Amantadine-resistance among H5N1 avian influenza viruses isolated in Northern China.

We tested the amantadine-resistance among avian influenza A (H5N1) viruses isolated from chicken in Hebei Province of Northern China from 2001 to 2005, and investigated the amantadine use in this area. Plague reduction assay in MDCK cells showed that 83.3% isolates (5/6) were amantadine-resistant strains. The M2 sequence analysis revealed that four of five resistant isolates contained the point mutations (Ser to Asn) at position 31 that could confer resistance to amantadine. These results indicated that the incidence of amantadine-resistant viruses isolated in Northern China was particularly high. In the investigation of amantadine use, we found that amantadine was used extensively in poultry farms in this area, which maybe was one of reasons of the high amantadine-resistance incidence.