Presence of African horse sickness virus in equine tissues, as determined by in situ hybridization.

In a retrospective study, a negative-sense digoxigenin-labeled RNA probe, corresponding to the gene encoding nonstructural protein-1 of African horse sickness virus (AHSV) serotype 4, was applied to formalin-fixed, paraffin-embedded tissue taken from horses in the terminal stages of infection with AHSV. Fifteen infected ponies and one noninfected control were studied. Ponies exhibited a range of clinical signs and lesions. Thirteen ponies were infected with serotype 4, one with serotype 1, and one with serotype 2. Ponies were monitored clinically and euthanatized when severely clinically ill. The following tissues were available for study by in situ hybridization and histopathology: lung, heart, spleen, neck muscle, and supraorbital fat. Histologically, the most striking changes were pulmonary edema and, in some, acute myocardial necrosis. In situ hybridization revealed virus distributed widely in sections of lung and heart examined, with relatively less in spleen, neck muscle, or supraorbital fat. Virus was localized to target cells with morphologic features compatible with endothelium in all organs except spleen, where it was found in both endotheliumlike cells and large mononuclear cells.