On 22 July, Singapore’s Ministry of Health announced that residents of four Housing and Development Board (H.D.B.) blocks along Sims Avenue had to undergo mandatory testing for Covid-19.
Interestingly, the four blocks in questions were located opposite the Geylang Serai Market, which was one of the several localities linked to the much larger Jurong Fishery Port cluster.
Obviously, living near a Covid-19 cluster wouldn’t be enough for authorities to order entire housing estates to get tested. If that were the case, a large chunk of Singapore would be in line to get tested right now, considering the small size of the island.
There needed to be something else which forced authorities to take this drastic step.
And in this case, the answer had to do with human poop.
What is wastewater surveillance?
Singapore has been using wastewater surveillance as part of its overall efforts to trace and contain Covid-19.
It was launched as a pilot programme in June 2020 to support the monitoring of Covid-19 transmission in 20 large dormitories. The programme was deemed effective as it could pinpoint specific dorms with potential Covid-19 transmission.
These pinpointed dorms would then undergo more swab tests leading to more detections and isolation of cases, including asymptomatic ones.
For dormitories where wastewater surveillance yielded a zero reading for the Covid-19 virus, the programme provided assurance that workers remained Covid-free, allowing them to leave their dormitories for work.
In July 2020, the wastewater surveillance programme was extended to residential sites and was first deployed in Tampines, leading to the detection of COVID-19 cases in a residential block.
Since then, Covid-19 viral fragments have been found in wastewater from several neighbourhoods including Bukit Merah View, Hougang and Yishun, prompting authorities to conduct swab tests on residents and visitors.
Today, more than 200 sites, ranging from workers’ dormitories to nursing homes and residential areas, are currently under surveillance, with over 2,500 samples of wastewater being tested each week.
At the start of this month, the government also announced its intention to double the number of overall testing sites from 200 to 400 by 2021. This signifies its interest in making wastewater surveillance a more important part of the country’s overall testing strategy.
But how does the programme even work? And how effective is it at catching Covid-19 positive cases?
Where does wastewater surveillance happen ?
Wastewater samples are collected at three levels.
Regional Level
On the first level, the National Environment Agency (N.E.A.) says that sewage samples from “regional nodes” are tested for viral fragments. While it declined to specify where these nodes are, it is likely that they test on a neighbourhood or regional level.
Since this type of surveillance casts the net extremely wide, it would be impossible to identify individuals with Covid-19. In other words, if the government finds viral fragments in wastewater from a neighbourhood, it would be nearly impossible to order the entire neighbourhood to get tested.
Hence, testing at regional nodes are not meant to pinpoint individuals or potential groups with the virus. Instead, they provide the government with “a sense of the prevalence of infections in the community”. This might be useful data in determining policy responses – lockdowns, enhanced social distancing and so on.
High Risk Level
On the second level, the N.E.A. seems to have set up permanent surveillance sites at high risk areas. As stated earlier, there are currently more than 200 of these sites at workers’ dormitories, hostels and nursing homes. In all of these examples, the groups in question are either living in close quarters with one another or are might develop serious illnesses if they catch the virus. As such, this level of testing might be to pre-empt large or deadly clusters.
Targetted Level
The last level of testing is more targeted. Authorities seem to be testing wastewater at places where they expect to see transmission. In the past, they have surveilled wastewater at housing blocks which are geographically close to known Covid-19 clusters. This seems to be how the four blocks at Sims Avenue were singled out. In other instances, authorities employed wastewater surveillance on housing blocks with known cases, even after these individuals are taken away to medical and isolation facilities. This way, it is possible to determine whether family members and the people who lived with the Covid positive individual get the disease themselves while being quarantined.
On the last two levels of testing, if viral fragments are found in the wastewater sample, authorities might order the group being surveilled to get swab tested at designated regional screening centres. Anyone who visited these groups’ households might also be encouraged (but not forced) to be tested.
If a positive Covid-19 case is detected, authorities will isolate the case, identify all close contacts and test and quarantine them to protect the community from further transmission.
The science behind wastewater surveillance
An individual who is infected with the Covid-19 virus will shed viral fragments in their stool, saliva, and mucus.
The viral fragments in question are ribonucleic acid (RNA) molecules which carry the genetic information of the Covid-19 virus. They contain all the instructions for making all the parts the virus needs to replicate itself. Since each RNA is unique and is associated with a corresponding virus, scientists can tell whether a person has been infected by Covid-19 by testing for its RNA.
Considering that wastewater contains the Covid-19’s RNA molecules, authorities can use it to test for the presence of the virus as well. Thus, the N.E.A. collects wastewater samples from suitable sites ranging from water reclamation plants to H.D.B. estates.
In residential areas, authorities insert a rubber sampler tubing into a manhole and connect it to a machine called the autosampler. The autosampler is programmed by the team to collect wastewater samples during specific time periods. Typically, wastewater is collected on an hourly basis during the diurnal sewage peaks (0500–1500 h and 1700–2200 h).
The samples, which contain about 45 millilitres of wastewater each, are then sent to laboratories. There, organic compounds that may interfere with testing are removed, and chemicals are added in order to concentrate the RNA molecules in the water.
Next, samples extracted from the previous step are measured using a Covid-19 PCR test, similar to those administered at test centres around the island. During PCR testing, substances known as DNA polymerase are added to the sample. These substances create numerous copies of the viral RNA present, so that there will be enough RNA to produce a positive result, thereby increasing the accuracy of the test.
The results also show scientists the amount of RNA fragments present in wastewater at any given time. This is useful for the regional node testing, which aims to gauge the extent of spread in a community.
The future
Across the world, wastewater surveillance is being used in various innovative ways to safeguard public health. In the United Arab Emirates, wastewater from incoming commercial aircrafts are being tested to determine whether they are carrying infected passengers.
Scientist in Singapore have also found ways to determine the variant and linage of a Covid-19 virus present in a group by using just wastewater. This allows authorities to better “trace infection clusters and consider suitable public health measures”.
Considering the N.E.A.’s doubling of wastewater surveillance sites and the various developments in the field, it is looking your poop might play a large role in tracing and containing Covid-19 in Singapore.
