TORONTO - Concurrent outbreaks of human cases of swine flu in the southwestern United States and a mystery respiratory illness in Mexico had health authorities across the continent struggling Thursday to figure out what is unfolding.
Laboratories in Atlanta and Winnipeg are looking at clinical specimens trying to determine if the mounting number of human cases of swine flu -- seven and counting -- in California and Texas, and an unusual explosion of severe respiratory illnesses in Mexico are pieces of the same puzzle or confusing coincidences.
Even on its own the human infections with swine flu viruses are significant enough to have experts wondering whether the world is watching the start of a flu pandemic.
But official pronouncements are being crafted with caution by authorities, who remember all too well the 1976 swine flu scare -- a feared pandemic that didn't materialize.
The World Health Organization, which has been monitoring the situation, said at this point it does not believe the threat of a pandemic -- the first since the 1968 Hong Kong flu -- has risen.
"We would not see any evidence yet for this being a phase change," said WHO spokesman Gregory Hartl, referring to the organization's six step pandemic alert scale. The world is currently at phase 3 -- occasional human cases with a novel flu virus -- because of ongoing sporadic human infections with the H5N1 avian flu virus.
"We never want to overestimate or underestimate something, so we're following it closely," Hartl said from Geneva. "But as of the moment we see no evidence to change the global pandemic phase."
The U.S. Centers for Disease Control, which earlier in the week revealed they had found two human cases of swine flu, announced Thursday that the number of confirmed cases has risen to seven. There are two clusters, a father-daughter pairing and two 16 year old boys who attended the same school.
Symptoms are similar to seasonal flu, though there are more reports of vomiting and diarrhea that is seen with regular flu. All of the cases have recovered, though one person required hospitalization. It appears none of the cases had contact with pigs.
"We believe at this point that human-to-human spread is occurring," said Dr. Anne Schuchat, director of the CDC's national centre for immunization and respiratory diseases.
"That's unusual. We don't know yet how widely it's spreading. And we certainly don't know the extent of the problem."
Genetic sequencing done to date reveals a seemingly unique influenza A virus of the H1N1 subtype made up of a distinctive mix of swine, bird and human flu virus genes. There have been no reports of this virus in pigs, said Dr. Marie Gramer, a swine flu expert with the University of Minnesota's college of veterinary medicine.
"It doesn't seem to be very similar to anything that is currently circulating, from what I have," said Gramer, who has an extensive library of swine flu virus isolates.
The CDC said they expect to confirm more cases in coming days as contacts of the cases are checked and doctors and clinics are put on notice the CDC wants samples of flu viruses that can't be identified by standard methods.
They also cautioned they can't currently tell if the events unfolding represent something new or something that happens occasionally but which has gone unnoticed in the past.
"We have improved and intensified influenza surveillance over the past few years. So it certainly is possible that events like this occurred in the past and we simply didn't detect them," said Dr. Nancy Cox, head of the CDC's influenza division.
The first of the known cases fell ill on March 28 and the most recent developed symptoms last Sunday. Most of the cases have been children or teenagers, though two are adults. Ages range from nine to 54 years. They live in and around San Diego and in San Antonio, Tex., cities that are nearly 3,000 kilometres apart. Except for the family pairing and the school mates, there don't appear to be links among the cases.
No infections with this virus have been found in Canada, Canadian authorities said.
Nor does it seem that any cases of an unusually severe respiratory illness plaguing parts of Mexico have turned up in this country, said Dr. Danielle Grondin, assistant deputy minister for the Public Health Agency of Canada.
One person from Ontario was ill after returning from Mexico but Grondin said it seemed unlikely the illness was related to the apparent outbreak, which is affecting mainly young, previously healthy adults.
Alerts that have been sent out to health providers and public health officials in Canada say 20 deaths have been associated with the outbreak, which seems to be centred around central Mexico and Mexico City.
Grondin said Canada would issue a travel advisory warning Canadians who plan to travel to Mexico of the problem, though it was not released Thursday.
And she said the National Microbiology Laboratory in Winnipeg is testing samples from Mexico looking for the cause or causes of the illnesses. Samples were also en route Thursday to the CDC's labs in Atlanta, Cox said.
Schuchat said there is no evidence yet that what is happening in Mexico is linked to what is happening in California and Texas.
"At this point, we do not have any confirmations of swine influenza in Mexico. We are working with them in terms of understand what they are seeing clinically as well as the virus characteristics of what they're seeing," she said.
A key test will be to see if any of the samples from Mexico contain the unusual swine flu virus being found in the southeastern United States.
"It will be critical to determine whether or not the strains of H1N1 isolated from patients in Mexico are also swine flu," said Dr. Donald Low, an infectious diseases specialist at Toronto's Mount Sinai Hospital.
