By Carl Zimmer
The world is eager to come out of lockdown. But if countries simply return to business as usual, new outbreaks of Covid-19 will follow. The only solution that public health experts see is to keep careful track of the coronavirus and clamp down on new flare-ups.
The trouble is that the most obvious way to monitor the virus — testing person by person — has already proved to be a huge, expensive challenge. Experts say we’re nowhere near the scale we need to get a good picture of the pandemic.
Now some scientists are looking for the virus not in our noses, blood or spit, but somewhere else: in our sewers.
“It’s the signature of a whole community,” said Krista Wigginton, an environmental engineer at the University of Michigan who has been finding the coronavirus in wastewater around the Bay Area in California.
Water authorities and governments are in discussions with scientists and companies about tracking the pandemic through the detection of viruses in the sewer. Wastewater monitoring could provide early warnings of outbreaks. It could potentially give governments some of the data they need about when to end lockdowns and when to ratchet them back up.
Measuring viruses in wastewater in effect tests an entire city or region at once. While only some people may get tested for the coronavirus on a given day, everyone uses the toilet.
“It’s a great leveler,” said Christobel Ferguson, chief innovation officer of the Water Research Foundation.
This week, the foundation sponsored a virtual research summit, during which Dr. Wigginton and other experts shared their early results and developed a road map for improving their surveillance.
For decades, public health workers have looked in sewage for signs of viral outbreaks. The World Health Organization has monitored polio viruses this way, to assess how well its vaccination campaigns have worked.
In the early days, researchers had to run painstaking tests to find viruses in wastewater. They had to mix the water with cells so that the viruses could infect them. Then the researchers had to wait for the new viruses to emerge.
Later, researchers were able to skip these experiments. They could simply fish out genetic material from the water, read its sequence, and determine what kind of virus they were dealing with. Even newer technology has made it possible to estimate the number of viruses by counting up the viral genes in a water sample.
Irene Xagoraraki, an environmental engineer at Michigan State University, uses this method to detect viruses in wastewater in Detroit. In a recent outbreak of hepatitis A, she found that the virus increased in the water about a week ahead of the rise in confirmed cases. “You can predict the outbreak,” she said.
When the coronavirus began spreading from China, Dr. Xagoraraki and other experts began wondering if they might see it turn up in wastewater.
The early reports about the coronavirus made the idea seemed plausible. Although the virus infects people’s airways first, it can eventually get into the intestines.
The coronavirus has been detected in some infected people’s feces. Some early studies suggest that the virus becomes inactive by the time it gets to the sewer system. But it still carries genes that researchers can detect.
“We started before the virus entered our country,” said Gertjan Medema of the KWR Water Research Institute in the Netherlands. He and his colleagues created a test for the coronavirus and began using it in wastewater in early February.
They didn’t get any positive results, which was reassuring. They could be confident that their test was specific enough not to be fooled by other viruses.
After the Netherlands saw its first confirmed case on Feb. 27, Dr. Medema and his colleagues went back out to run more tests. They found the virus in the sewers of cities like Amsterdam and Utrecht.
The researchers then went to remote towns without any known cases of Covid-19. They discovered the coronavirus up to six days before the first confirmed cases were found there.
Since then, Dr. Medema and his colleagues have continued to track the viruses in the sewer systems. As the confirmed cases of Covid-19 have gone up in Amsterdam and Utrecht, they have found more virus genes in the wastewater.
Researchers have reported similar results from countries including Australia, France, Spain and the United States.
At the meeting, the consensus of experts was that it’s not yet possible to use viruses in wastewater to estimate how many people are infected.
For one thing, researchers are still trying to figure out the average number of viruses that infected people shed in their feces. For another, it’s not clear how many viral genes survive the journey from a toilet to a wastewater treatment plant.
“I don’t feel like we’re at a point where we can say, ‘This is the concentration in the wastewater and this is the number of people with illness,’” Dr. Wigginton said.
Nevertheless, the experts who attended the meeting agreed that sewers have a lot to tell us about the pandemic.
The studies of Dr. Medema and others suggest that a weekly test of wastewater could serve as an early warning system for outbreaks.
When cities or states come out of lockdown, they could check the sewers to follow the virus trend. An increase would tell them that people were infecting each other. “Then you need to go back into quarantine,” said Eric Alm, a M.I.T. microbiologist and the scientific director of BioBiot, a comapany that tracks pathogens in wastewater.
Previous experience with other viruses has taught researchers to be careful about making sense of these apparent trends. If a huge crowd comes into a city to watch a football game, for example, the wastewater system may see a spike of viruses that has nothing to do with a new outbreak.
“It requires good information,” Dr. Medema said, “but it’s a doable thing.”
As their testing becomes more reliable and precise, Dr. Medema and other researchers hope to zoom in on future outbreaks. Instead of looking at a wastewater treatment plant that handles an entire city or county, they may go down into manholes to monitor changes in individual neighborhoods.
Conceivably, they might be able to zero in on nursing homes, factories and other places that have seen intense outbreaks.
“If we see a hot spot arising,” Dr. Xagoraraki said, “we can close down a particular area for a while, so you don’t kill the whole economy of a whole state.”