Dec 10, 2007 (CIDRAP News) – Scientists say they have found three distinct variants of H5N1 avian influenza virus in wild birds in Germany, two of which might have been brought in by wild birds migrating from Russia.
Researchers from the Friedrich Loeffler Institute in Insel Riems, Germany, analyzed 27 H5N1 isolates collected mostly from wild birds in widely scattered locations in Germany in 2006 and this year. Writing in the journal Veterinary Microbiology, they say the findings suggest that the virus was brought into the country on three separate occasions—two of them in early 2006 and the third in 2007.
The strains that appeared in early 2006 are closely related to viruses found in southern and central Russia, suggesting that wild birds on their winter migration from Russia might have brought the strains to Germany, says the report by E. Starick and colleagues.
In Germany in 2006, the report says, the highly pathogenic H5N1 virus was found in 343 dead wild birds, a black swan in a zoo, three stray cats, and a stone marten and on one turkey farm. In June and July of this year the virus was found in 96 wild birds in scattered areas of southeastern Germany and in one backyard goose. More recently, the disease killed ducks on a farm in Bavaria in late August (an outbreak not covered by this study).
The researchers collected 27 H5N1 viruses from 17 species of wild birds, the turkey farm, one stray cat, and the stone marten, the report says.
Previous study of the H5N1 viruses found in Germany indicated they all belonged to the strain that killed many wild waterfowl at Qinghai Lake in northern China in April 2005, called clade 2.2, the authors say. The new analysis of the 27 isolates showed that they fell into three groups that formed geographic and temporal clusters.
The viruses collected in 2006 formed two groups: one from northern Germany, designated subclade 2.2.2, and one from southern Germany, called subclade 2.2.1. The isolates gathered in 2007 formed a third type, which the authors called subclade 2.2.3, no examples of which were found in 2006.
Some members of both of the 2006 subclades were found in central Germany, and both types were involved in the poultry farm outbreak, the report says. In addition, one isolate of the "northern" type (subclade 2.2.2) was found in southern Germany, and one of the "southern" type was identified in northern Germany.
The authors write that the three subclades they identified match up with three clades described by other researchers who analyzed the complete genomes of 71 H5N1 viruses from Europe, the Middle East, and Africa (EMA). The northern Germany isolates fit in EMA group 2, those from the south in EMA group 1, and the 2007 isolates in EMA group 3, the report says.
"Our data suggest the simultaneous introduction in early 2006 of two closely related but distinct H5N1 variants into the wild bird population of Germany," the report states. "The source of these viruses and the exact time of introduction could not be identified."
But because the two subclades are closely related to H5N1 variants from southern and central Russia, the authors add, "an introduction, possibly via wild birds on winter escape from these regions, early in 2006 appears to be a highly likely scenario."
The separate subclade found in Germany in 2007 appears to represent a "new incursion," whose sources and routes of introduction remain unknown, the report adds.
David A. Halvorson, DVM, a veterinary pathologist and avian flu expert at the University of Minnesota in St. Paul, commented that the similarity between the German and Russian isolates doesn't necessarily mean the viruses were brought to Germany by wild birds.
"What is clear is that related viruses were introduced into Germany and they were observed in wild waterfowl before they were observed in domestic poultry," Halvorson told CIDRAP News. "This suggests that waterfowl may have been the source of introduction, but it doesn't prove it. This was known before the viruses were sequenced."
Starick E, Beer M, Hoffmann B, et al. Phylogenetic analyses of highly pathogenic avian influenza virus isolates from Germany in 2006 and 2007 suggest at least three separate introductions of H5N1 virus. Vet Microbiol 2007 (in press; early online publication Oct 18) [Full text]