China's agriculture authority has expanded the monitoring for the H7N9 strain of bird flu to pigs in order to have a better understanding of the virus that had infected at least 33 people by Wednesday.

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

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.

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

"These adaptations include an ability to bind to mammalian cells and to grow at temperatures close to the normal body temperature of mammals (which is lower than that of birds)," it said.

The monitoring plan is trying to find evidence of adaptation if there is 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 Sun.

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

"Since we share more similarities in cells and body temperature with pigs than poultry, the likeliness of receiving the virus from pigs is larger than from birds," he added.

According to the 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.