Germs’ persistence: Nothing to sneeze at

Years ago, I read (probably in Science News) that viruses can’t survive long outside their hosts. That implied any surface onto which a sneezed-out germ found itself — such as the arm of a chair, kitchen counter or car-door handle — would effectively decontaminate itself within hours to a day. A pair of new flu papers now indicates that although many germs will die within hours, none of us should count on it. Given the right environment, viruses can remain infectious — potentially for many weeks, according to one of the studies.

Its subjects may well qualify for the Guinness Book of Records. But that study is hardly the first to establish that viruses can live on surfaces for more than a few days.

A 2008 Swiss paper in Applied and Environmental Microbiology, for instance, quantified the infectious lifetime of flu bugs on banknotes. Three days was no biggie, it found, if the germs had been deposited in high enough concentrations. But its really disturbing news: The bugs’ survival could be almost sextupled — as in extended to 17 days — if high concentrations had landed on the money along with some respiratory mucus. And that’s exactly the environment to expect if someone sneezed directly onto a dollar bill or a drippy-nosed toddler had been handling the currency.

Viruses don’t like being naked because they risk drying out. But how quickly that happens remains poorly quantified, Jane Greatorex of Addenbrookes Hospital in Cambridge, England, and her colleagues point out in the Nov. 22 PLoS ONE. So they decided to investigate the issue, using two strains of H1N1, or swine flu, that had been isolated from people.

They swabbed all sorts of fabrics and hard surfaces (from wood to plastic and metal) and then went back hours later to look for viable virus. Throughout at least a day, genetic material from the virus persisted on all surfaces — except the wood (especially unsealed pine), where virtually all evidence of the virus disappeared within a few hours.

The germs’ disease-spreading potential disappeared more rapidly than did their genetic traces. The only household surface from which the researchers confirmed live virus 9 hours into their experiment was stainless steel. These new data are important, Greatorex’s team argues, because they point to the unreliability of using genetic residues — what had been a common survey technique — as a gauge of viral infectivity.

Virologist Amélie Dublineau of the Pasteur Institute in Paris and her colleagues offer related, but far more disturbing, data in the Nov. 23 PLoS ONE. They applied H1N1 bugs — both the pandemic strain that traveled around the world in 2009 and a more conventional laboratory strain — onto glass surfaces and into water whose salinities varied between that of freshwater and the super-salty concentrations encountered in the Dead Sea. The researchers also varied incubation temperatures for both the wet and dry conditions: from a chilly 4 degrees Celsius (which is about the same temperature as the inside of a refrigerator), up to a toasty 35 °C (or 95 degrees Fahrenheit).

Concentrations of viable flu bugs tended to wane as temperatures and salinity rose.

Germs also dried out promptly in the glass trials — typically within a day. Yet viable germs remained on glass for up to a week at the highest temperature, and for up to 66 days at 4 °C.

Water proved more conducive to virus survival. During the warmest trial, germs remained infectious for a day in even Dead Sea salt concentrations. Take the temperature down to 4° and cut the salt levels back to freshwater or low-salt conditions and the bugs remained live and infectious for “at least 200 days,” the virologists report. In one instance, live virus was still extractable after 595 days, observes coauthor Jean-Claude Manuguerra.

And that’s not an estimate, he emphasizes. “Our paper is the first one giving data on the long term survival of the virus — not from calculated data, but from observed data.”

His team also showed that compared to the lab strain they tested, the new H1N1 pandemic strain from 2009 proved much tougher at managing the environmental stresses they threw at it. “Did this play a role in its fabulous capacity to transmit,” he asks? “We don’t know!”

Perhaps the most intriguing data from the Paris team’s new study was what happened when the virus dried out. Conventional wisdom would argue that a desiccated virus is a dead germ. And Manuguerra concedes that as the virus dried out, the share of dangerous bugs diminished. “However,” he adds, even once they were fully dry, “there was still a very significant amount of viable virus left” — and this dried virus “remained infectious for an extended period of time.” He knows because his group later resuscitated the virus by plugging it into a growth medium.

Although the Greatorex team’s investigation of flu’s persistence “came up with sort of the expected answer, this new paper [by the Pasteur team] is tough,” says J. Owen Hendley of the University of Virginia School of Medicine in Charlottesville. “I’m not sure I’m ready to accept that influenza virus lasts longer than a day,” he says. Then again, he concedes, methods used by the French team show “It’s a serious piece of work. And I can’t see any holes in it. So it makes me consider the possibility that it might be right.”

Bottom line, he says: ”I’m eager for someone to reproduce this work.”