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COVID-19 arrives in Yosemite, sewage tests confirm


YOSEMITE NATIONAL PARK (KRON) — Yosemite National Park and its beautiful wilderness, filled with gushing waterfalls and granite mountain peaks, was an oasis away from the coronavirus. There were zero cases of COVID-19 for months.

That’s because Yosemite was one of the first national parks to shut down when the outbreak first began. “Literally, (Yosemite was) disease-free. There was no chance of there being cases in Yosemite from March up until June 11 because there just wasn’t travel into the park. And … we knew that there were no local cases,” said Mariposa County Health Officer Eric Sergienko.

But Mariposa County health officials knew they needed a way to detect when the virus inevitably arrived, especially when the park reopened to tourists in mid-June.

Visitors take pictures at the Tunnel View lookout on June 11, 2020 in Yosemite National Park. Only about half of the average June visitors will be allowed in, and they must make an online reservation for each car. (Photo by Getty Images)

While the county is home to just 18,000 residents, the park hosts 4 1/2 million visitors in a typical year. Seventy percent of the county’s economy is driven by tourism. The balancing acts of reopening to tourists vs protecting the health of local residents, is a difficult one, Sergienko said. He described the challenge as “threading the needle.” 

At first, health officials wondered, how can we track coronavirus cases, when so many people are just passing through as tourists? They found the answer in a dirty yet reliable place: Sewage.

“How do you look for disease in 4 1/2 million people that come up to either Mariposa, or the park, or both? They are not getting tested in our testing facilities, they are not going to our hospitals. But they are passing through. So that’s why we … started looking for the virus in wastewater,” Sergienko said.

Testing the park’s wastewater from visitors in July revealed that COVID-19 had definitively arrived in Yosemite, with a spike over Fourth of July weekend.

The gave health officials the evidence they needed to conclude that some tourists infected with the virus were visiting the park. COVID-19 has been hard to contain because many of those infected do not experience symptoms.

Park Ranger Maria Ayala hands out a daily permit at the entrance. (Photo by Getty Images)

Q&A with Mariposa County Health Officer Eric Sergienko

Question: What was the COVID-related subject focused on today at the Mariposa County Board of Supervisors meeting?

Answer: As far as COVID, the board is concerned that, given the Governor’s guidance from last Monday, shutting down restaurants, closing down bars, is, as a tourism-based economy, that impacts us significantly. So how do we thread the needle? What is the right level of activity? How can we support our business and at the same time decrease the risk (of infection) to our population, or at least mitigate the risk to our population? That was a good conversation we had with the board, in recognizing the concerns of the community. A third of our population in Mariposa are an at-risk population. So balancing their (health) concerns and balancing economic drivers — 70 percent of our economy drives from tourism. So it’s threading the needle continuously and guiding a path that is difficult. 

We have had at least two cases where people were working in the hospitality industry, that is, working with tourists on a regular basis. Those two cases came up over the past week. Given their role, and given the timing, and their travel history of not going outside the county, that those were most likely related to an encounter with a visitor from outside of the county. 

((Photo by APU GOMES/AFP via Getty Images)

Question: Do those two hospitality employees work in Yosemite National Park?

Answer: I can only tell you that they work in the county. Because under privacy act information (laws), I can only narrow it down to a population of 20,000. And the population of the entire county is 18,000. Those two individuals (infected with COVID) are residents of Mariposa. We have 18,000 residents, but we have 4 1/2 million visitors. So how do you conduct surveillance, how do you look for disease, in 4 1/2 million people that come up to either Mariposa, or the park, or both? They are not getting tested in our testing facilities, they are not going to our hospitals. But they are passing through. So that’s why we did the (wastewater sewage testing for COVID) and started looking for the virus in wastewater. 

Question: What was happening in Yosemite when the COVID outbreak was just beginning? Was Yosemite an oasis free from the virus for a while? 

Answer: We’ve had an incredible working relationship with the (Yosemite National) Park, and the other gateway counties, we formed the Yosemite Valley Gateway Area Coordination Team. The park was very proactive in closing the gates to the park, so they were one of the first national parks to shut down. So, literally, they were disease-free. There was no chance of there being cases in Yosemite from March up until June 11th because there just wasn’t travel into the park. And even travel in the local area, we knew that there were no local cases. But on June 11th, the park opened up. So we started to increase that risk. We have seen an increased transmission in the past month, from June 11 to the present. So as visitors come into our area, we have seen residents become sick with COVID. The thinking was, it will eventually get here, (so) how do we measure and monitor for that? 

A rainbow shines through the mist at Bridalveil Falls in Yosemite National Park. (Photo by APU GOMES/AFP via Getty Images)

Question: Since millions of visitors are coming and going from Yosemite National Park, is that why the county decided to use wastewater testing to find accurate numbers of COVID cases? 

Answer: Yes that’s exactly why we went with the wastewater testing. Because even if you don’t visit a testing site, you’re going to be contributing to our wastewater system in some fashion (using and flushing a toilet) if you’re in the county for at least a day. So getting to look at a large percentage of the visitors through wastewater sampling, we addressed the issue of (knowing) what’s going on with our visitors vs. our own residents. So that was the idea behind that. (COVID-positive test results) turned positive when I expected it would turn positive, which was our highest day of visitation. Around the July Fourth weekend. We got results back from the following weekend, and we actually saw a decrease.

The important thing about the wastewater testing is it’s just one part of the surveillance puzzle that we put together on a daily basis. We look at the wastewater, we look at our laboratory testing. We also have our employers, whether they are a small business or county agency, do daily febrile respiratory screening. The worst-case scenario here (would be if) we get a cluster of cases that pushes us into surge. The challenge (would be) we have one small critical access hospital. If all of the hospital in the San Joaquin Valley are full, there will be no place to move (patients) to.

Question: What is the plan for allowing visitors into Yosemite for the rest of the summer? Are any changes being made? 

Answer: The park itself is in federal jurisdiction so they have some degree of control over what they do. They have been in really close cooperation with us. They have been adhering to the state guidance, so restaurants are closed to dine in, bars are closed. They also reduced visitation. They are doing that by doing daily reservations. It limits it to about half the number of visitors that would be (in the park) on a normal July day. The overnight population of people in the park is about 11,000 people, so about half of (normal numbers). There is not going to be really any change … in terms of policy and procedure. But (COVID-positive test results) do make people aware that we do have COVID here, and it is a concern. It’s easy to ignore something when you don’t have physical evidence that it is present. So now we have that, people need to pay more attention. We will continue to monitor our own county and the counties around us and the counties where travelers come from. And if we see a need to level on more restrictions, we will do that. It’s not a decision made on one piece of data, but multiple pieces. 

A sign explains social distancing at Yosemite Falls on June 11, 2020. (Photo by Ezra Shaw/Getty Images)

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