Yosemite Sewage Tests Positive for Coronavirus

Test results suggest there were dozens of visitors carrying the novel coronavirus in the park over the Fourth of July weekend

Yosemite Valley
Yosemite Valley seen from the Tunnel View lookout point in the Yosemite National Park, California on July 08, 2020. The park's sewage has now tested positive for the presence of the novel coronavirus, suggesting that some of its visitors over the Fourth of July weekend were infected. APU GOMES/AFP via Getty Images

When Yosemite National Park reopened in early June, allowing about half the usual number of visitors to enter its open air cathedral of granite monoliths, the hope was that the glacier-carved valley could safely host them. But now tests of the park’s sewage reveal the virus is silently circulating in and around the park, reports Kurtis Alexander for the San Francisco Chronicle.

Prior to the virus’ detection in the park’s wastewater, no employees or residents had tested positive, nor had any visitors reported being sick, reports the Associated Press.

The positive test results, which were produced by a lab called Biobot based in Boston, Massachusetts, suggest that dozens of infected people were in the Yosemite area over the Fourth of July weekend.

“It’s one thing to live in denial: We live in the mountains, no one’s sick,” Eric Sergienko, the health officer for Mariposa County which is handling testing for the Yosemite area, tells the Chronicle. “But we can now confirm it’s here.”

The tests, which spanned the week of June 30 through July 6, rely on the fact that carriers of the novel coronavirus shed viral genetic material in their poop. For Smithsonian magazine, Katherine J. Wu reported in May that monitoring the virus in raw sewage can even act as an early warning mechanism in advance of a surge in COVID-19 cases. That’s because people tend not to get tested until they feel sick, if they feel sick at all.

"There's a lot of chatter now about doing wastewater testing, and for us it made sense. We have a highly mobile population that visits the area, and so it's difficult to capture that data in terms of lab testing and testing people," Sergienko tells Sara Sandrik of local broadcast station ABC 7.

In May, Mariposa County began testing sludge from wastewater treatment facilities in Mariposa, Wawona and, in June, El Portal which services Yosemite Valley, reports Carmen George of the Fresno Bee.

Once a week, the county sends samples to Biobot’s lab, which is also conducting testing for 400 other wastewater treatment plants in 42 states, according to ABC 7. Per the AP, Biobot told Mariposa County that, based on the amount of viral material they found, there may have been around 170 people infected in Yosemite Valley between June 30 and July 6.

Biobot is one of just a few labs conducting wastewater testing willing to provide such estimates, according to the Chronicle. While the testing can provide general trends, like whether cases of the virus are going up or down, researchers tell the Chronicle it’s not currently able to provide a precise measurement of prevalence. The Chronicle reports that some of the complicating factors include the fact that not everyone infected with the novel coronavirus sheds the virus’ genetic material in their feces and that signs of the virus can show up in the stool of people who have already recovered.

The positive test results came in one week before spiking cases of COVID-19 caused California Governor Gavin Newsom to reverse course on the state’s plans to reopen indoor businesses. Yosemite National Park shut down on March 20 and reopened with restrictions on June 11. Presently, the park remains open but requires visitors to secure reservations. The reservations hope to cap visitorship at around half what would be expected for this time of year under normal circumstances. In 2019, Yosemite received about 4.6 million visitors.

"It is a little bit scary to know that that the virus is around, which rededicates us to our processes for sanitization and for working with managing the visitors managing the crowds," Mariposa County Board of Supervisors Chair Kevin Cann, tells ABC 7.

Per ABC 7, the county plans to keep testing its sewage through January with the help of funding from the federal CARES Act to cover the roughly $88,000 cost.

The question of what would have to happen for Yosemite National Park to again close its gates to visitors is a murky, thorny question currently being faced by many of the 62 national parks across the country, report Elizabeth Williamson and Sarah Mervosh for the New York Times.

Carolyn Coder, an environmental health specialist for Mariposa County, tells the Chronicle that the wastewater testing will help inform whether more restrictions are imposed in the coming months.

“It’s part of the overall decision-making,” Coder tells the Chronicle. “Do we need to go back to shelter-in-place? Do we need to go back to stage one?”

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