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COVID-19: Wastewater test results mirror confirmed cases



Dr. Paul van Erp (seated), a postdoctoral fellow in the Walk Lab and Lila Bull Chief (standing), a graduate student in the Walk Lab, analyze collected wastewater samples for traces of SARS-CoV-2. PHOTO COURTESY OF WALK LAB

Big Sky sees new case, wastewater tests positive

By Brandon Walker EBS STAFF

GALLATIN COUNTY – Researchers in Gallatin County are finding a direct correlation between recent wastewater testing and confirmed cases of COVID-19. And as they recorded data showing positive results of the coronavirus in wastewater samples, county health officials also confirmed a new case of COVID-19 in Big Sky.

In a June 17 press release, the Gallatin City-County Health Department said four new cases were recorded between Big Sky and Gallatin Valley, with one of the four cases spurring from the Big Sky community. Coincidingly, on the same day researchers at Montana State University witnessed the first positive test result of Big Sky’s wastewater, showing the presence of COVID-19 in the sample.

MSU researchers in collaboration with the health department began testing county wastewater for COVID-19 in March with samples first collected in Bozeman on March 23. Testing began May 30 and June 6 in West Yellowstone and Three Forks respectively, and in Big Sky on May 14, after the Big Sky Resort Area District allotted nearly $80,000 to fund the testing. 

“What’s pretty interesting about the Bozeman results in particular is we saw … a detection in the sewage that coincided with a group of cases that we detected at about the same time,” said Gallatin City-County Health Officer Matt Kelley. 

Seth Walk, an associate professor at Montana State University in microbiology and immunology, confirmed Kelley’s outlook referring to results from Bozeman and West Yellowstone. “It correlates well with new cases getting confirmed and then the lack of cases being confirmed,” he said. 

Test results showed a detectable trace of COVID-19 in Bozeman wastewater in March, coinciding with the rising number of confirmed cases at the time. Walk said recently there were similar results in West Yellowstone after cases were confirmed there. The tests also have come back with undetectable traces of COVID-19 during periods in which no new cases were confirmed. Now, the Big Sky wastewater results mirrored the same trend with the confirmation of a case within the community and the first positive wastewater test on the same day.

The Centers for Disease Control and Prevention designates that a result lower than 40 Cycle threshold (Ct) is the quantity at which a test is considered positive for traces of COVID-19. Conversely, tests resulting in quantities above 40 Ct are considered negative. 

“It’s a cycle number, so every cycle that the sample is negative means that there’s less and less virus in the sample,” Walk said. “The higher that number, the less virus there is.”

It’s currently unknown what the 40 Ct quantity translates to in terms of how many infected people are in a given area. Walk said it depends on a number of factors including the magnitude of an individual’s case, how long a person has been infected and even the duration of time they’ve spent in the area. 

“The important thing to remember there though is the test itself was developed to test human samples,” Walk said. “They weren’t necessarily developed to test environmental samples, so, it’s really kind of unknown whether that 40 [Ct] is a hard and fast rule for the samples that we’re trying to test in wastewater.”

Walk also said the collected samples don’t allow researchers to determine if the detected virus is still transmissible within its host to another person.

Kelley and the health department are consulting test results as they continue to work toward a safe reopening for Gallatin County. He said that even if cases haven’t been confirmed through traditional testing methods in certain areas, wastewater results will continue to provide valuable insight. 

“But we start to see hits in the sewage load, that’s a clear [indicator that] maybe we need to start paying attention to that community a little bit more and pick up our testing there,” Kelley said.

Most Gallatin County communities, including Big Sky, are tested weekly with the exception of Bozeman which is being tested more frequently. In six weeks of testing, the most recent Big Sky test showed a detectable trace of the COVID-19 virus below the 40 Ct mark, resulting in a positive test, whereas in past weeks where the virus was undetectable.

“There’s something to watch there but it’s not [at] that level where we can say, conclusively, it’s showing anything,” Kelley said on June 16, prior to the most recent Big Sky wastewater test results.

Kelley recognizes the wastewater test results are valuable information to cross-reference as time progresses. “It’s a piece of the pie, it’s not the whole pie, so I think that’s kind of how we view it,” he said

Additionally, irrigation water samples from Big Sky have been tested for traces of COVID-19, all coming back negative without traces of the virus present.

In Big Sky, wastewater testing will continue on a weekly basis through the end of 2020 and possibly even through the winter months, according to Walk. “Right now, this looks like a really good indicator of virus in the community so as long as that’s the case I think it’s a very good resource to provide,” he said.

“We’ve now set a baseline [of] where we are in Big Sky, where we are in Bozeman, where we are in West Yellowstone,” Kelley said of the data collected from the wastewater tests. “Maybe the most important comparison to make is what it will look like in the fall and the winter in those places in the sewage testing, compared to where we are right now.”

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