Studies with Influenza Virus B of Recent Human Origin

The biphasic course of adaptation to the mouse lung of a type B influenza virus of recent human origin is described. Inoculation of the virus resulted in the production of extensive pulmonary consolidation and multiplication of virus as determined by hemagglutinin and egg infectivity titrations of bronchial washings. Continued passage resulted in a gradual decrease in consolidation and a depression of multiplication resulting in no demonstrable hemagglutinin and a decrease in egg infectivity titer. A subsequent gradual rise in consolidation and an increase of hemagglutinin and egg infectious particles were found. During the early phases of adaptation, a “functionally deficient” form of virus was produced. A study of the character of the lesion-producing capacity of the unadapted virus revealed that (a) the extent of lung lesions obtained paralleled the dosage of virus inoculated, and (b) the extent of consolidation produced was found to correlate with the production of hemagglutinin units. Continued passage in the chick embryo resulted in a diminished ability of unadapted virus, upon introduction into the mouse lung, to multiply and to produce consolidation.