Equine Immune Deficiency Syndromes – NAVLE Study Guide

Overview and Clinical Importance

Immune deficiency syndromes in horses represent a group of disorders characterized by dysfunction or absence of critical components of the immune system. These conditions result in increased susceptibility to infections and are important differentials in foals and young horses presenting with recurrent or persistent infections that fail to respond to conventional therapy. Understanding these syndromes is essential for the NAVLE, as they represent both diagnostic challenges and opportunities for genetic counseling in breeding programs.

Primary immunodeficiencies are congenital disorders where immune system components fail to develop properly, while secondary immunodeficiencies are acquired conditions resulting from external factors such as failure of passive transfer (FPT). This guide focuses on primary immune deficiency syndromes, which often have genetic bases and breed predispositions crucial for NAVLE examination questions.

Classification of Equine Immunodeficiency Disorders

Severe Combined Immunodeficiency (SCID)

Pathophysiology

SCID is an autosomal recessive inherited condition affecting Arabian and part-Arabian horses. The disease results from a 5-base pair deletion in the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) gene. This enzyme is essential for V(D)J recombination, the process required for generating diverse antigen receptors on B and T lymphocytes.

Without functional DNA-PK, lymphocytes cannot complete normal development. Both B and T cell lineages are affected, resulting in complete absence of adaptive immunity. The foal relies entirely on passively acquired maternal antibodies from colostrum, which decline over the first 2-3 months of life.

Epidemiology and Genetics

• Carrier frequency: Approximately 8-28% of Arabian horses are carriers
• Affected foal frequency: 2-3% of Arabian foals
• Inheritance: Autosomal recessive (both sexes equally affected)
• Carrier x Carrier mating: 25% chance of affected foal

Clinical Signs

Affected foals appear clinically normal at birth and during the first few weeks of life while protected by maternal antibodies. Clinical signs typically develop between 2-8 weeks of age as passive immunity wanes.

• Recurrent respiratory infections (most common presentation)
• Nasal discharge, cough, dyspnea
• Intermittent or persistent fever
• Diarrhea
• Poor growth, weight loss
• Failure to respond to antimicrobial therapy

Common Opportunistic Pathogens in SCID Foals

Diagnosis

Laboratory Findings

Definitive Diagnosis

DNA testing (PCR) is the gold standard for diagnosis of SCID and identification of carriers. The test detects the 5-bp deletion in the DNA-PKcs gene and can be performed on whole blood, hair follicles, or cheek swabs.

• N/N: Normal (not a carrier)
• N/SCID: Carrier (phenotypically normal, can transmit mutation)
• SCID/SCID: Affected (will develop fatal immunodeficiency)

Intradermal PHA Test

The intradermal phytohemagglutinin (PHA) test assesses T cell function. Normal foals develop a delayed hypersensitivity skin reaction (swelling) within 24-48 hours. SCID foals fail to respond due to absence of functional T cells. This test is not affected by maternal antibodies.

Treatment and Prognosis

Prognosis: GRAVE. There is no effective treatment for SCID. Supportive care and antimicrobial therapy may temporarily control infections, but affected foals invariably die or are euthanized by 5-6 months of age. One experimental foal survived to 5 years following bone marrow transplantation, but this is not a practical treatment option.

Prevention: Genetic testing of breeding stock is the key to prevention. The Arabian Horse Association (AHA) Code of Ethics requires disclosure of known SCID carrier status for horses offered for breeding, sale, or lease. Carrier x carrier matings should be avoided.

Foal Immunodeficiency Syndrome (FIS)

Pathophysiology

Foal Immunodeficiency Syndrome (FIS), formerly known as Fell Pony Syndrome, is a fatal autosomal recessive disorder affecting Fell Ponies, Dales Ponies, and Gypsy Cob horses. The disease is caused by a single nucleotide polymorphism (SNP) in the SLC5A3 gene (sodium/myo-inositol cotransporter), resulting in a missense mutation (c.1337G>T, p.P446L).

Unlike SCID, FIS affects both the immune system AND erythropoiesis. Affected foals develop progressive B cell lymphopenia and severe non-regenerative anemia. The exact mechanism by which the SLC5A3 mutation causes these defects is not fully understood but is hypothesized to involve impaired cellular response to osmotic stress during lymphocyte and erythroid development.

Epidemiology

• Carrier frequency: 39% of Fell Ponies, 18% of Dales Ponies, 9% of Gypsy horses
• Affected foal frequency: Approximately 10% of Fell Pony foals
• Mortality: 100% - all affected foals die or are euthanized by 3-4 months of age

Clinical Signs

Affected foals appear normal at birth. Clinical signs typically develop at 2-8 weeks of age (usually 3-6 weeks) as maternal antibodies wane and anemia progresses.

