Chicago health officials are now tracking the spread of COVID-19 infections through a non-traditional source: the city’s poop. The city recently joined statewide and national efforts to monitor our sewage for levels of SARS-CoV-2, the virus that causes COVID-19.
Since people with COVID-19 actually shed the virus in their poop, health officials now have another tool to help keep tabs on infections.
This week the Chicago Department of Public Health released its most recent wastewater COVID-19 surveillance report based on samples collected last December from three main water treatment plants and sewer sites they’re targeting in seven neighborhood areas spread throughout the city.
The results aren’t necessarily surprising. The wastewater data confirmed that during the last two weeks of December, COVID-19 infections increased across all the seven neighborhood zones where samples were collected. The data also showed an increase in the samples collected from all three treatment plants.
The bad news is that up-to-date analysis of wastewater surveillance for the month of January is not available to the public. However, the city’s downward trend in infections tracked by individual COVID-19 case counts can be found on the Chicago Department of Public Health website.
Still, wastewater surveillance is an important tool because it can help identify trends that might otherwise go undetected. As at-home tests become more prevalent our sewer water is likely to develop into an even more important source of information about how much and where the virus is circulating around Chicago.
First things first: How does it work?
After flushing the toilet, the feces and pee from symptomatic or asymptomatic residents travel through the sewer lines. Once in the sewage system, the poop water then makes its way to one of three wastewater treatment plants in the Chicagoland area: O’Brien on the North Side, Calumet on the South Side and Stickney on the West Side.
Each treatment plant gets waste from millions of city and suburban poopers, according to public health officials. Wastewater samples from each plant are then grabbed for further analysis.
Additionally, public health experts are monitoring wastewater from seven zones in Chicago including Chatham, Austin/Montclaire, Chicago Lawn/Ashburn, Roseland/West Pullman, Lakeview/Uptown, NorwoodPark/Jefferson Park and Lincoln Park/Near North Side. The wastewater from each of the seven neighborhood zones is usually on its way to one of the three main treatment plants. Every week, two samples are collected from each site and sent to the lab for testing and genetic sequencing.
Why is this a useful tool?
The wastewater surveillance program is an important development in the fight against COVID-19 because experts say they expect fewer people will seek testing at traditional testing locations as in-home tests become more popular and accessible.
“Normally, our case-based surveillance is built off of people getting laboratory tests which are reported through the state public health system,” said Dr. Isaac Ghinai, medical director of testing and laboratory-based surveillance with the Chicago Department of Public Health. “With at-home tests, you don’t get that kind of reporting. So we start to see a less and less complete picture of the pandemic.”
And wastewater surveillance can help detect trends before other data collection methods. The first time the Omicron variant made an appearance in Chicago’s poop water was on December 12, 2021, a few days after the first case was confirmed in Chicago. Experts say the wastewater indicated that more than one person was actually infected with the Omicron variant at that time.
“That’s really why we are doing wastewater monitoring is to give public health officials a heads up, a forecast that says there’s a wave and you’re going to start to see the impact in three or four days or seven days,” said Charlie Catlett, a senior research scientist at the University of Illinois Discovery Partners Institute, which is working on the wastewater surveillance with CDPH. The Metropolitan Water Reclamation District of Greater Chicago (MWRD) is also a partner in the wastewater surveillance program.
Catlett said the surveillance of wastewater from Chicago’s O’Hare’s domestic and international terminals could help health officials detect the arrival of variants.
Other benefits of wastewater surveillance include the ability to collect data from millions of asymptomatic residents who might not go and take a test or live in areas that lack access to health clinics and testing sites.
Wastewater surveillance can be tricky
There are a lot of complexities to wastewater COVID-19 surveillance. It isn’t possible yet to equate the presence of COVID-19 in a wastewater sample with a specific number of cases, experts say.
Catlett, the researcher, said there are several reasons why this is the case. First, everyone who gets COVID-19 will shed it at a different rate and some people shed more virus than others. And, he said, wastewater samples in Chicago not only contain human waste, but also stormwater and industrial contaminants.
“So even from one wastewater treatment plant to the other, a certain level of viral concentration doesn’t mean the same thing from a public health point of view,” he said. “And that’s why the CDC recommends that these numbers are used to calculate trends.”
Chicago is also one step behind in efforts to provide up-to-date public wastewater surveillance data compared to cities like Boston and Minneapolis. As the city moves past its latest peak in infections, updated wastewater surveillance data for the month of January is not yet available for public view. But in Minneapolis for example, residents can keep track of the daily viral load flowing into their region’s largest wastewater treatment plant each day. The city launched its wastewater surveillance efforts in May of 2020.
As wastewater surveillance programs gain popularity across the nation, the goal in Chicago is to expand these efforts at the neighborhood level.
“Because we’re supporting the local community surveillance in Chicago, one of the things we really need to do before we start releasing this data generally, in more real-time, is to try and understand exactly how it maps to true case numbers,” Dr. Ghinai said. “Putting out numbers that we’re not confident in what the public health interpretation is, is irresponsible.”
Instead of developing its own wastewater data portal for Chicago residents, public health officials say they plan to share the data with the CDC.
“The plan is for the CDC to collect data from cities all over the country, and to present all of [that] data in the same dashboard,” said Justin Hart, wastewater surveillance coordinator with CDPH.
The Chicago Department of Public Health plans to invest $2.14 million in its wastewater efforts over the next two years.
Adriana Cardona-Maguigad is a WBEZ reporter. Follow her on Twitter @AdrianaCardMag.