Coxsackievirus B-3 myocardiopathy was induced in weanling mice by intraperitoneal and intracerebral inoculations of the Nancy strain. Acute mortality was 5.5%. The cardiomyopathy is characterized by an early phase lasting about 9 days with myocardial necrosis, associated inflammation, and healing by fibrosis and calcification involving 25 to 50% of the contractile fibers in each affected mouse. Infectious coxsackievirus may be recovered from the heart during this phase. Continuing myocardial inflammatory lesions follow during the later phase, but infectious virus is no longer present.

When mice were forced to swim in a preheated pool (33°C) during both phases of their myocardiopathy, virulence was strikingly augmented. Fully half of the mice died of congestive failure, the majority while swimming. Hearts were dilated, hypertrophied, and grossly necrotic. The myocardium was transformed to a completely necrotic, inflammatory, calcifying mass.

At the peak of the infectious phase, myocardial replication of coxsackievirus was increased 530 times in nurslings which had been forced to swim. Myositis in hind limbs was more frequent, and inflammatory lesions in perirenal and pericardial fat were more severe in the mice which were forced to swim.

When swimming was begun on the 9th day after infection, the virulence and lethality (13.8%) of infection were moderately increased.

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