TORONTO - Doctors testing patients for the new swine flu virus are finding lots of cases of seasonal flu too, an odd development so late in the flu season, the U.S. Centers for Disease Control said Friday.
About half of people testing positive for influenza in the United States these days are infected with the new virus, the CDC's Dr. Dan Jernigan said during a media teleconference.
But that means the other half are testing positive for seasonal flu viruses at a time when flu activity generally wanes sharply in the Northern Hemisphere.
The CDC said part of the increase is certainly due to the fact that much more influenza testing is going on these days, because of concerns about H1N1 flu. But the agency said it seems that it's not just a case of more testing, but more flu too.
"We would be expecting to see the season slowing down or almost completely stopped from the kinds of surveillance systems that we normally monitor," said Jernigan, deputy head of the CDC's influenza division.
"But what we're seeing is that there are some areas that actually have reports of the amounts of respiratory disease that are coming into their clinics that are equivalent to peak influenza season, and so that's an indicator to us that there's something going on with the amount of influenza disease out there."
A spokesperson for the agency said flu season in the U.S. got off to a late start this past winter and that may be playing a part in this late season surge.
"For this time of year the level of activity we're seeing is unusual," Tom Skinner admitted.
"The bottom line is we are seeing an unusual level of flu activity for this time of year right now. That's the bottom line."
Parts of Canada are also seeing this unexpected late surge in seasonal flu cases.
Dr. Allison McGeer, head of infection control at Toronto's Mount Sinai Hospital, said about 20 per cent of people who are being tested for influenza-like illness in Toronto are testing positive for flu.
Twenty per cent is about the level of positive tests one would see during the peak of flu season, she said.
About half of those positive tests are for the new H1N1 flu virus, McGeer said, but the other half are for H3N2, one of the seasonal flu subtypes.
Others aren't as certain there is an unusually high level of flu activity now, feeling the surge in detected cases is mainly due to the fact that many more people are seeking medical care if they are suffering from flu-like symptoms.
Dr. David Patrick of the British Columbia Centre for Disease Control said his centre's surveillance shows a sharp increase in doctor visits, but no increase in things like deaths due to pneumonia or influenza, which would be expected if there was an increase in cases.
"Those haven't budged," he said of the mortality statistics. "What has budged has been a lot more people going to their doctors and from the labs for a period of about a week, 10 times the test volume we would have expected at this time of year."
"So if you just go on numbers of flu picked up, I think there's a pretty good explanation."
And influenza expert Dr. Arnold Monto also felt the answer to the puzzling increase may lie in heightened influenza surveillance.
"We've never looked this hard," said Monto, of the University of Michigan's School of Public Health.
