Acquired B lymphocyte deficiency and chronic enterocolitis in a 3-year-old quarter horse.

This case report describes a 3-year-old American Quarter Horse with acquired immunodeficiency. Clinical signs included chronic diarrhea due to Salmonella typhimurium and bacterial pneumonia. Characterization of the immunodeficiency involved in vivo phytohemagglutinin (PHA) intradermal testing, in vitro lymphocyte proliferation in response to concanavalin A, immunofluorescence flow cytometry data on blood lymphocytes, serum protein electrophoresis and immunoglobulin (Ig) quantification. A diagnosis of B lymphocyte deficiency with resulting deficiencies in serum IgG, IgA and IgM and a concurrent decrease in T cell function was made based on these tests. Postmortem examination revealed no evidence of lymphosarcoma. This case represents a variation of young adult-onset B cell deficiency not previously described in the literature.