Immunohistochemical demonstration of African horse sickness viral antigen in formalin-fixed equine tissues

The distribution of viral antigen was studied in various tissues of three ponies, aged 3-4 years, infected experimentally with a virulent strain of African horse sickness virus (AHSV) serotype 4. Tissues were collected from the animals in the terminal stage of the peracute form of the disease and from one noninfected horse, included as a control. A polyclonal antibody with specificity for AHSV, plus the nonstructural protein NS2, was used in a sensitive avidin-biotin-peroxidase-complex (ABC) method performed on formalin-fixed, paraffin-embedded tissue sections. AHSV antigen was located primarily in endothelial cells of capillaries and small venous and arteriolar vessels, particularly of cardiopulmonary tissues. Viral antigen was also identified in cells resembling macrophages and in reticular cells of spleen and lymph nodes. The pattern of viral antigen labeling in some lymph nodes along the mantle zone of lymphoid follicles was compatible with the morphology of cellular processes of follicular dendritic cells. In some tissues, viral antigen was detected occasionally in circulating cells, probably monocytes, within the larger vessels. These findings suggest that endothelial cells, and to a lesser extent mononuclear cells, are the main target cells of AHSV infection during the late stage of the peracute form of the disease.