Occurrence of temperature-sensitive influenza A viruses in nature

The origin and characteristics of the first naturally occurring temperature-sensitive (ts) strain of influenza A virus identified in 1973, Xia-ts, are described. Natural ts strains were found to occur in the early egg passage material of all influenza A subtypes examined, but the proportion of ts virus varied from 8.3% for old H1N1 virus (1949 to 1957) to 82.4% for recent H3N2 virus (1979 to 1980). A number of strains were found to be composed of a mixture of ts and wild-type (ts+) particles. Six natural ts strains with different shutoff temperatures and one ts+ strain of the H1N1 subtype were tested in antibody-free volunteers. Strains with a shutoff temperature of 38 degrees C or lower caused very mild symptoms, whereas those with a shutoff temperature of 39 degrees C and the ts+ strain were much more reactogenic. By complementation tests against a set of prototype WSN ts mutants with a defined genetic lesion, the ts lesion of two H3N2 viruses (HK/8/68 and Xia-ts) was located on the NP gene and that of two H1N1 viruses (Tianjin/78/77 and Beijing/1/79) was located on the M protein gene. The present study demonstrates the widespread occurrence in nature of influenza viruses of different degrees of temperature sensitivity and presumably of different degrees of virulence.