The World Health Organization has urged China to carry out further tests on the pig disease that official figures now show has killed 36 people since June.
Almost 200 people have become infected since June
The WHO said the high mortality rate could mean other factors were involved.
"Another bacteria, another virus, some sort of toxic substance or something happening in the environment, we don't know," said WHO spokesman Bob Dietz.
The agency praised China's diligence in informing them of the latest developments on the outbreak.
It is thought to be the biggest recorded outbreak of infection from the streptococcus suis bacteria.
However, correspondents say Chinese media coverage of the outbreak has been restricted.
China's health ministry says the disease is still spreading through the south-west province of Sichuan where the number of confirmed or suspected cases of people infected is now at 198.
Cases were originally identified near cities including Ziyang and Neijiang, but it has now spread to Chengdu and four other Sichuan cities.
China's health ministry is in the process of producing vaccine doses for 10 million pigs.
A further two cases have been reported in Hong Kong, reports the Associated Press news agency.
The health secretary there says that the only cause seemed to be streptococcus suis.
"At the moment we are reasonably sure it is caused by streptococcus suis. There is no evidence of any co-infection at the moment," York Chow said.
Sichuan is China's largest pig-rearing centre, producing more than 50 million swine annually.
Officials have distributed posters warning farmers not to eat sick animals and to alert the authorities if their livestock becomes infected.
Most cases have occurred in poor villages, where it is common for locals to butcher and eat sick animals rather than send the swine to market.
The streptococcus suis survives in faeces, dust and carcasses.