IMMUNOHISTOCHEMICAL DEMONSTRATION OF AFRICAN HORSE SICKNESS VIRAL-ANTIGEN IN TISSUES OF EXPERIMENTALLY INFECTED EQUINES

African horse sickness virus (AHSV) antigen was demonstrated immunohis tochemically in formalin-fixed, paraffin-embedded sections of tissues collected from three ponies suffering from the peracute form of the di sease and from one pony affected by the fever form. The pattern of the antigen distribution indicated a particular organ tropism characteris ed by an accumulation of AHSV antigen in cardio-pulmonary tissues of t he animals with the peracute disease and in the spleen of the pony wit h the fever form. AHSV antigen was identified in endothelial cells of small blood vessels, particularly capillaries and in large mononuclear cells resembling macrophages or reticular cells of lymphatic tissues. Occasional circulating mononuclear cells with the morphology of monoc ytes were also positively stained within the larger vessels. The immun ohistochemical results confirm earlier work suggesting that AHSV may h ave different tropisms to particular organs during various forms of th e disease and that different target cell populations exist in vivo. Im munohistochemistry may be an additional useful method for diagnostic a nd research purposes in AHS.