The new strain of bird flu that has killed 17 people in China has been circulating widely "under the radar" and has acquired significant genetic diversity that makes it more of a threat, scientists said on Friday.
Dutch and Chinese researchers who analyzed genetic data from seven samples of the new H7N9 strain say it has already acquired similar levels of genetic diversity as much larger outbreaks of other H7 strains of flu seen previously in birds.
"The diversity we see in these first few samples from China is as great as the diversity we have seen with a large outbreak in the Netherlands several years ago and one in Italy," said Marion Koopmans, head of virology at the Dutch National Institute for Public Health and the Environment, who worked on the study as part of a nine-member team.
"This means it (the H7N9 strain in China) has been spreading quite a bit and it's important to understand where exactly that is going on."
Its genetic diversity shows the virus has an ability to mutate repeatedly and is likely to continue doing so, raising the risk that it may become transmissible among humans.
Koopmans, whose research was published in the online journal Eurosurveillance, said the circulation would probably have taken place in either birds or mammals, but said exactly which animals were involved was not yet clear.
"Simply the fact that this virus is spreading under the radar - because that is what this data confirms - is of concern," she told Reuters in a telephone interview.
The H7N9 virus is so far known to have infected 87 people in China, killing 17 of them. Health officials raised further questions on Friday about the source of the new strain after data indicated that more than half of patients had had no contact with poultry.
A scientific study published last week showed the H7N9 strain was a so-called "triple reassortant" virus with a mixture of genes from three other flu strains found in birds in Asia. One of those three strains is thought to have come from a brambling, a type of small wild bird.
For their study, Koopmans and her team compared some data from the first two weeks of the China H7N9 outbreak with data from a large H7N7 flu outbreak in birds and people the Netherlands in 2003 and an H7N1 epidemic in birds in Italy in 1999 and 2000.
The Dutch outbreak resulted in infection of poultry on 255 farms and led to the culling of about 30 million chickens. Some 89 people were also diagnosed as having the H7N7 virus and one person, a vet, died as a result of the infection.
The comparison suggested that "widespread circulation (of the H7N9 strain in China) must have occurred, resulting in major genetic diversification", the researchers wrote in their study.
They added: "Such diversification is of concern, given that several markers associated with increased risk for public health are already present."
Flu experts in China and at the World Health Organization say there is no evidence so far that H7N9 is passing easily between people.
But scientists who analyzed the genetic sequence data from three early samples from China say the virus has already acquired some mutations that might make it more likely to be able to do so in the future, raising the risk of a human pandemic.
"Although human infections with H7 influenza viruses have occurred repeatedly over the last decades without evidence of sustained human-to-human transmission, the absence of sustained human-to-human transmission of H7N9 viruses does not come with any guarantee," Koopmans' team wrote in their study.