We surveyed the incidence of
amantadine-resistant influenza A viruses both at sentinel surveillance sites and at nursing homes, and verified their types of change by partial nucleotide sequence analysis of the M2
protein. Fifty-five influenza A viruses from 27 sentinel surveillance sites during six
influenza seasons from 1993 to 1999, and 26 influenza A viruses from 5 nursing homes from 1996 to 1999 were examined for susceptibility to the drug by virus titration in the presence or absence of
amantadine. While
amantadine-resistant viruses were not found in sentinel surveillance sites, a high frequency of resistance (8/26, 30.8%) in nursing homes was observed. Resistant viruses can occur quickly and be transmitted when used in an outbreak situation at nursing homes, where
amantadine is used either for neurologic indications or for
influenza treatment. Eight resistant viruses had a single
amino acid change of the M2
protein at residue 30 or 31. In vitro, all 11 sensitive viruses turned resistant after 3 or 5 passages in the presence of 2 microg/ml
amantadine, and they showed an
amino acid change at residue 27, 30, or 31. The predominant amino acid substitution in the M2
protein of resistant viruses is Ser-31-Asp (a change at 31,
serine to
asparagine). The results indicate that a monitoring system for
amantadine-resistant influenza viruses should be established without delay in Japan.