Yosemite national park officials suspect that hundreds of visitors this summer may have had Covid-19 thanks to an unorthodox approach – testing sewage.

The San Francisco Chronicle reported last week that the county health department has been collecting untreated wastewater flowing from the idyllic Yosemite Valley for testing. Prior to this effort, according to the Chronicle, no one had tested positive for the virus through nasal swab testing at the park’s health clinic.

Photograph: Ezra Shaw/Getty Images

Scientists at a lab called Biobot Analytics in Cambridge, Massachusetts, have examined the sewage water to determine if there are traces of genetic material from Sars-CoV-2, the coronavirus that causes Covid-19, in the human feces. From the traces in a given sample, they can estimate how many people passing through Yosemite might be infected with Covid-19 at that time.

“It just struck me as a really good idea,” said the Mariposa county health official Dr Eric Sergienko. He said that while the county was testing and monitoring local residents, “we really did not have a really good way of monitoring our visitors for Covid-19.”

The county began testing the wastewater from the Yosemite Valley, where thecelebrated granite Half Dome is located, in June. Based on their findings, health officials believe that an estimated 170 people in the park the week of the Fourth of July may have been infected with the coronavirus. The following week, that number dropped to 60.

In the neighboring community where wastewater is also tested, the estimate went down from 60 to 50 over that same two-week period. “For me that suggests that indeed the volume that we saw on the Fourth was directly related to visitors to the area,” says Sergienko.

Related: ‘Not a mask in sight’: thousands flock to Yellowstone as park reopens

For now, Mariposa county will continue testing the sewage water every week, and maintaining safety protocols currently in place.

The park closed its gates in March – partly, according to Sergienko, to avoid straining the local healthcare system with sick travelers. Now, the park requires visitors to make reservations to get in and has cut attendance by about half of what it was in June 2019. The park works closely with neighboring counties to implement safety measures. But since the national parks are under federal jurisdiction, it cannot force visitors to wear masks, for example.

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Kristen Brengel of the National Parks Conservation Association, a non-partisan group that advocates to protect national parks, said this decision came from the top. “The political appointees of the interior department have made it clear they do not want to make masks mandatory in parks. They’re doing this by choice.” Most parks are only able to recommend that visitors wear masks.

Brengel also says crowding at popular outdoor sites is a risk to visitor and employee health. Photos on the Yellowstone national park’s Facebook page show crowds lined up for its famous geysers, with about half the people wearing masks. In Yosemite, timed entry is helping space out crowds, and Brengel says a handful of other parks, including Denali national park and reserve and Rocky Mountain national park are implementing this tactic.

The National Park Service did not return a request for comment, but a public health update on its website asks visitors “to be our partner in adopting social distancing practices when visiting parks”.



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