Identification of naturally occurring temperature-sensitive strains of influenza A virus and location of their genetic lesions.

Influenza A viruses of different subtypes isolated in different years and from different parts of China were examined for temperature-sensitiveness (ts) in their early egg passages. The validity of ts character has been confirmed by the results of parallel tests in chick embryos and in cell cultures. From 12 strains of the old H1N1 subtype isolated between 1949 and 1957, no ts strain was detected. Two out of 6 strains of new H1N1 isolated after February 1979 were found to be ts. For the H3N2 subtype, only 8 out of 23 strains isolated between 1968 and 1978 were ts, but the proportion increased abruptly of 13 out of 15 strains isolated in 1979-1980. We also found 6 out of 16 strains of the H2N2 subtype to be ts. Two H2N2 and one H3N2 strains examined were found to be composed of mixed ts and ts+ particles. By recombination-complementation test against 7 standard ts strains of WSN virus with known genetic lesions, the ts lesions of the H3N2 strain Hong Kong/8/68 was located on the nucleoprotein gene, whereas that of the H1N1 strain Tianjin/78/77 was located on the matrix protein gene.