Scientists from Yale University found Covid-19 genetic material in sewage would surge before cases were positively identified through tests.
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The experts say that carefully monitoring waste could serve as an indicator for outbreaks of infection and allow officials to enforce more localised lockdowns to stop the spread.
It comes after studies from early in the epidemic found that humans can "shed" coronavirus particles in their faeces.
Researchers from Yale in New Haven, Connecticut, decided to start looking at a waste treatment facility in the city for signs of the virus.
They monitored sewage at the plant, which processes wastewater from around 200,000 residents, for around six weeks – between March 19 and May 20.
The team found that levels of genetic material would spike around seven days before there were similar increases in positive coronavirus tests and hospitalisations.
They also noted that every sample of sludge analysed contained traces of the virus' genetic material, known as RNA.
Surge in sludge
The number of people in hospital with coronavirus in the New Haven peaked on April 12.
But the experts found that the concentration levels of Covid-19 peaked in the city's sewage three days earlier – on April 9.
That was an even earlier indicator for positive cases which officially peaked on April 16.
The researchers notice a similar pattern when it came to a decline in cases and hospitalisations too.
Their findings have been published on the medical website medRxiv.org and haven't yet been peer-reviewed.
Writing in the study, the authors said: "Our study could have substantial policy implications.
"Jurisdictions can use primary sludge SARS-CoV-2 concentrations to preempt community outbreak dynamics or provide an additional basis for easing restrictions, especially when there are limitations in clinical testing.
"Raw wastewater and sludge-based surveillance is particularly useful for low and middle-income countries where clinical testing capacity is limited."
Wash your hands
Experts have previously found that Covid-19 can be detected in faecal matter – emphasising the importance of handwashing.
However, no one has yet been positively identified as catching coronavirus in this way.
The US health agency, the Centers for Disease Control and Prevention (CDC) states that the risk of transmission of Covid-19 from the faeces of an infected person is unknown.
But added: "However, the risk is expected to be low based on data from previous outbreaks of related coronaviruses, such as severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS).
"There have been no reports of faecal-oral transmission of Covid-19 to date."
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To prevent virus spread, experts recommend frequent handwashing, avoiding contact with people who are sick, and taking precautions to protect others if you become sick.
Meanwhile, a study from China found traces of the virus in stool samples of patients who had been given the all clear after testing positive for coronavirus.
Scientists from the Institute of Infectious Diseases at Beijing Ditan Hospital, in China, made the discovery after taking swabs to determine whether the patients were able to be discharged from hospital and whether they needed to continue to self-isolate.
Among 133 patients admitted with Covid-19 from January 20 to February 27, 2020, the doctors identified 22 with traces of coronavirus in their sputum or stools.
And they even found that traces may remain in this excrement for between 13 and 39 days – before they completely disappeared.
Despite this, the researchers cautioned that it is not known whether these positive sputum or stool results indicate that the patient could still be infectious to others.
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