High prevalence of amantadine resistance among circulating European porcine influenza A viruses

Genetic analysis of the M2 sequence of European porcine influenza A viruses reveals a high prevalence of amantadine resistance due to the substitution of serine 31 by asparagine in all three circulating subtypes, H1N1, H3N2 and H1N2. The M segment of all resistant strains belongs to a single genetic lineage. Whereas the first amantadine-resistant porcine strain was isolated in 1989, isolation of the last amantadine-susceptible strain dates to 1987, suggesting a displacement of amantadine-susceptible viruses by resistant strains soon after emergence of the mutation. Analysis of natural selection by codon-based tests indicates negative selection of codons 30, 31 and 34 which confer amantadine resistance. The codons 2, 11-28 and 54 of porcine and human strains exhibit differences in the patterns of substitution rates, suggesting different selection modes. Transfer of amantadine resistance by exchange of the M segment and viability of recombinant A/WSN/33 viruses with avian-like M segments raises concerns about the emergence of natural human reassortants.