Can COVID-19 be traced via the sewage system?

Coughs and sneezes traditionally spread diseases, but could coronavirus particles shed into the sewage system be another source of COVID-19 infection, and a valuable way to track the spread of the disease?

SARS-CoV-2, the virus that causes COVID-19, is shed in the faeces of the majority of patients, causing concern that the illness could spread this way^1,2 although, according to a recent report from the World Health Organisation (WHO), there are currently no recorded cases of this having happened. It is also not yet clear whether the virus detected in faeces is able to infect a new patient. Nevertheless, hygiene measures such as hand-washing and existing methods of wastewater treatment, they say, can help to reduce any possibility of transmission³.

But the finding of SARS-CoV-2 in faeces raises an interesting question: can we use virus shed into sewage to track the pandemic? The answer would appear to be "yes", if a recent study, by researchers at the KWR Water Research Institute in the Netherlands showing that SARS-CoV-2 can be detected in samples of wastewater across the country, turns out to be true⁴.

In the new study, sewage samples from water treatment plants in seven cities and one airport were tested for SARS-CoV-2 using a technique called RT-qPCR. Similar to the tests used to detect SARS-CoV-2 from throat swabs for COVID-19 diagnosis, this involves using the polymerase chain reaction to make millions of copies of specific parts of the genetic material of the virus. No SARS-CoV-2 signatures were detected in samples collected three weeks before the first COVID-19 case was reported in the Netherlands on February 27th. But six of the seven samples taken on March 5th, and six out of eight samples collected on March 16th, contained viral genetic sequences. By this point, the country had 1413 reported cases of COVID-19.

Recording levels of SARS-CoV-2 in sewage over time could be used to monitor the spread of the virus, providing useful information that may slip under the surveillance radar for COVID-19. Until recently, only patients with very specific symptoms were being screened, so anyone presenting differently, or even asymptomatically, was being overlooked. For instance, one city in the study had no reported cases of COVID-19 yet SARS-CoV-2 was detected in samples taken from its water treatment plant, proving that cases were going unrecorded. This “sewage surveillance” could potentially be used to detect re-emergence of COVID-19 in cities before clinical cases are reported, a method previously used in outbreaks of poliovirus.

One drawback is that it is difficult to know how sensitive or accurate this method is, because we do not know the true number of COVID-19 cases in the Netherlands. So it might be useful to gather data from countries where testing has been more widespread - and the number of reported cases is thought to be more accurate - so that a more valid comparison can be made. Additional information from more locations may also help to provide a more complete picture of how the levels of virus in sewage samples relate to the prevalence of the disease in a population. 

Critically, the method used in this study detects the presence of the genetic information of the virus, but this cannot tell us whether the virus is present in an infectious form. It could be that their trip down the toilet has rendered the viral particles defective and no longer capable of triggering an infection. Nevertheless, if the virus does remain intact, while it seems unlikely that the virus can survive the chemical treatment of wastewater, in countries with poor water-treatment infrastructure it is possible that SARS-CoV-2 could spread via wastewater. This possibility needs to be looked into urgently.

While we wait for clarity on this matter, what seems absolutely clear is that the presence of the virus in sewage systems is a useful tool for scientists tracking the spread of the disease and it could also be a handy early warning of re-emergence as countries seek to lift their lockdowns.