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Co-circulation of H5N1 and H7N7 Bird Flu in Europe?
August 27, 2005
The announcement of bird flu in a seagull in Oulu, Finland on Friday will probably be followed by an announcement that H5N1 has indeed invaded Europe. Evidence from southern Siberia suggests H5N1 wild bird flu is in northern Siberia, and birds from northern Siberia migrate over Finland, so addition sightings will probably be reported next week (see map). Although tests of the current isolates are projected to last 3 weeks, sequencing of the HA cleavage site is routine, and such a sequence will almost certainly show the 6 basic amino acids (RRRKKR) that are diagnostic for H5N1 from Asia. and HPAI (Highly Pathogenic Avian Influenza), a reporable disease.
H5 has been detected in Europe previously, and the H5 sequenced previously at Chany Lake in 2003 and Primorie in 2001 has many markers found in H5 in Europe. The European H5 reassorted with H7N7 in 2003 to generate H5N7 that was isolated from a mallard duck in Denmark, A/Mallard/64650/03(H5N7).
Because H5N1 from Asia has a multi-basic HA cleavage site, it more easily infects birds, including wild birds such as mallards. Reasortants are created when the same host is infected with two different viruses and H5N7 arose from H5 and H7N7 infecting the same host. The poly-basic cleavage site in H5N1 from Asia gives it a selective advantage and it will probably replace most of H5 from Europe.
The dual infection can also generate recombinants, which involve a mixing or portions of genes. The H5N1 from Qinghai Lake has acquired sequences from European swine via recombination, and its presence in Europe this year will lead to more dual infections and more recombination.
Co-circulation of H5N1 and H7N7 is particularly dangerous, because H7N7 is efficiently passed from human to human. Thus, H5N1 could acquire sequences allowing efficient human-to-human transmission, and this acquisition could happen in mallard ducks, which are known to be infected with H5 from Chany Lake, A/Anas platyrhynchos/Chany Lake/9/03(H5N3), or Primorie, A/duck/Primorie/2633/01(H5N3). Birds infected with H7N7 have also led to isolates from the Netherlands, A/avian/Netherlands/065/03(H7N7).
Although 30 million birds were culled in 2003 to halt the spread of H7N7, its potential return is quite real as is the possibility that H5N1 is already in Scandinavian countries, including Finland.
This potential co-circulation in Europe is clearly cause for concern.