Raw sewage may hold clues to case count

Miami-Dade to measure virus in wastewater

Figuring out how many people actually had the new coronavirus in South Florida is proving difficult, but Miami-Dade County is looking at its sewage as a way of measuring the pandemic. It’s a somewhat disgusting way of tracking the disease that doesn’t involve sticking a swab up the noses of hundreds of thousands of people, or trying to estimate the number of infected by randomly sampling the blood of thousands more for virus antibodies. And because infected people flush coronavirus down the toilet whether they have symptoms or not, measuring concentrations of virus in wastewater can clue scientists in quickly to just how big an outbreak might be — or when a second pandemic wave might be approaching.

“When you look at this data, you’re looking at what’s happening in the total population at once, rather than random people being tested,” says Dr. Aileen Marty, a professor of infectious diseases at Florida International University. “It really represents the overall status of the population.”

Miami-Dade’s wastewater virus monitoring works like this: raw sewage samples, taken about once a week at the county’s three wastewater treatment facilities, are sent to Massachusetts-based Biobot Ana

lytics for investigation. The company then calculates the number of coronavirus infections based on concentrations of the virus in the wastewater, and sends the county a report.

So far, the monitoring program, which has been operating since since March 25, shows only four snapshots in time during the peak of South Florida’s first pandemic wave, since samples have only been taken four times. But the highest readings, measured on April 9, show that up to 2% of the county’s almost 2.3 million wastewater customers, or about 46,000 people, may have had the disease.

That percentage “is substantially more than cases being reported in terms of individual testing,” says Doug Yoder, deputy director of the Miami-Dade Water and Sewer Department.

The 2% estimate is almost four times higher than the nearly 12,000 documented infections in the county, but below those predicted by Miami-Dade’s second round of antibody testing. That study, which took random blood samples from county residents, estimated that between 4.4% and 7.9% of the county’s nearly 3 million people had contracted the disease.

One reason for the difference in the estimate between the two methods might be because scientists are still learning to gauge how many people have been infected in an area just from the concentration of coronavirus in the wastewater, since the amount of virus people flush down the toilet can vary on a lot of factors.

“There are a lot of uncertainties [about] what the shedding rate of infected patients is,” Yoder said.

He notes that Biobot Analytics uses a model based on measurements taken in three scientific studies to estimate how much virus individuals shed, and when, but that the question requires more investigation.

In an email, representatives from Biobot Analytics said they did not have time to respond to a request for comment, and directed queries to their website.

But even if scientists still don’t know the average rate at which humans infected with the virus pass it in their stool, wastewater monitoring is becoming increasingly popular.

“Monitoring waste water is a good way,” says Dr. Mary Estes, professor of molecular virology and microbiology at Baylor College of Medicine. “When the virus is beginning to disappear, you will no longer be able detect it.”

“Sampling a population this way is much faster and multiple orders of magnitude cheaper [than testing],” said Rolf Halden, director of the Biodesign Center for Environmental Health Engineering at Arizona State University.

It is unclear if any other municipalities in South Florida have similar programs in place. Representatives from Broward County, Palm Beach County and the city of Hollywood said in emails that no testing was taking place at their wastewater facilities. The city of Fort Lauderdale did not respond to requests for comment.

“We’re starting to collect wastewater in Houston now to look for the virus and follow it over time,” says Estes, “and I think that’s going to be happening around the world.”