Experimental influenza A virus myocarditis in mice. Light and electron microscopic, virologic, and hemodynamic study

To elucidate the heart involvement associated with influenza virus infection, the authors studied the hearts of influenza A/PR/8/34 virus-inoculated ICR mice by light and electron microscopy, cardiac catheterization, virus assay, and indirect immunofluorescence. Light microscopy showed small necrotic foci with inflammatory cell infiltration spreading in the myocardium on days 3 to 7 and evidence of healing by day 9 after inoculation. Electron microscopy demonstrated that necrotic cell debris was phagocytosed by macrophages, and that degenerating cardiocytes, macrophages, and lymphocytes were often in close contact, suggesting immunologic interactions, and that platelet thrombi were present in some capillaries on days 3 to 5. Both systolic and diastolic functions of the left ventricle (LV) were impaired on days 3 to 9 and recovered almost to normal by day 14. The virus could be isolated from the heart on days 3 to 7. Immunofluorescent preparations showed virus antigens in the vascular walls and cardiocytes until day 7. These results suggest that the acute cardiac injury was related to cytotoxic immunologic interactions, virus-induced cytolysis and, at least in part, to ischemia due to intracapillary thrombosis. Compared with coxsackie B3 myocarditis in mice, the influenza myocarditis was mild in degree and short in duration, but the influenza infection is a most common and repetitive disease in humans. The clinical implications of this animal model with myocarditis are discussed.