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H5N1 Bird Flu Expands Host Range in Western Asia
August 7, 2005
The flu virus has been discovered among wild and barnyard fowls in Kurgan and Novosibirsk oblasts, reports the Ministry of Agriculture of Russia. As mentioned in a report with reference to Rosselkhoznadzor
[Russian Agricultural Directorate], there has been "confirmed, the circulation of the virus of the flu of subtype H5 in wild birds (ducks, gulls, and forest wildfowl) and in barnyards of citizens in two locations of Chastoozersky and Petukhovsky regions of Kurgan oblast".
In the Novosibirsk area the flu virus has been discovered among rooks, doves and wild ducks. The same pathogen is determined from pathologic material from wild ducks, which perished on a lake outside of the village of Urusy in the Sargatsky region of Omsk oblast.
The above comments from Russian language media provide more detail on the many species infected by H5N1. H5N1 in ducks, gulls, forest wildfowl, rooks, and doves indicates that H5N1 can infect and be transmitted via many species, providing further evidence that it will soon be endemic to Europe and Asia. This widespread transmission is possible because many species can be infected asymptomatically.
Some media reports have suggested that the H5N1 in some areas is low pathogenic. It seems likely that this diagnosis is based on lack of deaths and appearances of infected birds, rather than sequence data showing a low pathogenic cleavage site.
In the OIE report from Novisbisrk, many villages had H5N1 positive birds that did not appear to be ill. Reports of sequence data described 9 H5N1 independent isolates. There was no mention of mixtures between high and low pathogenic isolates. For H5N1, high pathogenic versions have a multibasic HA cleavage site. This sequence is well define and considered to be diagnostic. All H5N1 human cases involve isolates with a multibasic cleavage site. However, these same isolates that produce fatal infections in humans can infect laboratory ducks asymptomatically. These asymptomatic infections in Vietnam are widespread. 70% of water fowl in the Mekong Delta region are H5N1 positive.
Thus, finding H5N1 in birds that do not die or do not appear to be sick does not indicate that the isolates are LPAI (Low Pathogenic Avian Influenza). This designation is dependent on the sequence of the cleavage site as will as a pathogenicity index determined by experimentally infecting test chickens. The OIE report described the H5N1 as HPAI (High Pathogenic Avian Influenza), without mention of LPAI. Although LPAI H5 has been identified in Novosibirsk (Chany Lake) and Primorie in the past, it seems that most or all H5N1 isolates in the migratory bird-linked isolates are HPAI (all three outbreak in China were designated as HPAI based on the sequence of the cleavage site as well as pathogenicity index - experimentally infected chickens died with 20 hours).
HPAI in healthy birds is cause for concern because these birds can transmit the virus efficiently. In Vietnam, the virus grows to high titers in the ducks and then is excreted at high concentrations. Moreover., the excreted virus is unusually stable. Thus, asymptomatic birds can quickly establish H5N1 in an area and make it endemic. These infected birds can then spread the virus to other birds and mammals such as pigs or people. The dual infection of these mammals can allow the H5N1 to pick up mammalian sequences via recombination, which then allows H5N1 to expand its host range and more easily infect humans.