Differential patterns of amantadine-resistance in influenza A (H3N2) and (H1N1) isolates in Toronto, Canada.

BACKGROUND

Molecular methods were used to characterize influenza A (H1N1) and (H3N2) strains and to identify amantadine-resistance.

OBJECTIVES

To compare proportions of amantadine-resistant influenza A (H1N1) and (H3N2) isolates in the Greater Toronto Area.

STUDY DESIGN

Isolates of influenza A (H1N1) and (H3N2) were strain typed using molecular methods. Pyrosequencing for point mutations in the transmembrane domain of the M2 proton channel was undertaken. Proportions of amantadine-resistant and susceptible isolates were compared using the The Fisher's exact test.

RESULTS

96% of the 49 influenza A (H3N2) isolates and none of the influenza A (H1N1) tested carried a point mutation in the M gene coding for the M2 protein. Influenza A (H3N2) isolates were more likely to carry an amantadine-resistance associated mutation than influenza A (H1N1) isolates (Fishers's exact test, P<0.0001).

CONCLUSIONS

: Characterization of amantadine-resistance in influenza A (H1N1) isolates should utilize a variety of different methods including sub-typing, strain typing, and direct sequencing for point mutations associated with amantadine-resistance.