Apr 14, 2009 (CIDRAP News) – H5N1 influenza viruses found in chickens seized at Vietnam’s border with China in 2008 show a surprising level of genetic diversity, suggesting that the viruses are evolving rapidly and raising questions about disease surveillance and the effectiveness of prepandemic H5N1 vaccines, according to a team of Vietnamese and US scientists.

The findings suggest that the subgroup of viruses identified in the study has been circulating in the region either undetected or unreported for a “considerable amount of time,” the report says. The viruses are in clade 7, a group that has not been found in an outbreak since one in China’s Shanxi province in 2006.

Also, given the degree of difference between these clade 7 viruses and those used to make human H5N1 vaccines, “it is unclear whether or not humoral antibodies elicited by current vaccine candidates will have cross-neutralization activity against the new viruses,” says the report, published recently in Virology.

The study was conducted by scientists from Vietnam’s National Center for Veterinary Diagnostics in Hanoi and the US Centers for Disease Control and Prevention (CDC), with the CDC’s Ruben O. Donis as the senior author.

Vietnam bans the importation of poultry from H5N1-infected countries. The viruses analyzed in the study came from imported illegal chickens seized at border control stations in Vietnam’s Lang Son province, on the Chinese border. Authorities confiscated 495 chickens at the border stations in the first 5 months of 2008 and tested them for avian flu. Fifteen birds were found infected with H5N1 virus, and two others had an H9N2 virus.

The authors extracted RNA from the 15 H5N1 isolates, amplified it, and analyzed the sequence of the hemagglutinin (HA) genes. They found that all 15 HA genes fit within a subgroup of previously identified clade 7 viruses. They determined that 13 of the 15 HA genes fell into two further subgroups, which they labeled A and B.

The isolates within groups A and B were very closely related, but the groups differed considerably from each other, with an average divergence of 4.05% in HA nucleotide sequences and 5.69% in amino acid sequences, the report says. The authors also determined that the isolates differed by an average of 3.67% and 5.81% from the most closely related previous clade 7 isolate, which came from a chicken in China’s Shanxi province in 2006.

In comparing the viruses with representatives of other H5N1 clades, the investigators found they were most closely related a clade 4 virus (a 2006 isolate from Guiyang province of China) and most distantly related to a clade 1 virus, a 2004 isolate from Vietnam. The respective differences in amino acid sequences were 8.20% and 9.07%. The genetic distance between the HAs of these clade 7 viruses and the isolates used to make prepandemic vaccines raises doubt as to whether antibodies generated by the vaccines would react with the clade 7 viruses, (Snip)