Commentary
Asymptomatic
H5N1 in Germany Raises Surveillance Concerns
Recombinomics Commentary
12:55
October 27, 2008
Preliminary sequence and phylogenetic analysis of the HA gene from 4
duck samples gave evidence for a representative of a cluster 2.2
sublineage which had been detected in spring 2006 in Southern
Germany. In fact, a virus from a tufted duck (R1240/06; Starick et
al., 2008) found in 2006 40 km away from the present outbreak
location is very closely related.
The above comments on H5N1 in asymptomatic free range ducks in Saxony, Germany, leaves little doubt that the sub-clade has been maintained for two years in an undetected reservoir in Germany. This sub-clade was found in wild birds in southern Germany, Switzerland, the Czech Republic and France in early 2006. The sequence was readily distinguished fro other clade 2.2 sub-clades in Europe, by virtue of a number of polymorphisms, including NA G743A. However, after the outbreaks in early 2006, there were no reports of this sub-clade in poultry or wild birds.
Although it is possible that the sub-clade was maintained in domestic poultry, a wild bird reservoir is more likely because of the failure to detect this sub-clade during routine poultry testing between 2005-2008. As noted above, the infected birds were asymptomatic and most birds positive by PCR did not have sufficient levels of RNA to yield sequencing data, which again raises surveillance questions.
Detection of H5N1 in asymptomatic birds has been problematic. Almost all detection in Europe has been in dead or dying birds. These detection failures have been noted worldwide. A December, 2005 from a teal in Egypt yielded HA and NA sequence data for clade 2.2, but the sequence creation required multiple RNA extractions and virus was not isolated. However, the sequence was closely related to clade 2.2 from Austria, even though no EU country had acknowledged H5N1 infections in 2005. The first confirmed positives in the EU were in early 2006, once again supporting the view that surveillance approaches generally produce false negatives for live wild birds transporting and transmitting H5N1.
The failures have been confirmed in experimentally infected wild birds. These birds, when infected with lower levels of virus, yield H5N1 positive PCR in nasopharyngeal swabs when cloacal swabs are negative. Moreover, like the above data, most of the samples that were PCR positive failed to lead to virus isolation. Isolation was generally limited to a single 24 hour time frame several days after infection. Thus, experimental birds that are shedding H5N1 usually fail to yield virus, and most samples produce false negatives.
These false negatives have led to earlier pronouncements on the demise of H5N1 in wild birds. In June, 2007, wild birds in Europe was declared free of H5N1 based on negative data generated by a consortium of bird conservation groups collecting samples for FAO. Within minutes of this pronouncement, the Czech Republic reported in H5N1 in domestic poultry. This announcement was followed by detection of H5N1 in wild birds in the Czech Republic, Germany, and France. These isolates were clade 2.2.3, which were readily distinguished from the various clade 2.2 sub-clade reported in Europe in 2006. The strain had evolved over the summer of 2006 in Mongolia at Uvs Lake. The appearance throughout Europe in the summer of 2007, when long range migration was minimal, suggested this sub-clade had also entered Europe undetected in the 2006/2007 season, and was not reported until the summer of 2007. These outbreaks were followe3d by widespread detection of the Uvs Lake version of clade 2.2.3 in Europe in the 2007/2008 season.
Thus, current surveillance methods largely miss H5N1 in asymptomatic birds. The most recent data define a sub-clade circulating undetected in Europe between 2006 and 2008. These assays also failed to detect H5N1 in EU countries prior to early 2006, and also failed to detect the entry of clade 2.2.3 in late 2006, early 2007.
These detection failures remain a cause for concern.
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