Combined immunodeficiency of horses: a review

A syndrome of anaemia, immunodeficiency and peripheral ganglionopathy in Fell pony foals

Fell pony foals developed a syndrome of anaemia, immunodeficiency and peripheral ganglionopathy. They became ill in the second or third week, and died in the second or third month of life. Clinical and pathological investigations revealed severe anaemia associated with small numbers of late erythroid precursors in bone marrow, small thymi, an absence of secondary lymphoid follicles, a lack of plasma cells and neuronal chromatolysis involving trigeminal, cranial mesenteric and dorsal root ganglia. Some of the foals had cryptosporidial enteritis and adenoviral bronchopneumonia and pancreatitis. The clinical and pathological findings were compatible with an intrinsic defect.

Immunodiagnostic assays

The immune system is a complex interactive network. Defects in its function can be characterized broadly as being the result of actual deficiencies in the network or misdirection of normal immunologic functions. The assays that are available to detect deficiencies in the immunologic network barely scrape the surface of the possibilities. These assays primarily evaluate humoral immune function, but undetected defects in innate and cellular immunity are sure to exist. Although assays of humoral immunity have allowed the characterization of a number of immunodeficiency syndromes in horses, closer evaluation of the equine immune system using newer assays described in this text, as well as future assays yet to be developed are sure to determine new syndromes. Assays of misdirected immunologic functions have been limited to detection of misdirected antibody responses, but the dependence of antibody production on help from T cells could reflect an underlying defect of cellular immunity. Similar to immunodeficiency syndromes, misdirected responses of the innate and adaptive arms of immunity are sure to occur but will only be detected by more diligent surveillance of diseased horses and application of new immunodiagnostic technologies.

X-linked severe combined immunodeficiency in the dog

This study documents the occurrence of a form of X-linked severe combined immunodeficiency (SCID) in the dog with clinical, immunologic, and pathologic features similar to those of X-linked SCID with B cells in man. The disease in the dog is characterized by growth retardation and increased susceptibility to bacterial and viral infections in young pups. Affected pups have all died or were euthanatized by 5 months with signs of canine distemper, infectious hepatitis, or bacterial pneumonia. Laboratory findings include normal numbers of circulating B lymphocytes and normal concentrations of serum IgM, but low to absent concentrations of serum IgG and IgA, indicating a defect in the terminal differentiation of IgG and IgA B cells into immunoglobulin-secreting plasma cells. This is supported by the failure of peripheral lymphocytes to produce IgG or IgA plaque-forming cells in response to polyclonal activation. There are low-to-normal numbers of circulating T cells, but a severely depressed blastogenic response to T cell mitogens. Postmortem findings include thymic dysplasia and hypoplasia of lymphoid tissue. Family studies and breeding experiments are consistent with an X-linked recessive mode of inheritance.

Attempted reconstitution of a foal with primary severe combined immunodeficiency

A foal with primary severe combined immunodeficiency, diagnosed within the first two weeks of life, was maintained with its dam in semi-isolation. The foal received continuous prophylactic antibiotic therapy, plasma from a sibling hyperimmunised with equine adenovirus vaccine, and intensive general nursing care. A full sibling female was selected as a bone marrow donor on the basis of red blood cell cross-matching and mixed lymphocyte reactions. Cyclophosphamide was given before two bone marrow transfusions at 35 and 73 days of age. To prevent graft versus host disease graft versus host disease the foal was maintained on methotrexate therapy. Reconstitution was not achieved nor were there signs of graft versus host disease. The foal died suddenly four days after the second bone marrow transfer when 77 days old. It had remained clinically free of any life threatening infectious disease and at necropsy a remarkable degree of freedom from infectious disease was confirmed. The most notable necropsy findings were bilateral nephrosis and myocardial degeneration and fibrosis. The likely cause of death was an electrolyte imbalance, particularly hypokalaemia, which secondarily affected the myocardium. Renal toxicity caused by the cytotoxic drugs, especially cyclophosphamide, may have contributed to the electrolyte imbalance.

An evaluation of contribution derived from investigations of equine immunodeficiencies

Following the descriptions of immunodeficiencies in horses beginning in 1973, there has been considerable effort to develop methods for differential diagnosis and to determine the cause and prevalence of the disorders. In addition, the equine immunodeficiencies, especially combined immunodeficiency, have been studied from a comparative viewpoint with the goal of finding information applicable to similar diseases of children. Coincident with the development of knowledge about the immunodeficiencies per se, considerable information about several aspects of immunology has been obtained. It is the purpose of this review to focus on findings from experiments with equine immunodeficiencies concerning prenatal and neonatal immunology, lymphocyte function, secretory immunity, immunoreconstitution, graft-versus-host reactions and previously unrecognized diseases.