Mild avian flu strains cause disease in wild birds

AMSTERDAM (Reuters) - Mild forms of avian flu can cause illness among wild birds, slow their migration and spread faster than previously believed, a study found on Wednesday.

The findings alter the view that wild birds did not suffer at all from low pathogenic bird flu. Although milder forms of bird flu are themselves harmless to people, they can pose a serious threat if mixed with human strains.

"The study sheds new light on the transmission of these type of viruses but also of highly pathogenic viruses in wild birds," said Ron Fouchier of the Department of Virology at the Erasmus Medical Center in Rotterdam, who took part in the research.

Current concern over bird flu is focused on the highly pathogenic H5N1 form of the virus which originated in Asia and has killed at least 164 people since 2003.

Scientists fear that H5N1 could mutate into a form easily spread between humans, sparking a worldwide pandemic.

The report, compiled by Dutch scientists and published in the U.S. scientific journal PLoS ONE, monitored 12 swans, two of which had low pathogenic flu strains H6N2 and H6N8.

Its results showed the infected birds launched their migration later, fed slower and flew shorter distances than the healthier birds.

"The fact that the birds get sick is certainly no guarantee that they cannot transmit the virus amongst each other," Fouchier said.

Fouchier added it was previously believed that birds infected with highly pathogenic flu could not transmit the virus because they would be too sick to fly.

"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," said Jan van Gils of the Netherlands Institute of Ecology, which also participated in the study.

Last winter, the Dutch scientists attached GPS navigation chips to the neck collars of the 12 swans and monitored them closely with the help of volunteer bird watchers.

The two infected birds launched their migration to spring breeding grounds in north-west Russia in late January-early February, a month later than the healthy swans.

The sick birds flew only 35 km (21.7 miles) instead of the usual 250 km to their next stop and their digestion seemed to be impaired, the study said. Both birds returned to the Netherlands in the beginning of this winter and appeared fully recovered.

Van Gils said acquiring more knowledge about mild strains was important because they had formed the origin of past pandemics such as the 1918 Spanish Flu, the worst in modern history, which killed about 50 million people.