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Monitoring Poop for Covid-19

Why scientists are using sewage samples to track Covid-19 infections

April 15, 2022
By Carolyn Malkin

Paul Chinn/The San Francisco Chronicle via Getty Images

DIRTY JOB: Wastewater control inspectors in Southern California collect bottles of sewer water on July 14, 2020.

For the past two years, scientists across the country have been performing a gross—but important—job to keep tabs on Covid-19. They’ve been visiting sewage treatment plants in major cities across the United States to search for evidence of SARS-CoV-2. This virus, which causes Covid-19, can be found in the poop of people who have the disease. That makes sewage treatment plants ideal places to monitor Covid-19 infections in an area.

Wastewater monitoring has become an important tool during the pandemic. “It gives us a picture of the status of health within a community, from a college campus to the entire city of San Francisco.” says Laura Roldan-Hernandez, a member of the Sewer Coronavirus Alert Network, which monitors sewage plants in California.

SEWAGE SAMPLING

The sewage that ends up at a treatment plant comes not only from toilets, but also dishwashers, sinks, showers, and washing machines, as well as runoff from stormwater and industry. But “the vast majority of wastewater is human feces,” says Amy Kirby. She’s a program lead for the National Wastewater Surveillance System at the Centers for Disease Control and Prevention (CDC). 

Paul Chinn/The San Francisco Chronicle via Getty Images

SEWER SAMPLE: Bottles of wastewater will be tested for the SARS-CoV-2 virus.

At a sewage treatment plant, wastewater is pumped into large tanks. Poop and other solids separate from the liquid, sinking to the bottom of the tanks to form sludge. “It’s dark brown or black with the texture of oatmeal,” says Roldan-Hernandez.

Roldan-Hernandez and other scientists collect samples of this dark, oily substance to test for the presence of SARS-CoV-2. In a lab, they run a polymerase chain reaction (PCR) on the sludge samples. If the virus is present, this test makes millions of copies of its genetic material so it can be easily detected. The same test is used to detect the virus on nasal swabs taken from people (see Why Covid-19 Testing Matters).

VALUABLE DATA

Laura Roldan-Hernandez

POOP CLUES: Sewer sludge can contain SARS-CoV-2 virus particles, helping to reveal the levels of Covid-19 in an area.

As of March 2022, more than 640 wastewater surveillance sites in 39 states were contributing data to the CDC’s Covid-19 Data Tracker website. The site maps where levels of Covid-19 infection have increased or decreased during the previous 15 days. Health officials can use this tool to take appropriate action—for example, if numbers are going up, they might decide to increase testing of people in the area. 

Wastewater testing can also give health officials advance warning of the spread of a new variant, or form, of the virus. For example, the Omicron variant appeared at several wastewater testing sites in the U.S. nearly two weeks before it was first detected in a person.

Wastewater testing also has some drawbacks. About 20 percent of U.S. households are not connected to a sewer system and, therefore, aren’t included in the data collection. Wastewater testing also doesn’t reveal the actual number of Covid-19 cases; it only tracks trends in increasing or decreasing levels of the virus.

Because of these issues, says Kirby, “wastewater surveillance needs to be paired with ongoing clinical surveillance—testing of people for infections. Together, this gives us a very good picture of what’s going on in a community.” 

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