With flu season almost upon us, it's a good time to ponder why influenza strikes us hardest in the winter months. A new study chalks it up to the fact that the virus appears to be more infectious at colder temperatures and lower humidity. The findings could lead to strategies that help curtail transmission of the disease.
Several explanations have been put forth to explain the seasonal rise in flu cases. Some infectious-disease specialists have blamed the rise on the fact that people tend to have more contact during the colder months because they spend more time indoors. Others speculate that people's immune systems may become weaker during the winter. But so far there has been no good evidence to support these claims. So a team led by virologist Peter Palese of Mount Sinai School of Medicine in New York City tested whether temperature and humidity played a role in the spread of flu.
In a series of experiments, the researchers housed four guinea pigs infected with a human flu virus in cages next to four healthy animals. By holding the temperature constant and adjusting the humidity, they found that the greatest transmission occurred at a low relative humidity of 20% to 35%, the conditions under which 75% to 100% of the uninfected guinea pigs contracted the virus. Only one of the animals became infected when the humidity reached 50%, and transmission was blocked when the humidity rose above 80%, the researchers report in the October issue of PloS Pathogens. For temperature, the greatest transmission occurred at 5°C, with all of the healthy animals becoming infected. No transmission occurred at 30°C.
"This offers a nice explanation for why we have more flu in the winter, because it gets transmitted better in the cold," Palese says. The findings need to be confirmed in people, he says, but a possible reason for the increased transmission is that the flu virus is more stable at colder temperatures and lower humidity. Our physical barriers for keeping out pathogens, such as the mucous membrane in the nose, may also be impaired under these conditions.
Raymond Tellier, a microbiologist at the Hospital for Sick Children in Toronto, Canada, says the findings indicate that raising the humidity in buildings, such as hospitals and nursing homes, could help prevent transmission of flu. Still, this strategy could be tricky, he notes, because higher humidity could encourage the growth of other pathogens, such as mold or the bacteria that cause Legionnaires' disease. "You don't want to reduce one infectious disease only to increase another," Tellier says.
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