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UK tests could miss bird flu in migratory birds
21 May 2007
NewScientist.com news service
Debora Mackenzie

Discrepancies between levels of flu recorded in British birds and those elsewhere in Europe have surfaced yet again, following publication of the biggest study so far of bird flu in Europe. The findings raise concerns about whether British tests would pick up H5N1 bird flu if migratory wild birds carried it into the country.

Researchers in the Netherlands and Sweden analysed influenza A samples taken from 37,000 water birds in northern Europe over eight years. They confirmed that dabbling ducks, especially mallards, are the main carriers of flu viruses, although other birds, including geese, are occasional carriers. On average 6 per cent of ducks are infected, falling to a few per cent in spring and peaking at up to 20 per cent or more in autumn (PLoS Pathogens, DOI: 10.1371/journal.ppat.0030061).

In contrast, UK measurements of flu incidence in ducks remain low. In 2005 the Veterinary Laboratories Agency (VLA) in Addlestone, Surrey, found flu in 0.1 per cent of birds tested (New Scientist, 15 April 2006, p 12). In late 2006 it found flu in only 0.7 per cent of 1800 dabbling ducks. It did not test wildfowl wintering near the first British outbreak of H5N1 bird flu in East Anglia this spring, although some had migrated from Siberia, where the virus is known to circulate.

The main purpose of the European sampling programme is to map the prevalence of ordinary influenza A viruses in wild birds - usually benign gut infections in water fowl. The H5N1 variant, which has now killed at least 172 people, is a highly pathogenic mutant that is mainly spread in poultry but has also passed back into wild birds.

If H5N1 has managed to persist in wild populations, it is at a low level. It was not detected in the latest study, which included 13,000 dabbling ducks either migrating across Sweden after breeding in Siberia or on their Dutch wintering grounds.

The new study confirms that ducks mainly catch flu while breeding and clear it afterwards. "The farther you get from the breeding grounds, the fewer have flu," says Ab Osterhaus of Erasmus University Rotterdam, who led the Dutch team.

Even so, a third of Dutch wintering ducks - which are at a similar point in the migration path to British ducks - were infected, in contrast with the low UK findings. Similarly, a recent large-scale study of flu in wild African birds found flu in 6.6 per cent of dabbling ducks that had migrated from Eurasia, in spring, when flu is lowest (Emerging Infectious Diseases, vol 13, p 626).

One possible reason for the discrepancy is that in the UK faecal samples are stored dry, unlike elsewhere. This can allow viral RNA to decompose. However, Ian Brown of the VLA says his lab has found the virus has similar viability in wet and dry samples. He attributes the UK differences to variation in the place and time of sampling.

"The only way to tell is for everyone to work together, collect samples and compare techniques," says Osterhaus.

From issue 2604 of New Scientist magazine, 21 May 2007, page 12

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