An immunodeficiency in Fell ponies: a preliminary study into cellular responses

A putative immunodeficiency, causing mortality in UK Fell pony foals (Fell pony syndrome), was studied in affected foals and compared with healthy, ag e-matched foals. Differential cell counts of peripheral blood indicated tha t the syndrome foals were lymphopenic (P <0.05). Flow cytometric analysis o f circulating leucocytes showed a reduced MHC II expression (P <0.01) on ly mphocytes but not on polymorphonuclear cells in affected foals. There were no changes in the percentages of CD4(+) or CD8(+) T cells. There was an inc reased (P <0.05) expression of CD11a/18 by the lymphocytes of the syndrome foals, compared to the control foals, which is probably a response to syste mic bacterial infections. The syndrome foals' lymphocytes responded to mito gens (PHA, ConA, PWM) at normal levels. The data do not conform to any know n immunodeficiencies identified in any other species. Further analyses will be required, particularly on bone marrow function.