'We cannot discount that there might be other factors here. Another bacteria, another virus, some sort of toxic substance or something happening in the environment,' said a WHO spokesman.

The vaccines, enough for 350,000 hogs, were sent Sunday from a company in southern Guangdong province to Sichuan province, the China Daily newspaper said. It said a larger shipment would arrive later so that up to 10 million pigs can be inoculated.

The Health Ministry reported a total of 198 confirmed or suspected human cases in Sichuan linked to the bacteria streptococcus suis, which has swept through dozens of villages and towns. Some 30 people were in critical condition and 18 have been released from the hospital, the ministry said on its website.

No Transmissions Between People

One case of the disease has also been reported in Guangdong province, hundreds of kilometers (miles) southeast of Sichuan.

In Hong Kong, which borders Guangdong, two infections have been reported since the latest outbreak.

Health officials have not made it clear whether any of the cases are related to the Sichuan outbreak.

The infected in Sichuan have been farmers who handled or butchered sick pigs and have developed symptoms including nausea, fever, vomiting, and bleeding under the skin. No transmissions between people have been reported.

Fears had been building that the epidemic was a re-emergence of SARS -- severe acute respiratory syndrome -- or bird flu. The Chinese government has sought to reassure the public that it was doing its best to prevent the spread of the disease.

In Ziyang, the Sichuan city where human cases were first found, compulsory vaccinations for streptococcus suis in pigs will be enforced, An Weining, director of the local animal husbandry department, was quoted as saying by the official Xinhua News Agency.

More Than One Type of Bacteria?

The World Health Organization cautioned that the outbreak could be caused by more than one type of bacteria.

"We cannot discount that there might be other factors here. Another bacteria, another virus, some sort of toxic substance or something happening in the environment," WHO spokesman Bob Dietz said in Manila. "We have to keep an open mind. It's not been resolved yet."

While precautions still need to be taken, Dietz said the disease "doesn't look like it's going to explode."

"It doesn't look like it's going to rip across borders," he said. "It's not something that's going to get on a plane and travel around the world and infect other people."

In 2003, China was criticized for being slow to release information on SARS, which first emerged in the south and killed almost 800 people around the globe before it was declared under control in July 2003.