Equine common variable immunodeficiency: lessons from 100 clinical cases.

The clinical manifestation of recurrent fevers and infections alerts the clinician to the possibility of an underlying immunodeficiency. Common variable immunodeficiency (CVID) in the horse is a rare late-onset, non-familial immunologic disorder of B cell depletion and/or dysfunction with resultant inadequate antibody production. The most common clinical presentations in horses with CVID are recurrent upper and/or lower respiratory infections, meningitis and/or ataxia, cholangiohepatitis, infectious colitis, infectious dermatitis, and severe gastrointestinal parasitism. Immune-mediated and lymphoproliferative conditions are additional clinical features. The diagnosis of CVID in horses is based on persistent hypogammaglobulinemia primarily caused by a serum IgG concentration below 10.00 g/L in at least 2 different measurements, often accompanied by a serum IgM concentration below 0.50 g/L. Most horses with CVID show a persistent peripheral blood B cell distribution below 6% of total circulating lymphocytes, indicating severe B cell paucity or depletion, but the B cell distribution can be within the normal reference interval. Post-mortem findings add diagnostic information about the distribution of B and T cells in lymphoid tissues. Clinical management of horses with immunodeficiency is intense and expensive, and these factors weigh on the difficult decision of elective euthanasia. To date, no genetic mutation has been identified in horse patients with CVID, and the large number of breeds of single-affected individuals in a same herd or immediate lineage from various parts of the U.S. and the world do not point at an obvious inheritable mechanism of disease or environmental risk factors. This article describes the clinical and immunological findings in 100 cases, and comparisons with the disease in human patients.