Flu tracked to viral reservoir in tropics

Each winter, strains of influenza A virus infect North Americans, causing an average of 36,000 deaths. Now, researchers say the virus comes from a viral reservoir somewhere in the tropics, settling a key debate on the source of each season's infection.

"We now know where the influenza A virus comes from every year," said Edward Holmes, professor of biology at Penn State. "And because we now know how the virus evolves, we have a much better chance of controlling it."

Currently, there are many strains of the influenza virus that appear only in birds, which are natural viral reservoirs. So far three of these viral strains -- H1N1, H2N2 and H3N2 – have caused epidemics in humans as influenza A.

Of the three, H3N2 is the dominant strain, responsible for most influenza infections each winter, with lower levels of H1N1. However, little is known about how these two strains spread on a geographical scale, and how whole genome of influenza A virus evolves.

Holmes and his colleagues analyzed complete genomes of 1,032 strains of H1N1 and H3N2 viruses sampled over a 12-year period from New York state in the northern hemisphere and New Zealand in the southern hemisphere.

The researchers noticed that over time, both strains follow a distinctive pattern. In seasons where the H3N2 strain is dominant, H1N1 is not and vice versa.

"We found that the two strains peak at different times, and seem to be directly competing with each other" said Holmes, whose findings appear today online in Nature. The results also indicate that compared to the H3N2 strain, the H1N1 strain exhibits far less genetic diversity, although it is not clear why.