Mutation fears grow after human gives virus to pig

200 animals develop flu symptoms but none die on farm in Alberta, Canada

A pig farm in Canada has seen the first documented instance of swine flu being transmitted from humans back into animals, raising concerns that the H1N1 virus at the centre of the outbreak may mutate into a more virulent form.

A Canadian farm worker transmitted the H1N1 virus to the swine herd in Alberta after a trip to Mexico. About 200 pigs from a herd of more than 2,000 developed symptoms, but none have died, health officials said yesterday.

The World Health Organisation was informed of the human-to-pig transmission on Saturday and its officials have since called for increased surveillance of pig farms, especially in countries with H1N1 infection in humans. "If this happened once, it could happen again," a WHO spokesman said.

Scientists are concerned that the recycling of the H1N1 virus by repeated transmission between humans and pigs may increase the chances of it mutating into a form that could lead to a higher mortality rate in humans.

The WHO said that the virus isolated from the Alberta pig farm seemed to be the same as the one in humans. However, there is a risk that the current wave of "mild" flu may die down in the northern hemisphere this summer and return in the autumn in a more aggressive form.

As of yesterday, 787 confirmed cases of H1N1 flu had been recorded in 17 countries, though mostly in Mexico and the US, a WHO spokesman said.

Steps by China to confine Mexicans to hotels and other sites, irrespective of whether they were showing symptoms of having the virus, triggered a diplomatic row with Mexico on Sunday.

A Mexican embassy official in Beijing said Chinese authorities had quarantined more than 50 Mexican business people and tourists, fearing the spread of H1N1 flu, even though only one had symptoms. Mexican Foreign Minister Patricia Espinosa on Saturday described the steps as "discriminatory", saying: "Mexican citizens showing no signs at all of being ill have been isolated under unacceptable conditions." Beijing said its steps were justified and lawful.

Over the weekend, evidence emerged that the virus is spreading south and east. Hong Kong and South Korea reported their first cases, as did Costa Rica, the first case of H1N1 in Central America.

In Britain, the Department of Health announced three new cases: a man in Ayrshire and two children in London – bringing the total in the capital to five. "A 14-year-old child from Barnet has been found to have the virus following contact with a visitor to Mexico. An 11-year-old child in Wandsworth has been found to have the virus following a visit to the US," a health spokesman said. "Both are being treated at home with antiviral drugs and are responding."

However, the official figures may not be up to date. Kate Corbett, 29, from west London, was told yesterday by the Health Protection Agency that she has confirmed swine flu after a visit to Mexico, but her case had not yet been registered by the Department of Health.

The WHO said it did not plan to increase the pandemic level from phase 5 to phase 6, the highest alert, indicating that a pandemic is under way. For phase 6, sustained, community-wide transmission within at least two WHO regions has to be confirmed.

Sustained community transmission – the virus passing many times between people not part of the same family or circle of friends – has been documented only in Mexico and the US, a single WHO region. A surge in Spain, where 20 people were confirmed with the virus yesterday, is being closely monitored.

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