Story highlights There have been 77 confirmed cases and 16 confirmed deaths

Boy was detected from a target group of poultry vendors in Chaoyang District

Authorities say there is no evidence so far of human-to-human transmission

Beijing has begun monitoring susceptible groups more intensely

Doctors say the discovery of a 4-year-old carrier of the H7N9 bird flu virus who shows no symptoms of the potentially lethal virus is a worrying development that could make the spread of the infection more difficult to monitor.

The Beijing Municipal Health Bureau said the boy was detected from a group of close contacts of the first infection in Beijing and the laboratory results showed the boy was an asymptomatic carrier of the disease.

The boy emerged after authorities screened 24 poultry owners in Naidong Village, Cuigezhuang County in Chaoyang District, taking throat swabs of those in contact with the group.

The report said the boy's parents were engaged in poultry and fish trading.

According to Dr. Leo Poon Lit-man, associate professor, School of Public Health at the University of Hong Kong, while there is no evidence that the virus can be transmitted from human to human, medical authorities need to be on the alert.

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"It really depends on whether this H7N9 virus is transmissible between humans ... that key question needs to be answered," Dr. Poon told CNN. "So far, we do not know the full spectrum of the clinical presentations of these H7N9 patients.

"In the beginning, we were only able to detect it because there were some severe cases and some people actually died. They are trying enhance the surveillance so that's we can detect those cases where there is only a mild infection."

H7N9 is normally found in birds. Until last month, it was never known to infect people.

China has invited a team of international experts to assess the spread of the H7N9 bird flu virus in the country, the World Health Organization said Tuesday.

A team composed of Chinese experts on epidemiology, laboratory tests and clinical treatment, as well as experts and officials from the U.S, the European Union and the WHO, "will assess the outbreak and the response to guide further the prevention and control measures," Glenn Thomas, spokesman for the World Health Organization told reporters in Geneva.

Poon said evidence of human-to-human transmission would put the H7N9 problem into a different category.

"If it can (transmit human to human), then it's a very different scenario -- we would not be able to detect asymptomatic carriers and that would be a huge problem," he said.

The discovery of the carrier has meant Beijing has notched up a gear in its monitoring efforts, extending its action from a passive 'syndrome surveillance' to a more active 'pathogen monitoring'.

This involves actively seeking out and targeting populations at risk from the disease.

Health officials in China say there have been 77 confirmed cases of H7N9 and 16 confirmed deaths.

Several cities in eastern China have suspended trading in live poultry in an effort to contain the problem.