A rapid method for identification of newly isolated influenza viruses.

The hemagglutination inhibition test is commonly used for the identification of newly isolated influenza viruses. In this laboratory several difficulties frequently were encountered in typing these new strains after primary isolation by means of this technique. These drawbacks were: (1) Some newly isolated strains are susceptible to nonspecific inhibitors of hemagglutination (Francis, Salk, and Brace, 1946; Francis, 1947; Sigel, 1949). (2) The titer of a new virus is sometimes too low in first passage for successful hemagglutination inhibition testing. Consequently, this necessitates pasage of the virus to increase the titer and this results in delay of the final diagnosis. (3) The hemagglutination test is highly strain specific and hence, antisera against more than one influenza strain of a certain type or subtype are needed for typing. Even if a large stock of antisera to recent strains is available, typing would still be difficult, if not impossible, in the event of an appearance of a new subtype, as happened in 1947 when the A-prime subtype was isolated. In a report on the usefulness of hamster antisera in antigenic studies on influenza viruses (Sigel, Allen, Williams, and Girardi, 1949), mention was made of their use in rapid typing of newly isolated strains of influenza viruses, employing the complement fixation test. It is the purpose of this paper to extend these findings in the light of experience gathered during recent outbreaks of influenza and to describe a procedure which allows for identification of the viruses in 48 to 72 hours after collection of throat washings from patients.