With coronavirus testing still lagging behind targets, health officials are searching for other ways to assess the spread of the outbreak. One possibility? Looking at what we flush.

The coronavirus SARS-CoV-2 is often spread through sneezes and coughs, but it also leaves the human body through our waste. Scientists around the world are now testing sewage for the virus, using it as a collective sample to measure infection levels among thousands of people.

While the field of wastewater epidemiology existed before the coronavirus pandemic began, it's rapidly expanding in the hope it can become a front-line public health tool.

"Normally when I tell people I work with poo, they're not super-interested," Stephanie Loeb, a postdoctoral researcher at Stanford University, told NPR in an interview over Skype. But she said: "There's really a lot of information in our waste."

In the basement of a university building, Loeb pulls samples from freezers filled with vials of raw sewage, collected regularly from 25 wastewater treatment plants around California. Each is a snapshot of that community's health.

"It's this perfect mix, you know," said Krista Wigginton, a professor of environmental engineering at the University of Michigan who also is working on the Stanford project. "The entire community is putting samples in at the same time."

She said by the time the virus reaches wastewater treatment plants, it's still possible to read its RNA.

"These are virus particles that are mostly intact but that are no longer infective," Wigginton said. "That's what it looks like at this point."

The idea is that measuring overall virus levels in sewage over time could indicate whether an outbreak is growing or shrinking, potentially showing that trend earlier than patient testing would.

"That's a real-time measurement of what's happening in the community," Wigginton said. "Whereas some other tools we have, like the number of confirmed cases in clinics, sometimes those are delayed by quite a bit of time because people don't go get checked until maybe their illness has progressed by quite a bit."

The approach is already used for other diseases such as polio. Health officials are working to eradicate polio around the globe, and in Israel, an outbreak was spotted early through the wastewater system.

The Stanford team isn't the only group working on coronavirus detection in sewage.

"We have a lot of nicknames," said Newsha Ghaeli, co-founder of the startup Biobot Analytics. "I think some of our customers joke around that we're the 'sewer girls.' "

Biobot is testing sewage from about 150 communities across the United States. Originally, the company was using sewage to monitor the opioid crisis, but it quickly started offering coronavirus testing.

"It really caught fire," Ghaeli said. "Within 10 days, we hit internal capacity."

Ghaeli said that in some cities they've been able to detect the coronavirus in sewage the same week the first cases appeared. Other projects in France and the Netherlands have produced similar results.

In a more challenging scientific feat, Biobot is also working to estimate the number of individuals who have the coronavirus in a community, based on the levels found in sewage.

Calculating that depends on knowing how much virus individuals shed, and some people seem to shed for a longer time than others, complicating the math. Other things could also affect the virus levels, such as how long it takes for wastewater to reach a treatment plant and rainy weather, which causes runoff to flow into the sewage system in some communities, diluting the samples.

"There's a lot of research that needs to be done before we can say this number in wastewater means this many cases in the community," Wigginton said.

The advantage of testing sewage is that it may capture individuals who are less likely to go to a doctor's office.

"Every person that is using the toilet has a voice," said Mariana Matus, Biobot's other co-founder. "And they can be taken into account for public health resources and prioritization of resources."

While it's still early in the technology's development, some see it being helpful in detecting new waves of an outbreak.

"I think it is potentially a new role that utilities can play," said Doug Yoder, deputy director of the Miami-Dade County Water and Sewer Department in Florida, which serves 2.3 million people. "There has been, at the community level, not a whole lot of data about conditions communitywide."

For six weeks now, Miami-Dade County has been sending Biobot sewage samples, which have shown the area's virus levels going up and down a bit.

"We've seen in a couple [of] instances the virus counts increase by a factor of six," he said. "And then the week following, it went back down. This data may not yet be ready for prime time in terms of community decision-making, but it has potential and promise for being able to see trends."

Health officials are eager for the information, he said, as one more way to gauge what's really happening with their local outbreak.

Copyright NPR 2021.