Trying to destroy something that doesn’t live is complicated
Just as there are many different kinds of viruses, there are many different ways to destroy them.
As the spread of the new coronavirus continues, surpassing 70,000 confirmed cases and more than 2,000 deaths, there’s increasing pressure among scientists to figure out how long the virus might survive outside the human body.
How long a virus endures has implications for the scope of the outbreak: It matters for keeping hospital environments safe and minimizing the spread of the virus among people in public spaces.
A review of the scientific literature published last week reported that the family of viruses to which the new coronavirus belongs tend to persist around four days on surfaces at room temperature. Previously, researchers found that the coronavirus behind the SARS outbreak of 2003 could remain infectious on dry surfaces as long as six days. Scientists are now racing to understand how easily this new pathogen — now officially known as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) — can spread.
There’s a surprisingly wide range of how long viruses can last. On one end are viruses like the common cold–causing rhinovirus, which sometimes wither on surfaces within just two hours. At the other extreme are hardy parvoviruses, which can infect dogs and last more than a year on surfaces. Severely cold temperatures can also preserve viruses for longer: In an experiment published in 2014, scientists isolated a virus in material taken from 100 feet deep in the Siberian permafrost and showed that the virus — which they estimated to be more than 30,000 years old — could still infect amoebas.
Discovering the presence of genetic material of a virus doesn’t necessarily mean the virus is intact and infectious. When scientists are testing surfaces for viruses, they typically swab or run fluid over the surface in question to dislodge and collect any viral particles. They then use pipettes to transfer samples of that liquid to cells in a dish. The cells in the dish could be mucus cells, for example. (Basically, they’re the kind of human cells that scientists think are most vulnerable to attack by the virus.) The goal is to see whether the virus particles are still able to replicate and wreak havoc.
“We don’t think of viruses as truly alive.”
Even if someone encounters infectious viral particles, their body might be able to neutralize the virus before it fully takes root. In some cases, people need to be exposed to a high number of virus particles to get sick; in other cases, very few can cause infection. Vincent Racaniello, a microbiologist at Columbia University in New York, says that while as few as 10 particles of norovirus can cause a person to develop an infection and get sick, it might take as many as 100,000 particles of poliovirus for a person to fall ill from polio. “It’s a numbers game,” he explains. “One particle is not usually going to make it. You need a lot.”
Racaniello says the data is still lacking, but he speculates that a person would need to encounter thousands or tens of thousands of particles of the new coronavirus to be infected.
What is dead may never die
To say you have killed a virus, you must concede that it was alive to begin with. And that’s a complicated proposition. Viruses can’t replicate on their own — they need a host (like a human) for that.
“We don’t think of viruses as truly alive,” says Linsey Marr, who studies airborne disease transmission at Virginia Tech. “But we can talk about whether they are infectious or not, because some do get damaged enough that they can no longer infect a cell.”
Viruses don’t have the biological machinery needed to copy their genetic material on their own and replicate. Many viruses consist of as little as a strand of DNA or RNA bundled with a protective structure called a capsid. And just as there are many different kinds of viruses, there are many different ways to destroy them.
Tamar Kohn, head of the Environmental Chemistry Laboratory at the Swiss Federal Institute of Technology in Lausanne, has tried all sorts of different methods to obliterate viruses. In one study of a sample virus, her group found that UV light caused damage to the virus genome, making it impossible for it to replicate, while exposure to the disinfectant chlorine dioxide might inhibit the virus’s ability to recognize and bind to host cells.
Kohn says coronaviruses belong to a family of viruses that have an additional layer, known as an envelope. Envelopes can, in some cases, help viruses evade detection by the immune system, but paradoxically they also make viruses more vulnerable. The envelope can contain molecules necessary for the virus to enter cells. And once the envelope is disrupted, so too is the virus’s ability to be infectious. For this reason, Kohn says SARS-CoV-2 might be susceptible to cleaning products. “Generally, I would expect disinfectants to work very well against coronaviruses,” she says.
According to Axel Kramer, who heads the Institute for Hygiene and Environmental Medicine at the University of Greifswald in Germany, ethanol might be especially effective against SARS-CoV-2. This is because both ethanol and some proteins in the coronavirus envelope layer are attracted to molecules that have certain properties like water. Because of this similarity, ethanol is equipped to insert itself into the envelope and mess up the virus. There are also components of coronaviruses that are vulnerable to detergents like soap.
For reasons that are not completely clear, humidity seems to accelerate the degradation of viruses, Marr says. Heat also speeds this process. But there are many unanswered questions about the new SARS-CoV-2, including how it might survive in droplets or perhaps aerosols, which are smaller and travel by air. Virologists caution that SARS-CoV-2 has not been documented as an airborne virus as of yet; however, the coronavirus that causes Middle East respiratory syndrome was isolated from hospital air samples in an active form.
An unsealed sewage pipe is being investigated as a possible source of the several coronavirus cases in a Hong Kong building complex. Disinfection measures might need to broaden to guard against fecal contamination if it’s proven that SARS-CoV-2 is transmissible through this route. (The virus has already been associated with diarrhea.) Scientists have already documented that some coronaviruses, such as some that infect pigs, can withstand both the very acidic and very alkaline portions of the digestive system. “This is something known for many, many years about coronaviruses, but no one really has addressed this,” Racaniello says. Why they are impervious to these extreme conditions whereas some other enveloped viruses are more vulnerable is a longstanding mystery, he adds: “I think, in fact, it would be very interesting to know.”
Although only a handful of coronavirus cases have been confirmed so far in the United States, the country’s Centers for Disease Control and Prevention says that one key preventative step people can take against SARS-CoV-2 is simply following good basic hygiene, like regular hand-washing, “especially after going to the bathroom.” The CDC recommends using soap and water for at least 20 seconds — or an alcohol-based sanitizer in a pinch. For people concerned about exposure to SARS-CoV-2, the CDC suggests “everyday preventive actions” that one would normally take in environments with respiratory viruses: “Clean and disinfect frequently touched objects and surfaces using a regular household cleaning spray or wipe.”