Mum proved right... AGAIN!

New Scientist reports: [edited]

Scientists have finally confirmed what your mother knew all along – that flu spreads best in cold, dry weather.

As the first few cases of the northern hemisphere’s annual flu epidemic are trickling in this week, scientists may finally know why winter is flu season. It appears the virus lasts longer in cold, dry air, and our sluggish, cold-weather mucus cannot clear it out.

Astonishingly it has taken until the publication of research this week to settle the basic question about how flu spreads, and why it girdles each hemisphere every year during winter. Ironically, that research was made possible by the rediscovery of a report by army doctors in 1919.

Flu is hard to study in the lab because virtually no lab animals get it the way humans do. Mice, for example, do not get the same strains, or catch flu from each other. The most useful animal has been the ferret.

However, studying disease transmission requires too many animals to be practical with ferrets. “They’re big, they’re expensive, and they bite,” Peter Palese of Mount Sinai Medical School in New York City told New Scientist.

But in 1919, US Army doctors at Camp Cody in New Mexico reported that the 1918 flu pandemic had killed their guinea pigs – kept at the time for medical tests. “We didn’t know guinea pigs got flu,” says Palese. "They are no longer popular lab animals, and no-one had tried them."

So Palese’s team exposed hundreds of guinea pigs to a human flu strain at different temperatures and humidities, in cages that allowed only air to pass from sick animals to well ones. This settled a longstanding dispute over whether flu can spread solely as an airborne infection, or whether physical contact is needed. “It spread solely via the air they exhaled,” says Palese. “Guinea pigs with flu don’t cough or sneeze.”

At room temperature, they found flu transmission peaks at low relative humidity (20-35%) and again at 65%. It spread less well at around 50% humidity, and not at all over 80%. This parallels the stability of flu virus in aerosol droplets at different humidities, and also the droplets’ ability to remain airborne. At over 80% humidity, droplets containing flu virus themselves fall out of the air.

The effect also happened too quickly to be due to dry air damaging nasal tissue so that it is not as effective a barrier to viruses, which has been suggested as a factor in humans.

But temperature trumped humidity: at 5°C animals caught flu at humidities that stopped the virus when it was warmer, possibly because sick animals’ noses shed virus two days longer at cooler temperatures.

To find out why, Palese’s team charted 13 early immune reactions in infected animals, but they were unaffected by temperature. Mucus, however, normally flows up through our respiratory tracts to clear out contaminants. “Mucus becomes more viscous as cold air hits our upper respiratory tracts,” he says. “So we can’t clear the virus as easily.”
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