Southeast Asia the culprit in flu epidemics

Apr 17 2008

According to the World Health Organisation (WHO) each year influenza epidemics are thought to infect between 5 and 15 per cent of the world's population.

Anywhere between three million and five million are severe cases and between 250,000 and 500,000 deaths occur.

Researchers now suspect the annual epidemics originate from a viral reservoir somewhere in the tropics, particularly in places such as Southeast Asia.

An international team of researchers led by scientists from Cambridge University have discovered how strains of the flu virus emerge in Asia, circulate around the continent then migrate to start seasonal epidemics elsewhere.

The discovery may help scientists to select which strains to include in seasonal vaccines each year and improve immunisation programmes.

The scientists believe that countries in Asia with unique climatic conditions, such as China, South Korea, the Philippines, Thailand and Malaysia, could be the culprits for new outbreaks of influenza that spread around the world.

In general influenza normally causes epidemics during the winter in temperate zones, and during the rainy season in the tropics.

Scientists suspect this is because natural vitamin D levels, which are raised when people are exposed to sunlight, or possibly because greater indoor crowding during cold or wet weather.

As the timing of the tropical rainy season varies in different parts of East and SouthEast Asia, and the areas are close to temperate parts of the continent where flu thrives during the northern hemisphere winter, an overlap occurs.

This means that the virus is always active somewhere in the region and the constant circulation ensures that outbreaks in one country can spread rapidly to neighboring countries.

Added to this is the evolution of new strains which occur as the virus is carried around the world by international travellers.

The researchers say seasonal factors such as winter, or rainy seasons appear to be drive flu epidemics and cities which are only 700 miles from each other, such as Bangkok and Kuala Lumpur, have epidemics six months apart.

The study leaders Dr. Derek Smith and Dr. Colin Russell say in East and SouthEast Asia a great deal of variability exists which creates many opportunities for an epidemic in one country to seed an epidemic to another nearby country, much the same as a baton being passed by runners in a relay team.

For the study the scientists analysed more than 13,000 samples of the commonest strain of the influenza virus, H3N2, collected worldwide by the WHO Global Influenza Surveillance Network in 2002-07.

Their analysis showed that new strains tend to emerge first in East and SouthEast Asia, before reaching Europe and North America about six to nine months later, and South America several months later still.

It was also seen that viruses rarely circulated back to Asia; the team say although this is possible, by the time that they do so there is usually enough immunity among Asian populations to prevent fresh outbreaks.

Another study, by a team at Pennsylvania State University has also reached similar conclusions.

The study led by Professor of Biology, Edward Holmes also found that the annual influenza epidemics originate from a viral reservoir somewhere in the tropics, particularly in places such as Southeast Asia.

Experts say the findings are important, as improved surveillance of the spread of flu will assist efforts to select the right strains for seasonal vaccination.

Every year, eight months before immunisation programmes begin, scientists have to decide which three strains need to be included in the vaccine, which is almost a year before flu will actually strike.

Dr. Smith says the ultimate goal is to increase the ability to predict the evolution of influenza viruses.

The Cambridge study is published in the journal Science and the Pennsylvania study in the journal Nature.