Bengaluru waste water samples from 45 sites show 31% hike in Covid viral load

A reading of the sample collected.

A sewage surveillance system providing municipal authorities with data about the presence of the novel coronavirus has found 31 per cent of samples collected from open drain canals since November 28 are registering as positive.

With about 90 samples being collected per week from sites in 45 wards in the city, the last “high” recorded by the system was on June 20, when 35 per cent of samples collected came back as positive.

Cities in North America and Europe have adopted wastewater surveillance because scientists found that the people infected with Covid-19 shed causative novel coronavirus in their feces, irrespective of the symptoms.

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While the Bruhat Bengaluru Mahanagara Palike (BBMP) sources said the increasing viral load trends showing in the system have largely tallied with its own accounting of wards with high Covid-19 cases, experts cautioned that there are no hard-and-fast conclusions that can be drawn from the data.

Unlike in other countries, researchers in Bengaluru have not been provided access to sewer pipes. They can only collect samples from open drains managed by the BBMP.

This means “wastewater from a lot of households connected to the underground sewage networks drains out to sewage treatment plants (STPs) set up in the city, and may not enter the open canal system at all,” a scientist said.

Since many people are going uncovered by the wastewater surveillance, the system accuracy is affected.

Despite this, researchers from several private organisations involved in the project said the system provides between seven and 10 days’ advance warning to the BBMP about a potential increase in Covid-19 cases in several areas.

According to scientists involved in the project, the surveillance is more effective than other methods because it provides data from both symptomatic and asymptomatic infections.

“People infected with Sars-CoV-2 (which causes Covid-19) can shed virus particles through their feces days before respiratory symptoms show up. Hence, wastewater monitoring can provide an early opportunity to detect the virus even in the case of asymptomatic individuals,” a scientist added.

Assessing Covid-19 levels in the community by measuring the number of active cases may miss asymptomatic infections, especially as the government’s testing focus is on symptomatic cases such as people with Severe Acute Respiratory Infection (SARI) and Influenza-Like Illness (ILI), in addition to contacts of known Covid-19 cases.

As of Saturday afternoon, the surveillance system, as per online data, had noted that the viral trend increases at 25 collection locations in the 45 wards.

Two out of the 25 wards show up in the BBMP War Room’s list of top 10 wards reporting the highest number of positive cases in the last 10 days. At the same time, other wards in the top 10 such as Bellandur and HSR Layout showed declining viral load trends.

No certainties

Scientists explained that a high viral load from a collection site does not mean that there will be an imminent spike in cases in that particular ward.

“We have one collection point in each of the 45 wards. We collect samples from the same site twice a week. While the surveillance is not an indicator of the spread in a particular ward, it helps municipal authorities roll out a strategy to mitigate case numbers. The findings from all the sites cumulatively are a proxy for the city-wide wastewater surveillance,” a scientist said.

BBMP Chief Commissioner Gaurav Gupta acknowledged that the macro-level data coming from the system means that the source of the viral load increases cannot be pinpointed.

“However, it allows the BBMP War Room to coordinate with the individual wards to take steps such as increasing testing,” he said.

He added that ultimately the surveillance team would need to access the sewer pipes to help improve data being collected. “The permission is being sought,” Gupta said.

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