(NEXSTAR) — The Houston Health Department warned in a news release Monday that the U.K. variant of the coronavirus is likely spreading at an “ongoing and uncontrolled” pace in the community.

Samples from Feb. 22 found that the highly transmissible form of the virus showed up in wastewater at 31 of 39 of the city’s treatment plants, up from 21 plants 16 days before.

“The prevalence of the U.K. variant in our wastewater shows it’s actively spreading in our city,” said Dr. David Persse, chief medical officer for the City of Houston. “This is another clear indication that we must continue to mask up, practice social distancing, wash our hands, get tested and, get vaccinated when possible.”

The release came two days before Texas rolls back the statewide mask mandate on Wednesday.

While transmission can be tough to track without reliable, widespread testing, wastewater offers city officials another resource for monitoring COVID-19.

Regardless of symptoms, people who have COVID-19 will shed the virus in feces. The Houston Health Department and Houston Water started testing in May 2020 to help them identify outbreaks as they started and pinpoint where the infections were concentrated.

Of the sample results taken Feb. 22, nearly 1 in 5 samples contained the UK variant. According to the release, results for other variants from South Africa, Brazil and California are pending.