Age dependence of paralysis induced by echovirus type 9 in infant mice.

Newborn mice of different strains (including NMRI) infected with 2-5 x 10(6) pfu of echovirus type 9 strain A. Barty developed progressive flaccid paralysis five days after inoculation, followed by death. Suckling mice injected with the same infective dose two, four, or six days after birth disclosed at most transient paresis and survived. Infectivity titrations revealed in all groups a similar rate of virus multiplication exceeding 10(8) 50% tissue culture infective doses per mouse four days after virus inoculation. Histologically, polymyositis resulting in muscle fiber necrosis was seen. The percentage of damaged muscle tissue declined with increasing age of the experimental groups. It is concluded that in order for paralysis to occur, at least 50% of skeletal muscle tissue must be destroyed by virus-induced lesions before the sixth day of life. Electron microscopy demonstrated additional age-dependent, virus-mediated disturbances of postnatal muscle growth. Reparative events in destroyed muscle fibers seemed to be unaffected in all age groups.