Volunteer Carol Mahoney of Washburn checks a patient's temperature as part of the screening process during a COVID-19 vaccine clinic held by AR Gould Hospital at Northern Maine Community College in Presque Isle in spring 2021. Credit: Courtesy of AR Gould Hospital

AUGUSTA, Maine – Wastewater testing shows wide ranges in COVID-19 prevalence across Maine in recent weeks as the virus recedes across the country.

The geographic disparities have not been apparent in recent weeks because a backlog in positive tests rendered daily case counts unreliable and made it more difficult to tell where new infections were happening. Health experts have touted wastewater testing as a useful tool because it can reveal traces of the virus even if someone is not ill.

The data released by the Maine Center for Disease Control and Prevention on Tuesday suggest that Maine has continued to see high transmission in certain counties even as total infections seem to have waned. Concentrations remain highest in Aroostook County, where Presque Isle is seeing one of the nation’s highest average totals of COVID-19 in wastewater.

Maine has greatly expanded its wastewater testing capabilities in the past few months, with more than a dozen districts now participating in a state program. All of the municipalities that introduced testing in late December or early January have seen a steady decline in the virus since then, according to data shared by the Maine CDC on Tuesday from the company Biobot, which contracts with governments to analyze wastewater for COVID-19.

Testing from the Westbrook-Gorham Wastewater Treatment Facility as of Feb. 16, for example, revealed 355,000 copies of the coronavirus per liter of sewage, compared with more than 5 million copies per liter when samples were first collected earlier this winter.

That suggests a “dramatic” decrease in the virus within that community, said Dr. Gibson Parrish, an epidemiologist formerly of the U.S. Centers for Disease Control and Prevention.

“Within that group of people that are served by the Westbrook-Gorham treatment facility, it’s gone way down,” Parrish said, “but, that said, 355,000 copies per liter of sewage is still a fair amount of virus in the sewage.”

That reported virus level positions Westbrook-Gorham as having less virus in its sewage than about 63 percent of U.S. communities tested over the past six weeks. Several other communities, including Brunswick, Belfast and Blue Hill, report even better numbers.

But other Maine towns do not rank as highly, with the greatest virus concentration appearing in Aroostook County water districts. Presque Isle reported the highest virus levels, with more than 6 million virus copies per liter of wastewater. That was higher than 96 percent of U.S. communities tested in the past six weeks, but comparable to the virus concentrations that Westbrook and Gorham reported earlier this winter.

As of last week, Presque Isle and five other Maine wastewater districts — Lewiston-Auburn, East Millinocket, Calais, Houlton and Fort Kent — ranked worse than the median U.S. district in terms of wastewater, the data show.

Maine has seen significant geographic variation in coronavirus spread throughout the pandemic. Southern Maine saw the most cases during an initial wave in spring 2020, while mostly rural and less-vaccinated zip codes were hit harder by the delta variant wave this past fall.

Daily case counts have been the primary tool for comparing virus transmission between counties. But a backlog of positive tests that had reached 58,000 positive tests earlier this month distorted the state’s case numbers. The Maine CDC has pointed to wastewater testing as a key metric in recent weeks noting the backlog as well as the widespread availability of at-home tests, the results of which are not reported to the state.

As of Tuesday morning, the Maine CDC reported the backlog had shrunk to around 6,000 positive tests. The agency began automating part of its count last week as the rapid spread of the highly contagious omicron variant overwhelmed its ability to keep up with daily cases.