Cryptosporidium parvum, Pneumocystis jirovecii, and bacterial pathogens also isolated

Causes and Risk Factors!!navigator!!

  • Mutation in gene encoding catalytic subunit of DNA-dependent protein kinase
  • Loss of functional enzyme results in failure of gene rearrangement for T- and B-cell receptor variable region development, thus absence of T and B lymphocytes
  • Risk factor is breeding of Arabian horses not tested for genetic mutation

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Total or partial FTPI; foals with FTPI have low serum IgG concentration on day 1 or 2 of life, normal absolute lymphocyte counts and lymphoid tissues; foals with SCID often have normal serum IgG values until 4–6 weeks, but low absolute lymphocytes counts and lymphoid tissues
  • Foals with transient hypogammaglobinemia of the young have low IgM and IgG values and normal or low normal lymphocyte counts with normal lymphoid tissues
  • Foals with selective IgM deficiency have persistently low serum IgM and normal lymphocyte counts and lymphoid tissues
  • Agammaglobulinemia not reported in Arabian foals and is characterized by lack of immunoglobulins, B cells, plasma cells, and germinal centers in lymphoid tissues; absolute lymphocyte count often normal owing to presence of T lymphocytes

CBC/Biochemistry/Urinalysis!!navigator!!

  • Profound, persistent lymphopenia (<1000 lymphocytes/µL) over 1–2 weeks
  • Total white blood cell and neutrophil counts can be low, normal, or high depending on infections
  • Anemia may develop later in clinical course of disease
  • Biochemistry and urinalysis usually normal or reflect organ system involved in infection

Other Laboratory Tests!!navigator!!

  • Quantitation of serum IgM; absent before colostrum consumption or after 3 weeks of age owing to catabolism of maternally derived antibodies
  • Phytohemagglutinin intradermal test and/or in vitro leukocyte stimulation test are negative owing to absence of cell-mediated immune responses
  • Definitive diagnosis based on positive genetic test for DNA-dependent protein kinase from blood or cheek swab (VetGen LLC, Ann Arbor, MI); test determines if foal is free, heterozygous (carrier), or homozygous (affected) for genetic mutation

Imaging!!navigator!!

Radiographs/ultrasonography may reveal sites of infection.

Pathologic Findings!!navigator!!

  • Small thymus with fatty appearance and hypoplastic lymph nodes
  • Severe lymphoid hypoplasia, lack of germinal centers, and periarteriolar lymphocytic sheaths

Treatment ⬆ ⬇

TREATMENT

  • Medical management unrewarding as death is inevitable
  • Therapy is supportive and aimed at infection control
  • SCID can be corrected by transplanting histocompatible stem cells from a genetically matched donor, but technique is rarely performed

Medications ⬆ ⬇

MEDICATIONS

Drug(s) of Choice

Antimicrobials for the treatment of infections.

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

Affected animals may be monitored for development of infection.

Prevention/Avoidance!!navigator!!

  • Avoid production of affected foals by genetic testing stallion and mare before breeding
  • Breeding 2 carriers (heterozygous) will result in 25% of offspring with SCID, 50% carriers, and 25% normal
  • Breeding a carrier with a normal horse will produce nonaffected foals but with 50% chance of being carrier
  • All offspring should be tested and heterozygous animals used for nonreproductive pursuits

Possible Complications!!navigator!!

Recurrent infections, septicemia, and organ dysfunction.

Expected Course and Prognosis!!navigator!!

Grave prognosis; foals die from acquired infections by 5 months.

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

Infections with opportunistic organisms.

Age-Related Factors!!navigator!!

Only observed in young animals.

Abbreviations!!navigator!!

  • FTPI = failure of transfer of passive immunity
  • Ig = immunoglobulin
  • SCID = severe combined immunodeficiency

Suggested Reading ⬆ ⬇

Felippe MJB. Immunodeficiencies. In: Felippe MJB, ed. Equine Clinical Immunology. Ames, IA: Wiley Blackwell, 2016:193–204.

Author(s) ⬆

Author: M. Julia B. Felippe

Consulting Editors: David Hodgson, Harold C. McKenzie, and Jennifer L. Hodgson

Acknowledgment: The author and editors acknowledge prior contribution of Jennifer L. Hodgson.