MONDAY, June 29 (HealthDay News) -- Countries that received the most airline passengers from Mexico this spring were the most likely to see H1N1 swine flu infection, new research says.

The finding confirms that tracking global flight patterns to determine where an infectious disease may strike next could provide governments and public health officials with a means of preventing and dealing with such threats, according to an analysis by researchers in Canada.

"Infectious diseases don't respect national boundaries, and we live in an incredibly interconnected world," said Dr. Kamran Khan, an infectious disease physician and scientist at St. Michael's Hospital in Toronto. "Yet, infectious diseases do follow airline flight routes. If we can understand how people move around the world, we can understand how infectious diseases are likely to spread around the world."

The findings were published online June 29 in a letter to the editor in the New England Journal of Medicine.

Using an extensive database of global air traffic and passenger itineraries, the research team analyzed information on 2.35 million passengers traveling from Mexico to more than 1,000 cities in 164 nations in March and April 2008. Swine flu emerged this spring, but because passenger data from 2009 was not yet available, the investigators used 2008 flight information, noting that air travel patterns in March and April change little from year to year.

From Mexico, nearly 81 percent of air passengers flew to the United States or Canada, while 8.8 percent went to Central America, South America or the Caribbean Islands, 8.7 percent flew to Western Europe, 1 percent went to East Asia and 0.8 percent flew elsewhere, the researchers found.

The United States received the bulk of passengers from Mexico, with about 1.74 million arrivals, followed by Canada with 149,137 arrivals, France with 47,501, then Spain, Germany, Cuba, Argentina, Italy, Brazil, Guatemala, United Kingdom, Colombia, Japan, Chile, Venezuela, Panama, Costa Rica, Netherlands, Peru and Switzerland.

Cities receiving the most arrivals from Mexico were Los Angeles, New York, Chicago, Miami, Houston and Minneapolis-St. Paul.

Of the top-20 destination countries, only one -- Venezuela -- had no confirmed cases of swine flu as of May 25, 2009. Japan, Chile and Peru had confirmed swine flu cases, but there was no known association with travel to Mexico, according to the correspondence.

All of the others had confirmed cases of swine flu related to travel from Mexico as of May 25.

"The traffic was so strongly correlated with the importation of this disease," Khan said. "Where people go, infectious diseases of people will follow."

Conversely, the nations that were not among the top-20 destinations for passengers from Mexico had few or no cases of travel-associated swine flu.

"Almost all the imported cases were in the countries with high-traffic volumes of passengers from Mexico," Khan said. "Almost no importations were in the countries with low-traffic volumes."

On June 11, the World Health Organization declared swine flu a pandemic as infections climbed across North America, Australia, South America, Europe and elsewhere. An estimated one million Americans have been infected with the H1N1 swine flu, though in most people symptoms are mild, according to U.S. health officials.

The database used in the study, called the Bio.Diaspora Project, includes world air-travel patterns that represent 99 percent of the world's commercial air traffic, Khan said. The information, which was collected with the cooperation of several international airport and airline associations, includes itineraries from 2.2 billion passengers and flight schedules from 3,500 airports in about 250 nations and territories worldwide dating to January 2000.

Researchers plan to use the database, which does not connect itineraries with specific passenger names, to help determine where newly emerging infectious diseases are most likely to turn up.

In doing so, governments and public health authorities could work together to prevent the spread of disease and take steps such as determining where to marshal limited public health resources.

"We can't address all threats everywhere, but we can put all of our resources into locations where there is the greatest risk," Khan said.

Though the study shows how air travel contributes to the rapid spread of a disease, it's still unknown if travel information would help slow the spread of an emerging infectious disease, said Dr. Lisa Winston, an assistant clinical professor of medicine in the division of infectious diseases at the University of California, San Francisco Medical School.

Many diseases have already spread around the globe before epidemiologists and scientists learn of them, Winston said.

"It can be difficult to figure out where it started. Things can feel like they are popping up in different places," Winston said. "It's very difficult to control a disease that is very infectious."

More information

For more on swine flu, visit the U.S. Centers for Disease Control and Prevention.

SOURCES: Kamran Khan, M.D., infectious disease physician and scientist, St. Michael's Hospital, Toronto, Canada; Lisa Winston, M.D., assistant clinical professor of medicine, division of infectious disease, University of California, San Francisco Medical School; June 29, 2009, New England Journal of Medicine, online

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