A bacterium that rarely infects people but last year killed 38 in China is likely to have mutated into a more deadly form, according to the first study of the outbreak.
"The finding shows that there is potential for a pandemic," says lead author Tang Jiaqi of the Research Institute for Medicine of Nanjing Command, China, whose work is published in PLoS Medicine on April 11th.
The microbe Streptococcus suis is found in pigs worldwide, and usually only causes disease in piglets. Occasionally it infects farm workers who come into close contact.
But last year more than 200 people were infected in a major outbreak in Sichuan province, and nearly one in five of them died.
Despite no evidence of direct transmission between people, other features of the outbreak raised concern among researchers.
Until last year, only 200 human cases had been reported since 1968, and fewer than ten per cent died from the infection.
Previously, infected people have had meningitis or blood poisoning, but in Sichuan most infected people had symptoms of streptococcal toxic shock syndrome (STSS), which is usually caused by a different strain of Streptococcus bacteria.
The outbreak was similar to one in Jiangsu province in 1998 that killed 14 of the 25 reported patients.
According to Tang, most Streptococcus infections are easy to overcome with antibiotics, but STSS is hard to treat.
His team sequenced the genes from bacteria isolated from the two Chinese outbreaks and compared them with other harmful strains of Streptococcus suis.
The Chinese strains shared some genes that were not found in other strains.
"So far we do not have evidence for a direct link between the genetic difference and the higher lethality in China, but this possibility is very high," says Tang.
In an accompanying article in the journal, Shiranee Sriskandan and Joshua Slater from Imperial College in London say the emergence of a new animal disease that can infect and kill people is of global concern.
They say the international community must collaborate to identify differences between strains of the bacterium circulating in different parts of the world.