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.