March 5, 2008

Study Explains Why Flu Season Occurs in Winter

Researchers at the U.S. National Institute of Child Health and Human Development (NICHD) have completed a study that could reveal why flu season happens during the winter months.   

The study found that the influenza viruses coat themselves in a fatty, butter-like substance that becomes hardened in cold temperatures, providing protection for the virus that allows it to pass from person to person. 

Once in the respiratory tract, this protective coating melts and the virus infects the surrounding cells, the researchers reported.

The study could provide an explanation and lead to new ways to treat and prevent influenza infections, according to NICHD Director Duane Alexander.

The research team found that at temperatures slightly above freezing, the coating solidified into a gel.  However, as temperatures reached 60 degrees Fahrenheit, the gel covering gradually melted and turned into a liquid. 

"It's only in this liquid phase that the virus is capable of entering a cell to infect it," said Joshua Zimmerberg of NICHD, who led the study, in a Reuters report.

The scientists concluded that temperatures in the spring and summer were too high to allow the viral coating to enter its gel state.  As a result, at these temperatures the viruses dry out and weaken, and flu season ends. 

However, a British expert said the study does not provide an explanation of why some flu viruses flourish in tropical climates.

"I don't think this study provides anything like a definitive answer on the spread of the virus, there must be some other factors that come into play," said Professor John Oxford, a virology expert at Queen Mary College School of Medicine in London, in a BBC News interview.   He said although the report was interesting, it is premature to draw any solid conclusions.

"If this is the case why do we get flu in tropical areas, where the temperature is 35C (95F) all the time?  Places like Vietnam and Indonesia are predicted to the epicenter of a new outbreak of pandemic flu," he said.

Professor Oxford said researchers had tried to determine a definitive link between flu infection and cold weather since the time of the great Russian flu outbreak of 1890, but had not been able to find conclusive proof of a link.

The study was published in the journal Nature Chemical Biology. An abstract can be viewed here.


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