Qinghai H5N1 Wild Bird Surveillance Issues
Recombinomics Commentary 14:59
March 24, 2008
HPAIV (H5N1) infection in these wild ducks contrasts in pattern of excretion with that of low-pathogenicity avian influenza virus infection in wild ducks and contrasts in pattern of disease with that of HPAIV infection in chickens.
With regard to pattern of excretion, low cloacal excretion was associated with lack of evidence for HPAIV (H5N1) replication in intestinal epithelium of any of the 23 ducks examined (Appendix Table 2), in contrast to most low-pathogenicity avian influenza viruses for which intestine is the main replication site (30).
Severe clinical disease in the HPAIV (H5N1)–infected tufted ducks and pochards manifested itself mainly as neurologic signs at about 4 dpi, although pathologic examination also showed virus-induced lesions in organs other than the brain. These findings differ substantially from those of HPAIV (H5N1)–infected chickens, which are characterized mainly by widespread hemorrhage and edema and death by about 2 dpi (31). Again, this contrast can be explained by differences in tissue tropism. Whereas the cardiovascular lesions in poultry are associated with widespread replication of HPAIV (H5N1) in endothelium lining the blood vessels (31), no such endotheliotropism was detected in any of 23 ducks examined.
The above comments from a recent report on experimental infection of six species of wild birds with clade 2.2 (Qinghai) H5N1 help explain why the current surveillance systems are well into the abysmal category. The paper measured virus levels using PCR or virus isolation assays in pharyngeal and cloacal samples and found higher levels in the pharyngeal samples. The lack of replication in the intestines raises serious questions about negatives generated by various groups testing feces or cloacal samples. Similarly, assays that require virus isolation to confirm positives will also generate false negatives because of the time limits on virus isolation. In most instances, the time frame for virus isolation was limited to a single day, so the vast majority of daily collections from H5N1 infected birds were negative.
The paper identifies mallards as likely candidates for long range transport of clade 2.2 H5n1 because these birds can shed large levels of virus in the upper respiratory tract, but this testing was under laboratory conditions. Species that had low levels of shedding while healthy, might shed increased levels of clade 2.2 when stressed due to co-infections by other influence serotypes of other infectious agents. Moreover, some long range migratory birds can travel 1000 miles in 24 hours, so birds that are asymptomatic for multiple days can transport H5N1 over long distances prior to development of symptoms. Moreover, the transport of H5n1 is not dependent on a single member of a flock of birds, or a single species.
Thus, while these laboratory tests identify major differences between levels in birds infected with low path H5N1 or clade 2.2, or tissue distribution in domestic poultry, they leave open many issues linked to field conditions which can affect expression level.
Unlike low path avian influenza or other subclades of highly pathogenic H5N1, the vast majority of clade 2.2 isolates have PB2 E627K, which is linked to higher levels of polymerase activity at lower temperatures (33 C). Thus, conditions which lead to lower temperatures in the upper respiratory tract lead to higher levels of virus, which could affect transmission and detection frequencies.
The role of wild birds in the transport and transmission of H5N1 is easily seen in the spread of the virus as well as sequence analysis. Prior to the outbreak at Qingha Lake in central China in the spring of 2005, the “Asian” strain of H5N1 had not been reported by any country west of China. In the summer of 2005, H5N1 was reported for the first time in Russia, Kazakhstan, and Mongolia. In the fall, H5N1 was reported in southeastern Russia (Astrakhan near the Volga Delta), Romania (in the Danube Delta), western Turkey (near Black and Mediterranean Seas, and Crimea Peninsula (in Black Sea). H5N1 was also subsequently detected in a sample collected from a healthy teal in Egypt (Nile Delta) in December, 2005.
Although other countries in Europe, the Middle East, Africa, and south Asia denied H5N1 infections in 2005, almost fifty countries in these regions reported H5n1 in early 2006 and all reported isolates were clade 2.2 (Qinghai strain). In many instances, the H5N1 detection was limited to wild birds.
While H5N1 was being reported in dead and dying wild birds, conservation groups reported negative data on assays that focused on cloacal or fecal samples, which as noted above, have low levels of H5N1 due to the limited replication in wild bird intestines. Other agencies in North America require virus isolate for confirmation of H5 and N1 PCR positives and the vast majority of samples fail to yield H5N1 virus (11/44). Most testing leads to either no virus isolation, or isolation of other serotypes (including H5N3, H1N1, H3N2, H4N1, H6N2, and N1 isolates with H other than H5).
As noted above, the detection of low path H5N1 provides little assurance that the sample collection and testing has the sensitivity for the detection of high path H5N1 in field samples.
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