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Bird Flu Detected on Great Britain Farm
April 27, 2006
Chickens at a farm near Dereham in Norfolk are to be slaughtered after dead birds tested positive for a strain of bird flu.
Officials said preliminary test results indicated it was likely to be the H7 strain not the deadly H5N1 variation.
Last month a swan in Cellardyke, Fife, tested positive for the H5N1 strain
The above comments describe a bird flu outbreak in Great Britain. Although the infectious agent may be H7, co-circulation of H7 and the Qinghai strain of H5N1 is cause for concern.
Great Britain has reported one positive H5N1 case. However, results from test on over 7000 samples raise serious questions about the sensitivity of the surveillance testing. Bird flu is common in wild birds. Canada tested young ducks in August of 2005 and found H5 low pathogenic infections in many birds. 24% of the birds tested in British Columbia were H5 positive. The frequency of positives will vary by species, age and season, but over an extend time period 5-10% of birds tested should be positive for some form of bird flu. Testing in England only identified a few positive samples although over 7000 samples were tested. This low detection rate may be related to collection procedures which include storing collected swabs a 4 C, where they could dry out and reduce the detection or isolation of avian influenza.
The surveillance detected only one H5N1 positive bird and no birds positive for H7. The H7 or H5 infection of domestic poultry further suggests that the surveillance in England in particular, and Europe in general is inadequate. The only country to detect H5N1 in live wild birds has been Russia. H5N1 has been detected in dead birds in many European countries.
Co-circulation of H5 and H7 is cause for concern. In 2003 the Netherlands had an H7N7 outbreak, This strain was readily transmitted from birds to human and human to human. Over 80 cullers tested positive for H7 antibodies. However, a follow-up study found H7 antibodies in contacts of the cullers, indicating efficient human to human transmission. Most cases were mild with either no symptoms or mild flu symptoms. Most of the cullers also had conjunctivitis.
However a veterinarian developed pneumonia and died. The case is the only reported bird flu fatality by a serotype other than H5N1. The H7N7 isolated from the veterinarian had a number of unique polymorphisms, including PB2 E627K. E627K is universally found in H1, H2, and H3 isolates from humans. Prior to Qinghai Lake, all H5N1 isolates with E627K were from mammals. Many were from humans infected in Hong Kong in 1997 or Vietnam or Thailand in 2004. Most of these cases were fatal. E627K was also linked to virulence in experimental mice and increased polymerase activity at lower temperatures.
All 16 H5N1 isolates at Qinghai Lake had E627K. All reported PB2 sequences for Qinghai H5N1 isolates have also had E627K, including the buzzard isolate from Denmark this year. Dual infections involving H7 and H5N1 could allow the acquisition of E627K by H7. The acquisition could be driven by reassortment or recombination. H7 with E627K could be more virulent in humans and be easily transmitted between humans. The increased efficiency of human infections could also be acquired by H5N1 via recombination with the receptor binding domain on H7.
The H7 infection on a large poultry farm in England highlights the poor surveillance in England as well as the rest of Europe.
Countries reporting large numbers of negatives have serious surveillance issues, which are hazardous to the world’s health.