Ultrastructural degenerative changes of age-dependent ECHO 9 virus-induced polymyositis in infant mice.

Experimental inoculation of newborn NMRI mice with Echo virus, type 9, strain A. Barty *ECHO 9 virus AB) resulted in diffuse polymyositis 4 days later. The disease, after a delay of 1 day, increased in intensity and was accompanied by a rapid, progressive paresis leading to death 7-11 days following infection. As early as 24-48 h after inoculation, we detected initial ultrastructural degenerative changes, characterized by segmental dilation fo the sarcoplasmic reticulum. Disruption and breakage of myofibrils followed, and lead to the formation of increasing numbers of contraction bands. Ultimately, virus-mediated nuclear and other organellar injury resulted in muscle fiber necrosis. In addition, we observed some substructural cellular events related to virus propagation. Beginning on the 4th day after infection, pronounced proliferation of the sarcoplasmic-reticulum membranes became evident in perinuclear and subsarcolemmal areas in the mature muscle fibers, myoblasts, and post-fused myotubes of mice inoculated within 18 h after birth. These cytological alterations were not noted in the myoblasts and myotubes of NMRI mice injected with virus 2 or 4 days after birth.