It might be the least likely place to look for the coronavirus. But throughout the pandemic, groups across the country have been collecting and testing daily sludge samples from wastewater processing plants, hunting for fragments of SARS-CoV-2, the virus that causes Covid-19.
Wastewater-based epidemiology has proved to be so reliable in dozens of pilot projects across the US that the government has invested millions of dollars to create the National Wastewater Surveillance System, or NWSS, a network of 400 testing sites spread across 19 states that is coordinated by the US Centers for Disease Control and Prevention.
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The coronavirus is encased in an oily envelope. After it invades our bodies and begins to furiously clone itself, some of those copies are shed into our intestines, where the fatty parts of the virus stick to the fats in stool.
When we poop, genetic material from the virus gets flushed down the toilet into the wastewater stream, where it can be detected by the same kinds of tests labs use to detect the virus from nasal swabs: real-time polymerase chain reaction tests, or RT-PCR. This kind of testing is highly sensitive. It can pick up the presence of the virus when just one person out of 100,000 people in a given area, or sewershed, is infected.
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And because wastewater testing doesn’t depend on people to realize they’re sick and seek out a test, or even to have symptoms at all, it’s often the earliest warning that a community has a wave of Covid-19 infections on the way.
“As long as people are using a toilet that’s connected to a sewer, we can get information on those cases in that community,” said Amy Kirby, a CDC microbiologist who leads the NWSS project.
