The death toll from a new strain of bird flu in China rose to six on Friday as health officials in North America say the risk here is low.  

"We have no sign of sustained human to human transmission," World Health Organization (WHO) spokesman Gregory Hartl told a news conference in Geneva.

Doctors and scientists closely watch for any sign that a virus is jumping from person to person to person, a defining feature in pandemics.

The U.S. Centers for Disease Control and Prevention issued an alert to doctors to help them identify any human cases of H7N9 as a precaution.

"It's a virus we haven't seen before infect man," influenza virus expert Dr. Gregory Gray, chair of the environmental and global health department at the University of Florida, said in an interview. "It's called a low pathogenic virus in the sense that it doesn't seem to have the characteristics to readily kill a lot of poultry."

si-china-bird-flu-masks-220

There's no sign of sustained human to human transmission of H7N9. (Chance Chan/Reuters)

All of the 16 confirmed cases so far seem to be isolated, Dr. Thomas Frieden of the U.S. Centers for Disease Control and Prevention told reporters.

As a precaution, Shanghai authorities began culling more than 20,000 birds at a poultry market after a pigeon at a market was found to carry a virus that was genetically similar.

Health officials are scrambling to learn where the virus is in the environment and how humans are exposed.

"If we don't understand the reservoir and the ecology of the virus, it's hard to design interventions to protect humans," Gray said.

Dr. Barbara Raymond, director of preparedness planning division for the Public Health Agency of Canada in Ottawa, said Canadians should be aware of what's happening in China. Raymond suggested those travelling in China should avoid contact with poultry and practice common-sense hygiene measures like hand washing and coughing into the sleeve.

"There is really no concrete plan to move forward with vaccine production at this point in time," Raymond said.

Provincial and territorial laboratories are able to test for the virus if needed and the National Microbiology Laboratory in Winnipeg is working intensely to develop more sophisticated tests for rapid and efficient diagnosis if needed, she added.

With files from CBC's Kelly Crowe, Kim Brunhuber and The Associated Press