Viruses producing an unusual "sponge-like" cytopathogenic change in monkey kidney tissue culture were isolated from the pharyngeal swabs of 2 of 12 infants with croup. The infants from whom the viruses were isolated and 3 additional patients developed significant increases in neutralizing or hemagglutination-inhibition and complement-fixing or all 3 varieties of antibody during convalescence. The isolated agents appeared to be similar antigenically.

Fluid from infected monkey kidney tissue culture agglutinated chick erythrocytes and in lower titer human "O" red cells. Hemagglutination occurred best at 4°C. and pH 8.0. Agglutination was reversed at 37°C. but resuspension and sedimentation of red cells at 4°C. resulted in a restitution of positive patterns. In addition, the virus was capable of partially removing receptors from the erythrocyte surface, but only when large quantities of virus were incubated with red cells for 24 hours at 37°C. and small doses of hemagglutinin used to test the treated cells. RDE removed both the erythrocyte receptors and the inhibitor for hemagglutinin present in certain normal sera. Low level multiplication occurred at a slow rate in the amniotic cavity of the fertile hen' egg. Gradocol membrane filtration yielded a size of 90 to 135 mµ. The virus was stable at –70°C. but infectivity was lost after treatment with 20 per cent ether for 15 hours.

The properties of the isolated viruses were consistent with those required for classification in the myxovirus group. No antigenic relationship with influenza A, A', B, and C, Newcastle or Sendai viruses was found. The viruses were distinct from mumps virus but the existence of a common antigen was suggested.

The high incidence of infection with this new virus in one group of croup patients suggests that it may be at least one of the etiologic agents of this clinical syndrome, but more extensive control studies will be necessary to establish a specific etiologic association. For the present the group will be referred to as CA viruses; i.e., croup-associated viruses.

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