Four people in east China´s Jiangsu Province have been confirmed as being infected with the lesser-known H7N9 bird flu, bringing the total number of infections in the country to seven.

The four, from four cities in Jiangsu Province, are in critical conditions and under emergency treatment, the Jiangsu provincial health bureau said Tuesday in a statement.

The four were confirmed as human infections with H7N9 avian influenza by an expert team summoned by the provincial health bureau, based on clinical observations, laboratory tests and epidemiological surveys Tuesday afternoon, the statement said. No mutual infections were discovered among them.

A total of 167 people who had come into contact with the four showed no symptoms of fever or respiratory illnesses, it said.

The four included a 45-year-old woman from Nanjing, a 48-year-old woman from Suqian, a 83-year-old man from Suzhou, and a 32-year-old woman from Wuxi, it said.

The woman in Jiangning district of Nanjing, surnamed Xu, fell ill with flu symptoms on March 19. She was transferred to a hospital intensive care unit in Nanjing on March 27 after her condition worsened, the statement said. She is a poultry culler.

The woman from Shuyang county of Suqian City fell ill on March 19 and was transferred to a hospital intensive care unit in Nanjing on March 30.

Tests by the Jiangsu provincial center for disease control and prevention found the two women positive of the H7N9 strain on March 30 and further tests by the Chinese center for disease control and prevention confirmed the results on April 2.

The man from Wujiang district of Suzhou City became ill on March 20 and was admitted to a local hospital on March 29. He was first tested positive of H7N9 bird flu on April 1.

The woman from Binhu district of Wuxi City fell ill on March 21 and was transferred to an intensive care unit of a hospital in Wuxi after her condition worsened on March 28. She was first tested positive of H7N9 bird flu on March 31.

On Sunday, three H7N9 bird flu cases were reported, two in Shanghai and one in Anhui, the first human infections of the bird flu strain. The two in Shanghai died and the one from Anhui is in critical condition and under treatment in Nanjing.

It is unclear how the three got infected, and no mutual infections were discovered among them, said the National Health and Family Planning Commission Sunday. No abnormalities were detected among 88 of their closest contacts.

The subtype of H7N9 bird flu virus has not been contracted to human beings before. The virus shows no signs of being highly contagious among humans, according to the clinical observation on the cases´ close contacts.

However, as only three cases of human infection of H7N9 have been found, relatively little research has been done on it. The expert team is working to study the toxicity and human-infection capacity of the virus, the commission said Sunday.

There are no vaccines against the H7N9 bird flu virus either at home or abroad.