China has declared an alert against the swine virus infection, which has killed 19 people in southwest China's Sichuan province so far.

• Mystery disease kills 17 in China

Preliminary probe shows the unknown disease in Sichuan was caused by a kind of a swine virus known as streptococosis II.

The province has reported 80 cases of the infection as of Monday, including 67 confirmed and 13 suspected. Nineteen people have been reported dead and 17 people are in critical condition. Four have been discharged from hospital, the ministry of health said.

The patients came from 75 villages in 40 townships in cities and counties including Ziyang City, Jianyang City, Lezhi County and Zizhong County in Neijiang City.

All the patients had direct contact with ill or dead pigs before showing symptoms of illness, experts said. The patients first reported fever, weakness and sickness, and then got symptoms of endemic bleeding and shock. The white cell count in their blood too soared.

Judging from the symptoms and tests of the dead pigs, experts reached the preliminary conclusion that the disease was caused by swine streptococosis II, Xinhua reported.

With the enlarged searching, the number of the affected people has been increasing, but the cases were dispersed in different areas with no obvious signs of epidemic relationships.

Swine streptococosis was first spotted in the 1950s and has been reported in many countries including France [Images], Denmark, the United States and Canada [Images], the report said.