TORONTO — If Canadian scientists want to conduct research on H5N1 flu viruses modified to enhance their ability to spread, the work will have to be done in laboratories with the top level of biosecurity, the Public Health Agency of Canada says.

The agency said in a statement that any research on the viruses in this country would need to be done in labs designated as Containment Level 4 -- known as BSL4 labs elsewhere. The only Level 4 labs in Canada are located at the National Microbiology Laboratory in Winnipeg.

That requirement is a step up from the one governing research on regular H5N1 viruses. Canadian scientists are allowed to work on those viruses in Level 3 laboratories.

For the time being, the advice is moot; the viruses in question are locked up in labs in the Netherlands and the United States. And while controversy rages over whether the teams that created them should be able to publish their work in scientific journals, it's unlikely those labs will share samples, especially across international borders.

While it may not have immediate implications, the decision is interesting. There are calls in some quarters to limit future work on these viruses to Level 4 labs -- a move that would substantially restrict the number of teams that could work on them. There are fewer Level 4 labs in the world and neither the Dutch nor the American teams have Level 4 labs at their institutions.

The guidance relates to viruses that were created by Dutch and American researchers trying to see if H5N1 viruses could adapt to transmit easily among people. The viruses spread like wildfire among poultry, but they rarely infect people. And while limited human-to-human spread has been reported, to date the viruses haven't acquired the ability to spread easily to and among people in the way human flu viruses do.

The research groups took different paths, but both came up with viruses that did spread readily among ferrets, which are considered the best animal model for predicting how a flu virus will behave in people. (It is impossible to test these viruses in people to see if the ferret-transmissible viruses would spread person to person as well.)

When the researchers submitted their studies for publication, a committee of biosecurity experts who advise the U.S. government recommended the journals should be asked to redact key sections of the studies. The National Science Advisory Board on Biosecurity said publishing the full studies would be too dangerous.

The journals and the researchers have tentatively agreed to hold back the methods sections of the studies, as long as a system can be created by which the information could be shared with scientists and public health officials with a legitimate need to know.

A spokesperson for the National Microbiology Laboratory explained the biosafety advisory was drawn up following a risk assessment the Public Health Agency completed on the enhanced H5N1 viruses.

"No laboratories in Canada are currently working on or possess efficiently transmissible engineered influenza A/H5N1 viruses," Robert Cyrenne said in an email.

He added that any researcher who wanted to import samples of the viruses to Canada would need to obtain an import permit.