picture of an alley with a sewer cover in the street

The COVID pandemic has brought new and innovative ways to help public health experts track diseases. One technology that has received a lot of attention is wastewater surveillance. Wastewater surveillance isn’t brand new – it’s been used in the past for things like detecting levels of  opioids or diseases like polio – but COVID has really revolutionized the technology. By measuring sewage for pieces of the COVID virus, researchers can track virus trends over time. However, there are also limitations to this technology.

COVID wastewater data in Wisconsin is coordinated at the state level, not by us. We have received numerous questions from people who want to know more, so we talked to wastewater experts from the Wisconsin State Lab of Hygiene and the Wisconsin Department of Health Services about how this technology works and how people can use the data.

How Wastewater Monitoring Works

People who have COVID don’t just shed the virus through their respiratory fluids—they also shed COVID through their poop. Even people who have no symptoms still shed the virus. This allows researchers to track population-level COVID trends through the sewer systems. Wastewater monitoring helps us capture COVID trends from the whole population, even from the people who don’t get tested.

To test sewage, public health experts work with local wastewater treatment facilities. Samples are taken from the “influent” at a treatment facility—the mix of everyone’s sewage that comes in to be treated. The samples are taken over many hours so that the sample isn’t as affected by random ups and downs. Some of the millions of gallons of sewage that flow through the treatment facility end up in two small bottles that have a mix of sewage from many people over many hours. The treatment facility then sends the bottles to the Wisconsin State Lab of Hygiene for testing.

At the lab, workers save one sample as a backup and test the other one for genetic pieces of the COVID virus. They test the sample many times and average the numbers together to get a more accurate estimate. Then, they send the data to the Wisconsin Department of Health Services (DHS). DHS shares the data on their website and tracks it over time for each community that participates.

Each Local Wastewater System is Different

Many factors affect the levels of virus in each community’s sewage. For example, an area with many industrial businesses may have different chemicals in their sewage than a residential area. Some of those chemicals can lower the amount of virus that labs can detect in a sample. Areas that have more tourism or events may also see ups and downs in virus levels as people come in and out of the area. The temperature outside and the level of vaccination can also affect the levels of virus found in wastewater.

Because of these different factors, you can’t directly compare the amount of virus found in sewage between communities. But you can compare the amount of virus across time for the same community. For example, Oshkosh may have a concentration of 100 million genetic copies of the COVID virus, while Sheboygan has 150 million. But that doesn’t mean that Oshkosh has less COVID than Sheboygan. This is because Oshkosh and Sheboygan are different cities and have sewage with a different chemical makeup. But you could compare Oshkosh to itself. If Oshkosh previously had 100 million genetic copies but now has 200 million genetic copies, then you know that Oshkosh may have more COVID in the community than it did before.

How To Use Wastewater Datascreenshot of the WI DHS wastewater data page

The best way to use wastewater data is to look at the trend over time for your area. That way, you can see if your area is seeing an increase or decrease of COVID in its sewage. The DHS wastewater page shows this trend for each community that participates in sample collection.

Use a bit of caution, though--if your community shows an increase, that doesn’t always mean there’s a major COVID surge in your area. Sometimes, the data bounce around between increasing and decreasing because of all the different factors that can randomly affect virus concentrations. That’s why it’s better to look at medium and long term trends. If concentrations are going up for a sustained period of time, then it’s more likely to be because of a true COVID surge.

Wastewater surveillance is still an evolving technology, and it’s not a substitute for COVID case counts. It can give us an idea of future trends, but we can’t yet rely only on wastewater data to predict a COVID surge. It can also take a while to show a sustained decrease in COVID levels, as people are often shedding COVID virus in their poop for a while after they’ve cleared the virus from their respiratory system. But it is a helpful supplemental tool in addition to the regular COVID data we have. Especially as people are testing more and more with home tests, wastewater data can help us understand what is happening in our entire community. It may even have some uses for other diseases in the future, like the flu. Public health experts will likely be testing our community’s poop for a long time!

Thank you to our colleagues at the Wisconsin State Lab of Hygiene and the Wisconsin Department of Health Services for informing this blog post and for all your work to provide wastewater data to our community!

Learn More

Wisconsin Department of Health Services COVID Wastewater Data Page
CDC COVID Wastewater Page

This content is free for use with credit to the City of Madison - Public Health Madison & Dane County and a link back to the original post.