Deaths of ducks in different barn units of farm B, Bavaria. A) Barn B/3–5; duck hatched April 25, 2007, slaughtered June 14, 2007, at an abattoir in Lower Saxony: no material available for testing. B) Barn B/3–5; duck hatched June 20, 2007, slaughtered August 1, 2007, at farm A): 2 highly pathogenic avian influenza virus (HPAIV) A (H5N1)–positive samples detected in retained samples of frozen duck carcasses; virologic and serologic results suggestive of a recent HPAIV (H5N1) infection are listed in the Table. C) Barn B/1–1; duck hatched June 29, 2007, slaughtered August 11, 2007, at farm A; subtype H5N1–specific RNA was detected in 1 retained sample of a frozen duck carcass, but the pathotype could not be determined due to low viral loads. D) Barn B/2–4; duck hatched July 20, 2007, culled September 9, 2007; virologic and serologic results suggestive of a recent HPAIV (H5N1) infection are listed in the Table.

Possible pathway of transmission of highly pathogenic avian influenza virus (HPAIV) (H5N1) from farm B, Bavaria, to 3 backyard chicken holdings in Brandenburg (gray house symbols) based on phylogenetic and circumstantial epidemiologic evidence. Viruses of these cases were virtually identical, although they were separated by 4 months (August and December, 2007) and ≈400 km without linking outbreaks. In contrast, other viruses occurring at the same time (August) in Bavaria in wild birds or in farm A were distinguishable from those of farm B. The same was true for viruses detected in Poland (close to Brandenburg) in December. Therefore, a direct epidemiologic link between farm B and the outbreaks in Brandenburg was suspected (dashed arrow). From June 14 through June 22, 2007, three fattening flocks from farm B were slaughtered in Lower Saxony (angled arrow). These included flock B/3-5 with elevated proportion of deaths (Figure 1, panel A). Slaughtering lots 724/725, which contained ducks from farm B flocks with an elevated cumulative proportion of deaths, were distributed in Brandenburg supermarkets (horizontal arrow). Frozen duck carcasses from these lots had been purchased by the owners of the Brandenburg chicken holdings, and in 2 of the 3 outbreaks, owners admitted that chickens had access to uncooked offal from these carcasses before the outbreaks.

Phylogetic tree of the hemagglutinin (HA) gene (full-length sequence) of highly pathogenic avian influenza virus (HPAIV) (H5N1) detected in poultry from Brandenburg and Bavaria, Germany, in 2007, including sequences of wild birds and poultry from neighboring countries. Sequence of the Czech poultry isolate is supported by GenBank. The tree was constructed by using a minimal-evolution algorithm; numbers represent bootstrap values after 1,000 replications. A maximum-likelihood (ML)–based tree resulted in a similar topology; italicized numbers indicate bootstrap values of the ML tree after 200 replications. Scale bar indicates substitutions per site. The HA sequence of A/chicken/Czech Republic/11242–38/2007 (H5N1) was extracted from GenBank accession no. EU 443553. LPAIV, low pathogenicity avian influenza virus.