— H5N1 bird flu was found in a closed turkey farm in the UK. How did it get there? (Graphic: David Johnston)
On 3 February 2007, confirmation came that the deadly strain of H5N1 had made it to Europes biggest poultry plant, the Bernard Matthews turkey farm in Suffolk, eastern England (see H5N1 bird flu outbreak confirmed on English farm). It was the second confirmed case of this strain of the virus in a European Union state in 2007, following an outbreak in Hungary.
The H5N1 virus is known to have infected 270 people and killed more than 160 worldwide since 2003, most of them in Asia, and over 200 million birds have died from it or been killed to prevent its spread. (Find out more in our Bird Flu special report.)
The H5N1 bird flu was found in a turkey production plant with active biosecurity measures how did it get in and where did it come from? New Scientist investigates (see graphic, right).
Suspect: East Asia
Evidence: The highly pathogenic H5N1 bird flu that has killed 166 people so far first evolved in poultry in southern China, and it has spread with poultry across the region. It has become widespread in poultry and has caused the most cases so far in people Indonesia is now hardest hit.
Verdict: Extremely unlikely to have spread directly from here to the UK. The H5N1 circulating in Europe is descended from a strain first isolated in wild birds at Qinghai Lake in central China in 2005. This is genetically distinct from the H5N1 circulating in poultry in the Far East.
Evidence: Dabbling ducks can carry H5N1 and stay healthy. Many dabbling ducks now wintering in Europe spent the summer nesting in Siberia. Nesting grounds are where birds contract bird flu most often. 20 kilometres from Holton is a nature reserve where wigeons gregarious dabbling ducks that summer across Siberia are wintering. Those ducks could well have nested alongside birds now wintering in Hungary. They all could have carried the same virus from Siberia to both England and Hungary, where it circulated at a low level among other birds. Then some local poultry farm just got unlucky.
Further evidence: Recent H5N1 outbreaks in poultry in Krasnodar on the Russian Black Sea coast, and in eastern Turkey (major destinations for the same ducks) plus suspected human cases in nearby Azerbaijan.
Verdict: Likely. How did H5N1 get from wild birds into closed turkey barns? A worker could merely have stepped in duck faeces then walked into a barn, say scientists. Turkeys are incredibly sensitive to H5N1.
Evidence: Hungary had an H5N1 outbreak in geese in Csongrad on 19 January, and the virus is described as genetically very similar. The British outbreak started on 27 January 2007.
Verdict: Just because one happened after the other, it doesnt mean the first one caused it. Bernard Matthews, the company that owns the British turkey farm, owns a turkey processing plant at Sarvar, Hungary, which sends 38 tonnes of partly processed turkey to the British plant weekly. But Sarvar is 260 kilometres from Csongrad making direct spread unlikely. Some Sarvar turkeys were slaughtered at a plant also handling Csongrad birds, so carcasses might have been some cross-contaminated. A smear of turkey on a workers shoe could have got it from processing plant to turkey barn.
Evidence: Britains only previous case of H5N1 was a dead whooper swan in a harbour in Cellardyke, Scotland, in March 2006. Could the virus have persisted in British birds? No other birds with H5N1 have been found in Britain, and officials have suggested that the swan was infected in Germany, and died while migrating over Scotland. But questions surround Britains wild bird surveillance. Scientists in Sweden find influenza viruses of all types, on average, in 14% of dabblers tested, with higher levels in autumn after nesting season. But last autumn the British testing programme found influenza in a tiny 0.8% of thousands of dabblers tested. This suggests the methodology is missing infected birds, and potentially H5N1. On the other hand, H5N1 is clearly rare; the best way to detect it might be to leave sentinel turkeys outdoors.
Verdict: Likely. The same wild British ducks that infected the Scottish swan may have infected the Bernard Matthews turkeys. A worker could merely have stepped in duck faeces then walked into a barn, say scientists.