Cities may be sources of influenza epidemics

A new study has shown that tropical areas in Southeast and Eastern Asia are not the only regions that are a source for seasonal influenza epidemics.

Researchers from the Duke-NUS Graduate Medical School in Singapore found, contrary to a commonly held theory, any one of the urban centers involved in the study could act as a source of flu epidemics found in other localities.

"We found that these regions are just one node in a network of urban centers connected by air travel, through which flu virus circulates and causes a series of local epidemics that overlap in time," Dr. Gavin Smith, the study’s senior author and an associate professor at Duke-NUS, said.

The study, which appears in the online edition of the journal Proceedings of the National Academy of Sciences, looked at RNA samples of influenza A taken from 2003 to 2006 in Australia, Europe, Japan, New York, New Zealand and Southeast Asia, including more recent samples from Hong Kong.

The virus populations from tropical Southeast Asia and Hong Kong had lower levels of genetic diversity than expected and no seasonal fluctuations in comparison with epidemics from more temperate regions.

Although the virus patterns showed that migrations throughout Southeast Asia persisted over time, infection patterns also depended on an influx from regions that have distinct seasons. None of the regions they analyzed were a single source for all of the flu strains that appeared in a given year.

The data showed that multiple lineages of a virus could source annual flu epidemics and that each region could potentially serve as a source population.