The wastewater project is led by Dr Joanne Hewitt, head of ESR's Environmental Virology Laboratory and ESR Science Leader, Dr Brent Gilpin. Initial work on the project was funded by the MBIE’s Covid-19 Innovation Acceleration Fund, with the Ministry of Health now funding further application of the work as part of New Zealand’s COVID-19 response.
Wastewater testing is currently occurring at sites around the country.
How do you collect it and test it?
The preferred option is the automatic composite sample. This is where a pump automatically collects a small volume of wastewater every 15 minutes over 24 hours. These are already setup at most wastewater treatment plants and most of the samples are taken this way. It means you get a sample which is reflective of multiple points in time over the day. While many treatment plants have these samplers, some don’t and if you are looking to investigate a smaller area, sampling from a manhole for instance, you have to use grab sampling, which is exactly as it sounds. You are taking a sample directly from the wastewater at one point in time. Which means you basically get a snapshot in time which is dependent on diffusions and flows, so it’s less reflective of the overall virus in the sewage for that time.
Once we receive the one litre samples at ESR’s laboratory in Wellington, our laboratory staff have become very good at processing them quickly. First, we take up to 500 ml from those samples and store the rest. As viruses can be attached to the solids (poo) and be in the water, we use methods that will recover viruses from both parts, so we don’t lose any virus. This is important if we don't expect a high level of virus in the sample. Then we concentrate it all down to about half a teaspoon, basically removing all the water and leaving the virus behind. From there it’s the same process as you go through for regular testing for the virus of someone’s swab, we extract the viruses, turn the RNA into DNA and run it on a PCR to tell us if the sample is positive or negative for the virus.
We also add a cat coronavirus and a mouse norovirus to every sample we test as a control to see how good our methods are. The cat coronavirus is a good control for SARS CoV-2, while the murine norovirus is more like the enteric viruses we usually look for in wastewater. We store the other 500 ml to confirm an unexpected result. Remaining viral nucleic acid and concentrated samples are also being stored for future work on human viruses in wastewater.
Can we trust the results?
There are two factors – specificity and sensitivity. How do we know we’ve detected what we want to detect and how sensitive is the test at detecting it. We have been working for many months of both as part of the ongoing research.
In terms of specificity – we are very confident in the precision. When we detect viral RNA in a sample, we are very confident that it is actually from SARS-CoV-2. PCR is a widely used methodology, is extremely sensitive and can produce false positive results particularly when we are looking for things at very low levels. This is something we constantly work to guard against. We have a lot of positive and negative controls, and lots of replicates. Before reporting a positive, particularly a very low level unexpected positive, we would always repeat the PCR, and then the whole extraction on a stored replicate sample. It’s tough on the lab as this would invariably require working late into the night, but we are committed to results that we are completely confident in, and which can inform decisions made by Government.
The second part of this is sensitivity. It is theoretically possible to detect a single person shedding virus, but unlikely in practice – at least in a reproducible manner. Based on what we have observed so far, our test is sensitive enough to discover approximately 10 cases in an area of 100,000 people. That’s not a hard and fast rule as some people shed at different rates based on time of infection, and there are many other factors that affect detection. Generally, we would need more cases in large catchments to increase the likelihood of detection because as more people contribute to a sewerage stream there will be dilution in the wastewater which will alter the level of sensitivity.
It’s kind of like sharks detecting blood in the water. If we think of the blood as the virus and the sharks as our sampling. People say that sharks can detect a single drop of blood from hundreds of metres away. The more drops of blood in the water, the more likely they detect it. On the same line, the more sharks in the water in the right area, the higher likelihood they detect it.
Does the 10 in 100,000 guideline always hold true ?
This a guideline not a hard and fast rule. There is evidence of the detection of SARS-CoV-2 in wastewater where there were only three known cases in the catchment, and ESR consider that a single person shedding the virus may be detected as one person can shed a large amount of the virus.
A positive detection in the wastewater indicates that at least one person has been shedding SARS-CoV-2 into the wastewater in the 24 hours before each sample was collected. Just as a negative finding does not necessarily guarantee an absence of COVID-19 in the community, it should be noted that a positive finding does not necessarily mean that active COVID-19 infections are present in the community. A positive detection in the wastewater could also be due to:
* a case in a managed isolation facility
* a non-infectious historical case or a non-infectious recently recovered case who is still shedding low levels of SARS-CoV-2.
Detection sensitivity can depend on a number of factors including the size of the sewerage network and the presence of recently recovered cases that are still shedding the virus.
Can you get infected via wastewater?
Wastewater sampling can detect these fragments, but there is no evidence that the wastewater itself is infectious.
What are the limitations to it - what can't it tell us ?
Wastewater testing can tell us someone in the network shedding virus, but not specifically where or who is shedding the virus, or the exact number of people who have contributed to a positive detection. Shedding levels can vary significantly between individuals and at different stages in an infection and not every infected person may shed the virus.
When do people "shed" the virus?
People who have been infected may continue to shed virus after they are no longer infectious. Published data suggests that this can continue for weeks after the initial illness. However, most of the data we have in this space comes from hospitalised individuals, most of whom are very sick and some with ongoing immune issues. Not really the situation we have in New Zealand with those in managed isolation, where a third of cases are asymptomatic. Also, most are younger and fortunately most have milder symptoms. Therefore, how long shedding in faeces occurs in most of the COVID-19 cases in New Zealand is an unknown. What we do know is that this post infectious shedding by people will produce much lower levels of virus in faeces, which is good for New Zealand’s unique situation with the small number of cases we currently have.
We have tested now several hundred wastewater samples, many of which will have had people who previously had COVID-19 contributing to them. What we see when an outbreak occurs is a consistent detection of virus at relatively high levels. That’s one of the reasons we like to collect a series of samples. Historical cases could produce the occasional sporadic low-level result, but they are not going to produce ongoing detectable levels.
What happens when a low-level result is returned?
The protocol we developed with the Ministry of Health is that if you have a very low-level unexpected detection, the team immediately analyize a second sample taken at the same time, and then take a repeat sample a day or two later.
It is also important to remember that not everyone’s home is connected to a municipal wastewater system. Many of these people may however work or socialise in places that are connected to sewer and therefore still get captured by wastewater testing. And those in most isolated places are less likely to get infected, and less likely to spread the virus to others, than those living in higher populated centres.
There are always complicating factors, but we continue to learn and build upon our findings.
Who is ESR working with on this?
ESR is part of ColoSSoS (Collaboration on Sewage Surveillance of SARS-CoV-2)(external link) a project led by Water Research Australia and operating in parts of Australia, New Zealand and the Mekong countries. The project has the goal of ensuring experts are liaising and using the most reliable and robust methods and techniques to find a way to conduct this work for the good of communities.
We also have to remember, that this research is going to be useful for other viruses longer-term. The whole area of wastewater epidemiology is a burgeoning area worldwide. Illicit drugs and infectious viruses are just two of the possible areas that wastewater analysis may allow us to understand. The smart toilet of the future may analyse what you deposit in your toilet to help with your personal wellbeing and health, while on a societal level wastewater analysis may allow us to better understand the behaviours and health of New Zealanders across a whole range of health and wellbeing issues. You might say there really is “information gold” in wastewater, and wastewater epidemiology is something you are only going to see more of in the coming years.