Searching sewage for coronavirus
You’ve probably heard of tracking the coronavirus by swab, or by app - but what about by sewage? That’s the “delightful” proposal from scientists who want to figure out how many people are infected in - for example - a city, based on the amount of virus those people have flushed away in their waste. It’s an unpleasant but promising new weapon in the covid-hunting arsenal, and in theory you can test sewage water the same way as saliva: by using a technique called qPCR to look for the virus’s genetic code, or ‘RNA’. Andrew Singer at the Centre of Ecology and Hydrology is helping lead this project, and Phil Sansom heard from him about the samples being provided by water companies from their treatment plants.
Andrew - They provide a litre worth of the sewage. We're effectively going to concentrate the RNA from the sample. And that goes into the qPCR, which is to quantify the amount of RNA that is present. That is the shortest version of the pipeline that we're going to be using.
Phil - That's a much better reaction than most people would have if you gave them a litre of sewage.
Andrew - Well, it's amazing how much information is embedded in our wastewater.
Phil - How much can you get out of that amount of sewage - a litre of sewage?
Andrew - The amount of virus that would be shed by a person might be in the hundreds of millions of virus particles per gram of feces, and millions per litre of water. What we're trying to understand is, what are the limits of the detection? How few people in any one population can you detect? There were some sewage works where you have only a couple hundred people, and so it might be that one person is infected and you could see them. But one person in a sewage works of 200,000 people is probably going to be in the noise.
Phil - What are the things that are complicated when you're trying to figure out how many infected people equals how much coronavirus in the sewage?
Andrew - The numbers of things that complicate this seem to have no end at the moment. What we'd love to be able to do is have confidence about the direction of travel: that there are fewer cases, or is it stable, or is it going up? It's obviously more helpful if you can then put numbers on that. And the real strength of this approach - the Holy Grail - you want to be able to say that there's a hundred people in this catchment who appear to be shedding the virus and traditional surveillance can only account for 75 of those hundred, which means that there's 25 people who don't know that they're infected, but they're shedding the virus. And so having the asymptomatic shedders included in your sample gives you the power of knowing the size of the beast, so to speak.
Phil - How far away are you from that Holy Grail of finding out the actual quantitative numbers?
Andrew - Well, we're definitely having a go at it! It's a very difficult challenge to overcome. There's a number of different factors that can affect the shedding rate, and all of that will be important to feed into the model for then making better estimates.
Phil - In the meantime, right, you're planning this huge operation to do this kind of testing at, what, every sewage plant across the UK?
Andrew - Ah. So even in the national scale surveillance, which is being run by the government, they'll only be sampling 50-100 different sewage works. Because logistically it's not going to be possible to sample the 9,000 sewage works that are within the UK. So we have to take this triage, canary-in-the-coal-mine approach of saying, "well, where do we think would give us the best information about COVID, and also the likelihood of it transmitting to another area of the UK?" So maybe commuter populations would be a really interesting place to be sampling.
Phil - How useful is sewage for that? Because when I think about the coronavirus, I think about coughing it out; I don't - excuse me for this - necessarily think about pooing it out.
Andrew - I would imagine virtually no-one does. There's a lot of infectious disease in poo, which is why we have the problems that we do in the world with sanitation. And then you have coronavirus, which you would not predict is going to be persistent in wastewater; but the reality is that it is.