Uvs Lake H5N1 in Asymptomatic Pochard
Recombinomics Commentary 19:32
March 28, 2008
Animal health officials in Switzerland today said samples from an asymptomatic duck were positive for H5N1 avian influenza
Animal health officials said the wild duck was a European pochard, according to Swissinfo, a Swiss Broadcasting Corp. Web news service. Authorities said an analysis of the H5N1 virus showed that it resembled strains found in other European birds in 2007.
The above comments indicate earlier reports that the asymptomatic H5N1 positive duck in Switzerland was a European pochard. These data highlight the limitations of lab research on wild bird susceptibility to H5N1. Recent data suggested that a strong candidate for transport and transmission of H5N1 was mallards because they were asymptomatic and shed relatively high levels of the clade 2.2 virus isolated in Turkey in 2005.
Pochards were not considered likely sources because they developed symptoms that would limit transport by infected birds. However, the presence of H5N1 in an asymptomatic pochard contradicts that conclusion and highlights variables that may limit conclusions.
Although the failure of the H5N1 to grow to high levels in the intestines of 23 species of wild birds may explain the general failure to detect H5N1 in cloacal swabs or feces from wild birds, those levels may be linked to the strain of H5N1. Clade 1 grew to high levels in the intestines of asymptomatic waterfowl in Vietnam and much of the clade 2.3 detected in Asia is found in cloacal swabs or fecal samples.
However, as noted in the recent paper on wild birds, the clade 2.2 from Turkey in 2005 (Qinghai strain) did not grow to high levels, but clade 2.2.3 (Uvs Lake strain) was found in fecal samples collected from areas in South Korea frequented by wild birds. Thus, detection may be related to how samples are collected and tested, or to changes in the H5N1 itself.
The H5N1 that was in the pochard, as well as all recent outbreaks in central or western Europe has been the Uvs Lake strain, which may be more common in asymptomatic birds. In Germany, H5N1 was found in a healthy mute swan in the summer of 2007 or in England recently. Similarly, recent poultry outbreaks in Turkey have been linked to discarded tissues from hunter killed wild birds, which likely represents H5N1 in healthy wild birds.
Alternatively, the effect on the infected birds may be linked to immunity from earlier exposure to high or low path H5 or the dominance of the Uvs Lake strain may be linked to viral evolution leading to spread linked to asymptomatic infections.
Thus, identifying species involved in the transport and transmission of H5N1 based on lab testing with a limited number of H5N1 species may not be predictive. Lab experiments showing resistance in pigeons has not been supported by H5N1 from dead pigeons in a number of countries in the Middle East, Africa, and Asia. Unfortunately, these lab results may lead to lack of testing, as indicated by comments by heath agencies on testing of various species, including limited testing of pigeons.
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