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Deadly effect of flu in Mexico puzzles experts

By Scott LaFee Union-Tribune Staff Writer — April 29, 2009

As the worldwide number of swine flu cases climbs, one question confounding infectious disease experts is why the unusual strain seems more virulent in Mexico than elsewhere.

At last count, nearly 2,500 people in Mexico were reported sickened by the virus and it is suspected in 159 deaths. Countries from Canada to New Zealand have confirmed cases, but none has reported fatalities from the outbreak except Mexico.

In the United States, the swine flu has been scattered and mild, with cases seemingly no worse than seasonal flu.

Early testing indicates the same strain of swine flu is afflicting Mexico, the United States and other nations. To Dr. Douglas Richman, a professor of infectious diseases at UCSD Medical Center and the San Diego VA Healthcare System in La Jolla, that suggests the difference in the severity of the outbreak may be a matter of numbers.

"We don't know yet how many people in Mexico have been infected with this virus and how many recovered without any problems," he said. "If that total number is thousands more without a huge increase in fatalities, then the death rate would decline considerably and this strain would be more typical of other flus."

Joan Nichols, an infectious disease specialist, agreed.

"Right now, we're in a state of imperfect knowledge," said Nichols, an associate director of the Galveston National Laboratory at the University of Texas Medical Branch. "There's no doubt that there's swine flu in Mexico, but is it responsible for all of the attributed deaths? There may be other causes. We just don't know yet."

Keiji Fakuda of the World Health Organization said the reason for the dichotomy between Mexico and elsewhere was among flu scientists' more pressing questions. "This will be the object of a great deal of research and attention, but at this time we can't say why there appears to be a difference," he said.

Nonetheless, many hypotheses have emerged.

Maria Teresa Cerqueira, chief of the U.S.-Mexico border office of the Pan American Health Organization, part of the World Health Organization, said she suspected Mexico's greater severity was due in part to limited public health preparedness and communication.

"There were problems getting information out of Mexico when the first cases came in," she said. "There was difficulty confirming that it was a new strain of swine flu. Those first samples had to be sent to Canada for testing."

When the crisis erupted, only two laboratories the U.S. Centers for Disease Control and Prevention in Atlanta and a Canadian national lab in Winnipeg were capable of identifying the new strain. Yesterday, California opened its public health lab in Richmond and Mexican officials said they are starting swine flu testing at a few facilities, including one in Mexicali.

Cerqueira also said the Mexican outbreak was likely worse because nobody initially recognized the threat or its potential consequences.

"I don't think (infected) people were going to their doctors because they didn't know they should," she said. "And I don't think doctors or health officials were immediately aware of the scope of the problem."

Another likely explanation is that there have been too few cases outside Mexico for the full spectrum of the disease to emerge elsewhere. Richard Besser, acting director of the CDC, predicted that U.S. deaths would be reported as more domestic cases are confirmed.

Less likely are other suggested theories, such as the Mexican flu strain being more lethal because of the presence of another unidentified but exacerbating microbe, nutritional deficiencies in the victims or improper medical treatment. Richman expressed doubt about those possibilities.

"That would make this virus different from all other flu viruses, and flu viruses don't behave differently," he said.

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