The infection of pigs with swine flu does not by itself pose a threat to human health.

Though nervousness about the virus has prompted several countries, including Russia and China, to restrict pork inputs from affected nations, and Egypt is slaughtering its entire pig herd, there is little evidence that it can be passed on through the food chain.

The influenza virus is very efficently killed by heat, and the World Health Organisation (WHO) advises that so long as pork, bacon and ham are properly handled and thoroughly cooked, there is no danger.

Peter Ben Embarek, a food safety scientist with the WHO, said: “This is not a food-borne disease, you do not get this disease from eating pork. As long as it is cooked in the way you normally cook pork, there is no danger of getting the disease.”

Cured pork products such as Parma ham and chorizo are also safe to eat. “There is no reason to start destroying these wonderful meat products,” Dr Ben Embarek said. “To produce a ham takes a long time and the processes involved are also believed to inactivate any virus that might have been on the raw material. You can continue to eat your prosciutto safely.”

The WHO has also said that import bans and preventative slaughter are unnecessary, and even changed the offical name of the virus to influenza A (H1N1) to try to reassure people that it was not being spread by pigs.

While genetic analysis indicates that both the viruses that gave rise to H1N1 originated in swine, and thus that they were almost certainly mixed in a co-infected pig, no pigs that carried it had been identified before the Alberta incident.

The cross back to pigs raises some concern because it could create a fresh opportunity for the virus to evolve in new and potentially unpleasant ways.

When any animal becomes infected with two different influenza strains at the same time, these can swap genes to create an entirely new virus with unpredictable qualities.

This process, known as reassortment or antigenic shift, gave rise to the new swine flu strain, and a sustained spread to pigs could increase the chances of another such mutation arising. This sort of evolution, however, could also occur in a human host infected with, for example, both swine flu and a seasonal flu strain.

The biggest worry would be if a person or a pig became infected with both swine flu and H5N1 avian flu at the same time. As the former is highly transmissible but does not appear to be particularly lethal, while the latter is highly virulent but does not spread easily, a reassortment between the two could generate a very dangerous strain.

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