According to the Ministry of Agriculture, apart from poultry and wild birds, monitoring pigs may provide a deeper comprehension of H7N9, including its origin, host range, route of transmission and the degree of harm. The Ministry issued an emergency monitoring plan this week in response to the outbreak.

Experts said that pigs have been included in the plan in an effort to better explore the transmission route, since it is still unknown why and how the bird flu virus infects human beings.

WHO (World Health Organization), on its website, said that its analyses of the genes of influenza A H7 viruses have shown signs of adaptation to grow in mammalian species.

The monitoring plan is trying to find evidence of adaptation, if any, said Sun Quanhui, a science adviser at the World Society for the Protection of Animals.

"By expanding the exclusion range of virus infection, the authorities will provide more scientific evidence that will help control and prevent human infection of H7N9," said Mr Sun.

According to Yao Jiezhang, a veterinarian and spokesman for the Beijing Bureau of Agriculture's animal husbandry and veterinary station, if pigs were found carrying the virus, people are likely to get infected more easily.

According to the monitoring plan, the animal disease control and prevention center of the Agriculture Ministry will take samples nationwide from live bird markets, slaughterhouses, poultry farms and wild bird habitats, and test them for viruses.