The Netherlands banned poultry exports and started culling hundreds of thousands of chickens on Monday after an outbreak of avian flu was discovered 28 February. The avian influenza virus is usually restricted to birds, but European health authorities are watching the situation closely because bird-to-human crossover has happened before and can be fatal.
After working around the clock, a team of scientists at Erasmus University in Rotterdam led by virologist Ron Fouchier announced on Tuesday that the culprit behind the outbreak is a highly pathogenic mutant form of a less dangerous avian influenza. The mutation alters a surface protein of the virus, allowing it to dodge the immune system and infect cells throughout the bird's body, often resulting in fatal pneumonia. The sudden appearance of the mutant is "very surprising," says Eric Gingerich, an avian pathologist at the University of Pennsylvania School of Veterinary Medicine, because it usually appears only after the less pathogenic form has circulated among poultry for months.
The less pathogenic form of avian influenza is carried at low levels by wild birds. But within crowded chicken farms, the disease spreads like wildfire through drinking water contaminated by virus-loaded feces. Once the mutation transforms the virus into the highly pathogenic form, most infected birds die, often within days. The Dutch team believes that a duck or goose may be to blame for infecting the chickens, which have suffered up to 80% mortality on the 18 farms believed to be infected so far. Almost 50 farms surrounding the afflicted properties are also slated to be culled.
Although it's extremely rare, avian influenza viruses can spread to humans if they swap genes with the human influenza virus. This rare event is possible in pigs, which are vulnerable to both human and avian influenza. In 1997, such a strain killed six people in Hong Kong.
Like Hong Kong, the Netherlands has a high density of animals, including chickens and pigs. For this reason, and because the Netherlands is the top exporter of eggs in Europe, the Dutch government has moved rapidly with its poultry culling.