SHANGHAI – Medical experts have determined that the rare H7N9 type of bird flu that has infected at least seven people around the Shanghai area, leaving two dead and the others critically ill, is likely a pathologically virulent strain that could cause serious medical complications once infected, sources close to the study said Wednesday.

The World Health Organization has said so far there has been no evidence of human-to-human transmission, but medical experts believe the H7N9 virus could easily mutate into a strain making person-to-person infection possible.

“There are abnormalities in the DNA of the H7N9 virus, which is likely to make it easy for get people infected,” said Masato Tashiro, director of the Influenza Virus Research Center under Japan’s National Institute of Infectious Diseases.

The seven H7N9 infection cases reported by Chinese authorities since Sunday are the first known cases of human infection, according to the WHO.

There has been speculation that the dumping of thousands of dead pigs into Shanghai’s main river last month could have been related to the outbreak of the H7N9 infection, but Chinese authorities say they have yet to pinpoint the source.