Colby Cosh: The gold mine of population health data flowing through the sewers

We all hope the the virus won't get out of control where we live. But if it did, I would rather like my municipality to have been establishing the kind of baseline measurement that Ottawa has

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Ottawa, rightly or wrongly, is not a city especially noted for enterprise. But this week its public health unit began publishing measurements of SARS-CoV-2 viral RNA in wastewater samples from the city’s sewers, which shows a spirit of scientific adventure not otherwise much in evidence as this country handles the pandemic. Any big city is equipped to do this sort of experimentation, but Ottawa has the advantage that an enormous majority of its sewage outflow passes through one facility, the Robert O. Pickard Environmental Centre.

The results indicate that through four and a half months of sampling, counts of viral material in wastewater roughly track the number of case reports of COVID-19 in the capital region. I wrote about an early experiment of this kind in March, and there have been a few more since, mostly showing the same kinds of results.

Stop me if you’ve heard this before in COVID-19 articles, but it is rather a good news-bad news story when it comes to this method of epidemiological surveillance. You can see one problem in Ottawa’s chart: sewage analysis seems to have “predicted” a mid-July upturn in cases, but the sewage graph then diverges from the case count, and when it comes to the “second wave” that struck a few weeks ago, wastewater looks like a lagging indicator.

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One of the most thorough studies of SARS-CoV-2 detection in poo was published as a preprint by Montana researchers in July. They were looking at wastewater from the city of Bozeman, and they, too, found that the overall graph of virus counts in their samples matched the COVID-19 epidemiological curve during March and April pretty well.

Unfortunately for the hope of using the sewer as an early warning system, when they analyzed the twin charts they deduced that patients seemed to be having COVID-19 symptoms about a week before their viral shedding appeared in wastewater samples. In individuals, tests for SARS-CoV-2 and admissions to hospital are also a few days behind the first COVID-19 symptoms, so the “detection” in wastewater may end up being more or less simultaneous with the real-world consequences of COVID-19 spread. Poop data might just be telling you something your intensive-care nurses already know.

There is still some hope that wastewater might be useful, however. In situations where there has been a superspreading event in a young crowd, for example, the poo measurements would detect the growth in asymptomatic cases that is bound to transmit to the vulnerable later. In places where the overall capacity for testing has not kept up, the changes in the viral load in wastewater might help in assessing and calibrating lockdown measures or understanding the seasonality of COVID-19. We all hope the apocalypse scenario of absolutely uncontrolled and unsurveilled spread of the virus won’t happen where we live. But if it did, I would rather like my municipal government to have been establishing the kind of baseline measurement that Ottawa has.

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And as long as you are sequencing the RNA of the virus from wastewater samples anyhow, you can check which strains of the virus are prevalent in your community from day to day. The Montana researchers were able to establish the ancestry of the genotype of the virus then circulating in Bozeman; it descended from early cases in Australia and California, but had developed an all new mutation, one perhaps particular to Bozeman. Sewage sampling may turn out to be a handy shortcut in monitoring the evolution of SARS-CoV-2, or even identifying the most lethal varieties.

But, you know, this method isn’t destined to drop out of sight if and when the novel coronavirus does. Statistics Canada has already been messing around with the use of wastewater to survey drug use without having to ask anybody awkward survey questions. The use of wastewater monitoring for infectious disease was thought of because experiments had been done before with enteric diseases and other coronaviruses, including both the dangerous cousins of SARS-CoV-2 (MERS and classic SARS) and its less harmful ones (doing business under the name of the “common cold”).

It’s absurd but true: there are a zillion health, diet and substance-use parameters concealed in our feces, all of which can conceivably be accumulated without invading anyone’s personal privacy. Most have not yet been thought of. Public health units of the near-ish future may have to stop thinking of it as “waste,” and start thinking of it as a gold mine of population health data.

National Post
Twitter.com/colbycosh

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