BioEd Online

The notion that flu epidemics start in areas of high population density and spread outwards may not hold true for the tropics, hints a study from Brazil.

In that country, new research reveals, flu starts in the less densely populated north and moves towards cities in the south. The result indicates that climate, rather than population density, plays a bigger part in the spread of the disease in Brazil. And that could have implications for how flu is managed in the tropics.

"This flips our understanding of influenza in tropical regions on its head and will hopefully improve control strategies," says Mark Miller of the Fogarty International Center (FIC) for Advanced Study in the Health Sciences in Bethesda, Maryland, who worked on the study.

Nobody really understands how seasonal influenza epidemics start, but human behaviour is thought to play a major role. In temperate climates, as autumn arrives people tend to gather together inside and children return to school, creating more crowded conditions in which disease is easily transmitted — leading to the winter flu season. Airports and other major transport links compound the problem by helping illness to spread. In the tropics, flu seasons are less well defined.

When Miller — along with FIC's Wladimir Alonso and other colleagues — looked at Brazil, they expected to find that the largest cities with the busiest transport links (São Paulo and Rio de Janeiro) would develop seasonal influenza epidemics first and that the disease would then spread to less populated areas.

But after trawling through two decades' worth of records on influenza mortality, along with clinic data on patients with influenza-like illnesses from 2000 to 2005, they found the opposite. The southern states, containing Sao Paolo and Rio de Janeiro, developed epidemics 2-3 months after epidemics had already appeared in the northern jungles of the country.

"We were really surprised," says Miller. "The fact that influenza starts in the north may mean that seasonal epidemics arise from the tropics where the virus persists all year round." The work is slated for publication in the American Journal of Epidemiology later this month¹.

Warm or wet

The team thinks that different weather patterns in the north and south have a role in how the disease migrates south from the Equator. The north has a rainy season from November to May, whereas the south has hot summers and frosty winters in July and August. The specific role that these climatic regions play in disease transmission is not yet understood.

"More delicate experiments are definitely needed to tease apart the complex relationships being uncovered here," agrees Eddie Holmes, evolutionary virologist at Pennsylvania State University, University Park. "This research takes a nice first look at viral behaviour in the tropics," he says.

The researchers note that their results show how the influenza vaccine, which is typically given to 70% of the elderly across Brazil, could be more effective if given at different times in different places. At the moment the jab is being administered just before the southern sub-tropical winter — but this comes too late for people in the north, where influenza will have already hit its peak. Staggering the vaccine administration to be earlier in the north would be more effective, they say.