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LPAI H5N1 Detected in Wild Ducks from Manitoba
November 19, 2005
Canadian scientists who have studied big chunks of the genetic code of the two proteins on the surface of the Manitoba H5N1 viruses have confirmed these viruses are from the family of North American H5N1 viruses, not the strains circulating in Southeast Asia.
North American H5N1 viruses have so far proven to be much milder viruses than their distant Asian cousins, avian influenza experts say.
"We've got 32 years of surveillance work that says these North American strains in wild birds in the past have never been a threat," Dr. Richard Slemons, an avian influenza expert at Ohio State University, said from Columbus on Saturday.
"Does that mean they won't be a threat in the future? No it does not. But history says they aren't a risk."
Recent history is more useful than 32 years of surveillance work. In May of 2005 HPAI H5N1 was found in long range migratory birds at Qinghai Lake. This discovery was followed by similar sequences detected for the first time in Russia, Kazakhstan, Mongolia, Romania, Turkey, and Croatia. All of these outbreaks were linked to migratory birds and many of the migratory leave their summer locations when the weather turns colder. Included are birds that migrate from Siberia to North America.
Canada reported H5 in Manitoba, Quebec, and British Columbia, with results pending in the remaining provinces. The widespread detection of H5 raised the possibility of HPAI H5N1 migration into North America from Europe or Asia.
The five sequences reported above (only 5 of the 47 H5's were from Manitoba) do little to address the question of HPAI H5 in the positive birds. The delay in releasing data was due in part to mixtures of viruses in several birds. Thus, finding LPAI in some birds does not preclude HPAI from the same birds or other H5 positive birds.
HPAI H5N1 in migratory birds has been well characterized via full sequences from Qinghai Lake, Novosibirsk, and Mongolia. Like HPAI H5N1 in domestic birds in Asia, all wild bird isolates have a multi-basic HA cleavage site containing the sequence RRRKKR as well as an N1 with a 20 amino acid deletion.
Because of the size differences between HPAI and LPAI HA and NA genes, the two types of sequences can be easily distinguished one the basis of size. Thus, appropriate inserts can be run on gels to immediately determine detectable HPAI inserts in HA and NA. Sequencing could then be used to verify that larger HA inserts or smaller NA inserts contained sequences that matched the published sequences for HPAI H5N1.
Instead Canada merely proved that two H5 positive birds contained LPAI H5N1, which doesn't address the presence or absence of H5N1 HPAI in any of the H5 positive birds, including those with LPAI H5N1.