OTTAWA -- As they continue to analyze the new swine-flu virus, researchers are grappling with a puzzling question: Why are the Canadian and U.S. cases milder than those in Mexico?
It's not just an interesting question for scientists to ponder. Over the coming days and weeks, public-health officials will be watching like hawks to see if the milder cases are a sign the outbreak is dissipating, or whether such cases are just the prelude to a fullblown pandemic.
Infectious-disease experts note the Spanish flu pandemic started out mildly in the spring of 1918 before reemerging with a vengeance in the fall.
"We cannot be complacent, because this particular influenza strain, just like any other virus or flu strain, can still continue to mutate and it can mutate in either direction," said Dr. Colin Lee, associate medical officer of health at the Simcoe Muskoka District Health Unit.
There have been more than 150 suspected deaths in Mexico.
On Wednesday, the United States announced a toddler from Mexico City had died after catching the swine flu.
To date, most of the cases in countries other than Mexico have been relatively mild. Several theories are emerging to explain the discrepancy.
One is purely statistical.
In any flu outbreak, there will be a large number of mild cases and a much smaller number of severe, or even fatal, cases. Given that Mexico is the epicentre of the outbreak, it makes some sense more deaths would have been confirmed there so far, noted Dr. Gerald Evans, chief of the infectious-diseases division at Queen's University.
Experts also say the virus could have mutated, becoming milder as it migrates to other parts of the world.
It's also possible the strain found in places such as Canada and the U.S. is at an earlier stage of mutation than the original strain in Mexico, which would suggest Canada and the U.S. are in store for much more serious cases.
"The question really is are we looking at the earlier stage of the disease, which means the strain we are seeing now (in Canada and the U.S.) is just an earlier version of the Mexican strain," said Bhagirath Singh, scientific director of the CIHR Institute of Infection and Immunity at the University of Western Ontario.
There is also the possibility something in the genetic makeup of Mexicans is making them more susceptible, although some experts dismiss the hypothesis.
"We do not differ genetically that much from our Mexican brothers and sisters as some people might speculate, and genes very rarely have anything to do with the expression of infectious diseases," said Evans. Some point to poor health care infrastructure in some parts of Mexico, which would make it more difficult for some Mexicans to receive care, or even air pollution in Mexico City.
"This is a respiratory disease, and in parts of Mexico, especially Mexico City, there's a lot of pollution, and it may exacerbate respiratory infections," said Lee. It will likely be weeks before researchers have a solid hypothesis.
"The Mexican data may not be very reliable because of their testing and healthcare infrastructure, so they may not be seeing the milder cases," Lee. "It's so early right now that nobody has really confirmed the data."
-- Canwest News Service