LESIONS OF EXPERIMENTAL EQUINE MORBILLIVIRUS PNEUMONIA IN HORSES

Laboratory examinations of equine morbillivirus included experimental reproductions of the disease caused by the virus by transmission of mi xed lung and spleen taken from two field equine cases into two horses and by inoculating tissue culture virus into a further two horses. The most distinctive gross lesions of the diseases that developed in thre e of the horses was that of pulmonary edema characterized by gelatinou s distension of subpleural lymphatics. Histologically, the lesions in the lungs were these of serofibrinous alveolar edema, alveolar macroph ages, hemorrhage, thrombosis of capillaries, and syncytial cells. Clea rly defined vascular lesions in three horses that became clinically af fected within 8 days of inoculation of virus included intramural hemor rhage, edema, and necrosis and syncytial cells in the endothelium of p ulmonary vessels (similar to 40-70 mu m in diameter). Vascular lesions accompanied by parenchymal degeneration were also seen in the heart, kidney, brain, spleen, lymph node, and stomach. A fourth horse, which survived for 12 days, had detectable lesions only in the lungs, which were more chronic than those in the other three horses, a greater degr ee of cellular infiltration, and fewer well-defined vascular lesions. Sections stained by an indirect immunocytochemical method showed equin e morbillivirus antigen was present in the vascular lesions and along alveolar walls. When endothelial cells were examined by electron micro scope, cytoplasmic virus inclusion bodies containing filamentous struc tures were seen that reacted to an immunogold test to equine morbilliv irus antigen. The presence of the syncytia in the small blood vessels in the lungs and other organs was interpreted as an important characte ristic of the disease and consistent with a reaction to a morbilliviru s.