Immunodeficiencies

Several specific immunodeficiency syndromes occur in the foal and may be manifest in the neonatal period or later in life.

I. Clinical Signs of an Immunodeficiency [1,2]

1. Onset of infections early in life.
2. Repeat infections that are poorly responsive to therapy.
3. Infections caused by commensal organisms or organisms of low pathogenicity.
4. Disease resulting from use of attenuated live vaccines.
5. Failure to respond to vaccination.

II. Severe Combined Immunodeficiency of Arabians [3-5]

1. Autosomal recessive (Chromosome 9) requiring carrier to carrier breeding. Primarily Arabian, but Appaloosa and crosses have been reported.
2. Illness from 2 days to 4-1/2 months of age but usually when the colostral IgG begin dropping.

1. Adenovirus is a significant pathogen in SCID foals
2. Other pathogens associated with SCID are Pneumocystis carinii, Cryptosporidium parvum, R. equi, Influenza virus and E. coli.
3. Affected foals rarely survive beyond 5 months of age.

3. Presuckle SCID screening test.

1. Serum IgM: absent in SCID foals, low level of IgM is found in normal foals.
2. Lymphopenia <1000/ul.

4. Diagnosis requires multiple factors

1. Lymphopenia <1000/ul (usually less than 500/ul).
2. Decreased levels of immunoglobulins (IgM and IgG) after 6-8 weeks of age and declining with age.
3. Lack of lymphocyte response to mitogens.
4. Lymphoid hypoplasia, thymus, lymph nodes, spleen on post mortem.
5. γ-interferon deficiency
6. Genetic testing is also available: https://vgl.ucdavis.edu/test/scid

5. Treatment and prevention

1. No practical treatment
2. Genetic testing of the breeding animals

III. Selective Igm Deficiency [4,5]

1. Most common in Arabians and Quarter horses.
2. Increased susceptibility to infection – 2 clinical syndromes.

1. Foals or weanlings less than 1 year of age. Typical signs of immunodeficiency present (persistent or recurring infections) which responds to antimicrobials but recurs.
2. Older horses (2-5 years of age) which have long history of recurring infections and some of these horses also have evidence of lymphosarcoma.

3. Diagnosis after colostral antibody declines.
4. Serum IgM more than twice the standard deviation lower than age-matched controls with normal or elevated levels of IgG, IgA.
5. Peripheral lymphocyte count is normal.
6. Adult horses with selective IgM deficiency should have further investigation to assess if lymphoproliferative neoplasia is present.

IV. Agammaglobulinemia

1. Described in Standardbred, Thoroughbred and Quarter horses.
2. Rare; inability to produce B lymphocytes, seen only in males.
3. Recurrent infections in first year of life.
4. Diagnosis - Older foal (>3 months) with normal lymphocyte count but no detectable B-lymphocytes, no IgA or IgM, and very low or absent IgG.
5. Treatment: Associated infections usually respond well to antibiotics and plasma, although treatment is not recommended since the disease is hereditary.

V. Transient Hypogammaglobulinemia [4,5]

1. Normal fetus starts to produce IgM at 190 days gestational age. Delayed onset of immunoglobulin synthesis (up to 3 months of age). Has been reported in Arabians and Thoroughbreds.
2. Genetic basis is suspected, but unproven yet.
3. Clinical signs start when maternal antibodies begin to wane.
4. Diagnosis is made with Gamma-globulin quantification. Band T-cell numbers are normal.
5. Essential to differentiate form agammaglobulinemia, since the prognosis is excellent if they receive appropriate supportive care.

VI. Fell Pony Syndrome (Anemia, Immunodeficiency and Peripheral Ganglionopathy) [3,4,6]

1. Recently described in Fell-ponies. No sex predilection. Affected foals are normal at birth. This is a congenital fatal disease. Inheritance is suspected but has not been proven yet. Exact etiology is unknown.
2. Clinical signs at occur at 2-4 weeks of age. Foals become weak and lethargic, and start losing weight. Diarrhea, coughing, hypersalivation and chewing motions are common.
3. Presumptive diagnosis can be made on the presence of clinical signs and on significant reduction of immunoglobulin levels. Affected foals are anemic that is unrelated to hemolysis or blood loss. Opportunistic infections include bronchopneumonia and cryptosporidial enteritis.
4. Other immunodeficiencies should be considered and ruled out. Anemia has not been reported in any other primary immunodeficiencies. Secondary immunodeficiencies manifest earlier in life.
5. Foals fail to respond to therapy with recurrence of clinical signs. Progressive anemia always present. Affected foals die before 3 months of age.
6. Postmortem lesions show bone marrow hypoplasia, thymus hypoplasia, and lymphoid depletion of the lymphoid tissues, bronchopneumonia, enteritis and glossal hyperkeratosis. Peripheral ganglionopathy has been reported in some cases.

VIII. Secondary Immunodeficiencies

1. Failure of Passive Transfer (See Plasma Therapy)
2. Congenital Equine Herpes Virus Infection
   It causes lymphoid depletion in the thymus and spleen. Foals that are born alive may be premature and have severe lymphopenia (< 1000/ul) and neutropenia.