Low-pathogenic forms of bird flu do cause illness among birds: Mild form of bird flu slows down migration of swans


Bewick's Swan: (photo: Steve Round).

Migratory swans carrying a mild form of avian influenza depart from The Netherlands more than a month after their healthy counterparts do. They also feed more slowly and fly shorter distances according to research published today in PLoS ONE, the International, peer-reviewed, open-access, online publication from the Public Library of Science (PLoS) by scientists from the Netherlands Institute of Ecology (NIOO-KNAW) and the Department of Virology of the Erasmus MC. This contrasts with previous ideas that mild forms of bird flu do not cause illness among wild birds. Moreover, these patterns can affect the rate of spread of avian influenza.

Wild birds were thought not to suffer from mild forms of avian influenza. But new data suggest that so-called 'low-pathogenic' avian influenza viruses do affect the lives of birds. The Bewick's Swan is a migratory bird that breeds in northwest-Russia and overwinters in northwest-Europe, especially in The Netherlands. In this species, a team of ecologists and virologists showed that infected individuals initiate their migration by the end of January or early February, while uninfected individuals have already done so by the end of December. Also, their next 'fuelling station' is only 35 km away instead of the usual 250 km. Sick birds fuel at a lower rate: they take fewer bites per day and their digestion seems to be impaired. Presumably, this is due to their need to channel more energy towards their immune system.

Content continues after advertisements

Being ill is no fun for us, but for a migratory bird being ill could potentially mess up its whole calendar for the rest of the year. According to scientist Jan van Gils from NIOO, "Infected swans clearly suffer from their 'mild' disease. The late departure from the wintering grounds could lead to late arrival in the breeding grounds, and thus to a lost breeding season. All in all, these low-pathogenic viruses have a much greater impact than previously thought. Because of their slower migration, ill birds get in touch with many more healthy birds passing by them on migration. In this way the virus can spread itself more rapidly than previously thought."

Acquiring more knowledge about mild but illness-causing avian influenza viruses is very important. Van Gils adds, "These mild virus-types always formed the origin of massive pandemics such as the Spanish Flu. Only such viruses that are non-lethal to birds can be spread easily by (wild or captive) birds, simply because the birds stay alive." Only after mixing with human flu can such a low-pathogenic avian flu cause the nightmare of a deadly pandemic among humans. "High-pathogenic avian flu that causes death among birds seems to originate from intensive poultry farms."

In the Dutch study 25 swans were investigated. The birds were caught in polder Wieringermeer (northwest-Netherlands) last winter and were measured, weighed and screened for the prevalence of avian influenza viruses. Twelve birds were equipped with a GPS attached to their neck collar (80 g in total). This enabled the researchers to follow the birds' movements in great detail. When within 300 m of the bird, all "travel data" were downloadable by modern wireless technology. Birds were found by volunteer birdwatchers reading and reporting the code on each bird's neck collar. Among the twelve neck-collared birds, two were infected with bird flu: type H6N2 and H6N8. These mild virus types are common among free-living waterfowl. From antibodies found back in blood samples, it could be concluded that a few more individuals had been infected before but were fully recovered and behaved as the other healthy birds. Both birds that were infected last winter have returned this winter and seem fully recovered too.