• Progressive anemia: Pale mucous membranes, weakness, lethargy
• Recurrent infections (diarrhea, pneumonia)
• Failure to thrive, poor growth
• Dry, dull coat
• Reduced appetite, failure to suckle
• Infections resistant to treatment

Diagnosis

Treatment and Prevention

Treatment: There is no effective treatment. Foals may initially respond to supportive care and antimicrobials, but infections recur and anemia progresses. Euthanasia is recommended to prevent suffering.

Prevention: DNA testing has been available since 2011 and has significantly reduced the incidence of affected foals. Carrier x carrier matings should be avoided. However, due to the small population size and limited genetic diversity of Fell and Dales Ponies, carrier x clear matings are still recommended to preserve the breed gene pool.

Selective IgM Deficiency

Overview

Selective IgM deficiency is characterized by persistently low serum IgM concentrations (less than 25 mg/dL) with normal IgG and IgA levels. It has been reported in several breeds but is most common in Arabians and Quarter Horses. The underlying cause is unknown but suspected to be genetic.

Clinical Syndromes

Three clinical presentations have been described:

• Severe neonatal form: Foals present at 2-8 months with life-threatening pneumonia or septicemia; most die before 10 months
• Chronic form: Stunted growth, recurrent infections that temporarily respond to antimicrobials; death by 1-2 years
• Adult form: May be associated with lymphoma, stress, or immunosuppressive therapy; clinical significance unclear if IgG is normal

Diagnosis

• Persistently low serum IgM (less than 25 mg/dL) on multiple measurements
• Normal serum IgG and IgA concentrations
• Normal lymphocyte counts (unlike SCID)
• Must rule out SCID, FPT, and transient hypogammaglobulinemia

Treatment and Prognosis

There is no specific treatment. Prolonged antimicrobial therapy and supportive care may extend life. Rare cases of spontaneous recovery with sudden increases in IgM levels have been reported. Prognosis is generally poor for foals presenting with severe disease. Rebreeding the dam or sire of an affected foal is not recommended.

Agammaglobulinemia

Agammaglobulinemia is a rare primary immunodeficiency characterized by complete absence of B lymphocytes and inability to produce immunoglobulins. All reported cases (n=5) have been in male horses (Thoroughbred, Standardbred, Quarter Horse), suggesting possible X-linked inheritance similar to Bruton's agammaglobulinemia in humans.

Key Features

• Absent or severely reduced serum IgM, IgG, and IgA
• Absent B lymphocytes in peripheral blood and lymphoid tissues
• Normal T cell numbers and function
• Absence of plasma cells, germinal centers in lymph nodes
• Infections develop at 2-32 weeks as maternal antibodies wane
• All reported foals died by 19 months despite plasma transfusions

Transient Hypogammaglobulinemia

Transient hypogammaglobulinemia results from a temporary delay in endogenous immunoglobulin production by the foal. It is rare (only 2-3 reported cases) but important because, unlike other primary immunodeficiencies, foals can recover.

Clinical Features

• Presents at 2-5 months of age when colostral antibodies decline
• Recurrent bacterial infections (respiratory most common)
• Low serum IgG (below protective levels)
• Normal T and B cell numbers and function
• Serum IgG increases over time (diagnostic confirmation)
• Treatment: Prolonged antibiotics; plasma transfusion if severe

Common Variable Immunodeficiency (CVID)

CVID is unique among equine immunodeficiencies because it is a late-onset, non-familial disorder affecting adult horses. It is characterized by progressive B cell depletion, hypogammaglobulinemia, and recurrent bacterial infections.

Epidemiology

• Age: Adult horses (average 10.7 years, range 2-19 years)
• Sex: Both males and females affected equally
• Breeds: Multiple breeds (Thoroughbred, Quarter Horse, Warmblood, Pony)
• Inheritance: No genetic mutation identified; non-familial pattern

Clinical Signs

• Recurrent fevers
• Recurrent upper and/or lower respiratory infections
• Bacterial meningitis, ataxia
• Cholangiohepatitis
• Infectious colitis, diarrhea
• Severe gastrointestinal parasitism
• Immune-mediated conditions (in some cases)

Diagnosis

• Serum IgG: Less than 1000 mg/dL (often less than 200-800 mg/dL)
• Serum IgM: Less than 25 mg/dL
• B cell distribution: Less than 6% of peripheral blood lymphocytes (severe B cell depletion)
• Response to vaccination: Failure to produce antibodies to tetanus toxoid
• Necropsy: Absence of B cells in bone marrow, spleen, lymph nodes

Treatment and Prognosis

Prognosis: GRAVE. Management is intensive and expensive. Prolonged parenteral antibiotic therapy can temporarily control infections. Periodic plasma transfusions to maintain serum IgG above 500 mg/dL are impractical for adult horses. Immunosuppressive drugs (corticosteroids) are contraindicated. Most horses are euthanized due to severe pneumonia, meningitis, or septicemia. Some horses may be managed for 1-5 years with intensive care.

Differential Diagnosis Summary