In early March, as the novel coronavirus started to make itself felt in the homes and hospitals of Paris, it was also visible in the sewers under the city — to those who knew how to look.
On March 5, doctors knew of only 91 coronavirus cases in Paris, a city of 12 million. But researchers testing the city’s wastewater that day could already detect the virus: people who had it were sending out a warning every time they flushed the toilet.
The coronavirus increased in the city as the weeks went on, shown on the surface by tests of people and also in the sewer by tests of wastewater.
On March 17, Paris locked down, a harsh but in the end successful measure that flattened that city’s COVID-19 curve.
As it had signalled the danger, sewage testing also told researchers that the lockdown had been successful. The virus, which had been steadily increasing in Paris’s sewage, peaked on April 9.
“The thing that’s really important about the French study … is that the trends in the wastewater tend to match what you see in the community,” says says Bernadette Conant of the Canadian Water Network, a non-profit.
It takes a while after infection for someone with the coronavirus to show up in the statistics, and many never do. They have to feel sick, seek out testing, get tested — if tests are available — and then the results have to come back. Only after that does the person’s illness show up in public health statistics.
Much earlier than that, though, they’re shedding the virus into the sewers with every bathroom visit. So on a community level, sewage has the potential to track coronavirus much faster than adding up positive tests on individuals.
“They actually saw it in a city of 12 million people when there were no deaths yet and only 91 clinical cases at that point,” Conant says.
“They could see both the cases rising and the amount of COVID in the community rising before that sort of steep rise in the amount of cases, or deaths, occurred. After lockdown, they saw what one would expect if this was a good indication of circulation of prevalence of COVID in the community — it started to trend down. ”
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Why is that important?
Shutdowns have closed schools and devastated economies in many countries, including Canada. Governments are under pressure to reopen society, though history teaches us to fear a second wave of infection.
So while we are headed to some form of reopening, we also need a way of learning quickly if it is a mistake.
“If you think your communities have flattened the curve, do those curves stay flat when we reopen?,” Conant says. “Ideally what you want in these cases of reopening the economy is a canary in the mine shaft to tell you when it’s starting to spread again, or when it’s come back.”
“Just that value, that it can give us an early warning, to see the virus in the community even if we’re not seeing a lot of clinical cases, is huge.”
The information that could give us warning — or reassurance — is flowing under our feet, but so far no government in Canada is moving to analyze sewage for the coronavirus, says the Public Health Agency of Canada.
“The Public Health Agency of Canada has been engaging with a range of stakeholders to assess current activities related to wastewater sampling including testing and sampling,” spokesperson Natalie Mohamed wrote in an email. “An evidence review is underway to better understand the scientific merits of this approach and learn from any successful initiatives undertaken to date.”
“Research on testing sewage and wastewater is at an early stage of development. At this time, PHAC is not aware of any Canadian studies collecting sewage samples for the detection and identification of COVID-19.”
Conant’s ideal would be a “pan-Canadian surveillance system,” she says.
“There is clearly not a national program.”
Testing in Brisbane, Australia in March and April showed levels of the coronavirus in sewage loosely matching rates of positive tests on people in the broader community.
Israeli scientists had similar results, with higher levels of the virus found in sewage in a known hotspot in Tel Aviv.
Scientists in the United States and the Netherlands are also looking to wastewater to learn more about coronavirus outbreaks. In March, Dutch scientists watched the coronavirus appear in the Netherlands more or less in real time: the virus was detected in sewage at Schipol airport just four days after the first known Dutch case was reported.
More recently, officials in Houston, Tex. have started testing sewage weekly for coronavirus. They hope to get an idea of how many people in that community have the virus but are asymptomatic, and also to be alerted to local hotspots that they didn’t otherwise know about.
In all, Conant says she knows of eight countries that are testing sewage for coronavirus.
In Canada, federal grants are funding 99 coronavirus research projects, but none concern wastewater testing.
Federal health minister Patty Hadju’s office did not respond to questions about whether the federal government had plans for sewage surveillance, or why none of the funded research related to it.
Ideally, sewage testing would reach down to the institutional level — individual long-term care homes or schools, for example, Conant says.
“Definitely that potential is there. The potential is there, but we’re not there,” she says.
“What’s an ideal situation is that we have a surveillance network where all these facilities regularly submit samples for testing. That’s what everybody wants to build towards.”