analysis

Updated: Apr 19, 2020 17:02 IST

The fight against the coronavirus (Covid-19) pandemic is at a critical juncture. To gauge the extent of the infection, the Government of India now plans to expand random testing in areas with no reported cases. It will use a combination of the Reverse transcription polymerase chain reaction, also known as RT-PCR, on pooled samples and antibody detection tests.

Another strategy that can help delineate the areas with silent transmission is testing sewage samples for the coronavirus (Sars-CoV2).

By now, we know that the Covid-19 disease is caused by Sars-CoV2, and that only in 20% of the cases, the severity of the infection requires hospitalisation. In 30% of the hospitalised patients, the virus has been detected in samples from the respiratory tract as well as from faeces. We also know that about 80% of the infections are mild and do not require hospitalisation. Among mild infections, the symptoms are mainly low-grade fever, slight cough, sore throat, running nose, but some also a few gastrointestinal symptoms such as diarrhoea, loss of appetite, nausea and vomiting. These symptoms may occur alone or in combination with cough and sore throat. Patients with gastrointestinal symptoms have a longer duration of illness and shed virus in stools. In nearly 25% of patients, nucleic acid particles are detected in stool as early as within three days of the infection – even before it appears in the respiratory tract. (However, the virus is detected in stools even in those who have only respiratory symptoms.) These findings suggest the importance of testing sewage samples to detect Sars-CoV2.

In past, sewage or waste-water monitoring has been used successfully to detect the outbreaks of norovirus, antibiotic-resistant bacteria, measles and polioviruses. Similarly, a few countries have already started monitoring sewage/waste-water for Covid-19. In Amsterdam, the virus was detected in waste-water at Schiphol airport, just four days after the Netherlands confirmed its first case, well before the infection was reported in the community. Its samples have been found in raw sewage in Tucson (Arizona). Sweden and Germany are also doing sewage analysis.

Sewage surveillance can be an effective method to ascertain the presence of the virus a community even if individuals are asymptomatic for Covid-19. Moreover, the density of the viral particles can give us an idea of the infection’s spread. This can also help us verify if the interventions such as physical distancing reduce the transmission. Regular and periodic sewage surveillance can be an early warning tool to alert public health authorities of the likelihood of a rise in cases, giving them crucial headway for preparation.

As it is not possible to test everyone in a community, sewage surveillance has the potential to unearth the positive cases within it. Such monitoring can also provide a better estimation of the infection’s spread as it can account for those who have not been tested, or show only mild symptoms, or are asymptomatic. Sewage sampling could also serve as an early warning about the emergence of clusters. It can also act as a sensitive indicator for the reappearance of the virus. The sewage surveillance for Covid-19 in India is possible and feasible.

India has, however, domestic examples to emulate. It is one of the first countries to use the sewage surveillance (also known as the environmental surveillance) to detect the silent transmission for the poliovirus through shedding in the community sewage system. India used to rely on acute flaccid paralysis (lameness survey). But it is a well-known fact that the paralysis happens only in severe cases; many infected children shed the virus without having the illness as well. In 2001, the environmental surveillance was initiated, with sewage sampling conducted weekly from three sites in Mumbai, and by 2017, there were 42 sewage sampling sites spread over Maharashtra, Gujarat, West Bengal, Uttar Pradesh, Bihar, Punjab, Telangana and Delhi. We have trained human resource to carry out the sewage surveillance for Covid-19. It can be an effective tool, especially in states with a large number of cases.

Since the infection was reported in December in China last year, we know some of the critical information about the virus, but there are still a large number of unknowns. One such unknown is if the virus particles are detected in stool, can they transmit through faeces? Scientists have detected high concentrations of viral particles in stool, but have so far not observed replication in the virus in it. Though, current evidence does not support faecal transmission of Covid-19 through stool, sewage surveillance can act as an infection-related marker of many things. It is worth giving it a try.

Dr Lalit Kant is an infectious disease epidemiologist. He is the former head of the Division of Epidemiology & Communicable Diseases and senior adviser, Infectious Diseases, PHFI

The views expressed are personal