David Hirschberg has a simple explanation for why a growing number of scientists are looking to sewage to help track the spread of the novel coronavirus in communities around the world.

“S--- is a great source of information,” said Hirschberg, founder of a nonprofit biotech firm and professor at the University of Washington at Tacoma. “This is the kind of early warning system you want to have. When people start showing up at hospitals and start dying, that’s not the indicator you want to have. That’s too late.”

Researchers say the virus can be detected in untreated wastewater within days of infection and as much as two weeks before a person grows ill enough to seek medical care — that is, if symptoms ever materialize at all.

Hirschberg and his colleagues, who have been monitoring raw wastewater coming into treatment plants in Tacoma and surrounding Pierce County for evidence of the virus over the past month, are among researchers in the United States, Europe, Australia and elsewhere who say the approach allows a glimpse at the curve of probable infections before confirmed cases begin to rise.

As the lack of adequate testing in many places has made it difficult to keep pace with the highly infectious coronavirus, scientists say that monitoring sewage for the presence of the virus can provide public health officials with a relatively cheap and reliable tool that could remove some of the guesswork about when to impose local lockdowns — or when to ease them.

“We’re hopeful this info can really be a valuable addition to all the other information they are looking at to help them decide in the safest, most responsible — but also the fastest way possible — when to open up our economy and our cities,” said Newsha Ghaeli, president of Biobot, a Massachusetts-based start-up that analyzes wastewater.

Her firm — working alongside researchers from the Massachusetts Institute of Technology, Harvard University and Boston’s Brigham and Women’s Hospital — has undertaken one of the most ambitious efforts to ramp up sewage surveillance as the pandemic persists. More than 170 wastewater facilities across 37 states, representing about 13 percent of the U.S. population, have been sending regular samples for analysis.

So far, the samples have consistently shown a higher concentration of virus in places with more intense outbreaks. And the firm’s modeling, which estimates the likely number of cases in an area based on the amount of virus in wastewater, corroborates what other researchers are also finding: Many more people have covid-19 than official counts suggest.

“Our estimates are about 10 times higher than the cumulative [confirmed] cases up to that date,” said Mariana Matus, a Biobot co-founder.

Case in point: New Castle County, Del.

Based on sampling Biobot conducted there in mid-April, researchers estimated there were roughly 15,000 cases of covid-19 in the county — more than 15 times the 974 cases that had been confirmed. More recently, as the number of confirmed cases has continued to climb by several hundred, the firm estimated there were actually thousands of more cases across the county.

New Castle County Executive Matt Meyer said the findings offered valuable evidence that the local outbreak was still intensifying, that many infected residents were asymptomatic and that people should continue social distancing, wearing masks and staying home.

“When you’re dealing with imperfect facts, it’s really important to get as much data as you can,” Meyer said, adding that the wastewater data can be helpful to policymakers who are wrestling with uncertainties as they try to figure out when and how to ease current restrictions.

“You have this virus, which really is not understood that well. There’s a lot of data, there’s a lot of misinformation, and there’s a lot of things we just don’t know,” he said. “The worst enemy is an invisible enemy.”

The novel coronavirus uses a number of tools to infect our cells and replicate. What we've learned from SARS and MERS can help fight covid-19. (The Washington Post)

The ongoing pandemic is not the first time that epidemiologists have looked in wastewater for clues about the spread of an infectious disease.

In 2013, for example, Israel used a sewage surveillance system, which had been put in place decades earlier, to detect the circulation of the polio virus — a situation that threatened to set back global efforts to eradicate the crippling disease.

Because of the early detection in sewage, officials were able to pinpoint the most likely areas for infection and work quickly to ramp up a vaccination campaign to head off a more serious outbreak.

Monitoring wastewater for the novel coronavirus is hardly a panacea. The approach faces a number of challenges, including the logistics of deploying it on a massive scale and winning buy-in from government officials. Effective surveillance would need to be ongoing, and the results would probably need to be available more rapidly than they sometimes are now. The turnaround for Biobot’s results, officials there said, is roughly five days because samples are mailed to the firm from around the country.

Laurent Moulin, a microbiologist with Eau de Paris, the French capital’s publicly owned water utility, believes wastewater surveillance could prove especially useful in helping detect the second wave of covid-19 infections that many public health officials have warned is likely.

“When we stop the lockdowns, a lot of people could have interactions, and the virus could start to spread again,” said Moulin, who recently published findings, which have yet to be peer-reviewed, in which he and colleagues detailed how the rise and fall of confirmed infections in Paris correlated with the amount of virus detected in sewage. “If we monitor the wastewater, we can have an early warning system,” he added.

Zhugen Yang, a professor at Cranfield University in the United Kingdom, who is working to develop a rapid, paper-based test for the coronavirus in wastewater, said such technology ultimately could act as more than just a way of alerting authorities to the need for more restrictions as infections rise.

It also could be deployed on a hyperlocal level to give officials some measure of comfort in reopening schools and businesses. If cases are falling and the virus begins to disappear from wastewater, he said, sewage could help decide when it’s okay to inch back toward normality.

“If we can do proper detection, we could know in some communities there is lower risk,” Yang said. “It could be a signal to say that certain communities are not affected, and give them more freedom.”