The new coronavirus, SARS-CoV-2, may have been in Europe for longer than previously thought. Recent studies have suggested that it was circulating in Italy in early December 2019. More surprisingly, researchers at the University of Barcelona found traces of the virus by analyzing untreated sewage samples dated March 12, 2019.
The study was recently published on a prepress server, medRxiv. The document is under critical review by external experts in preparation for publication in a scientific journal. However, until this peer review process is complete, the evidence should be treated with caution.
So how was the experiment conducted and what exactly did the scientists find?
One of the first findings about SARS-CoV-2 is that it is found in the feces of infected people. As the virus makes its way through the intestine, where it can cause gastrointestinal symptoms, it loses its outer protein layer, but fragments of genetic material called RNA survive the journey intact and are eliminated in the faeces.
At this point, it is no longer infectious, from what the current evidence tells us.
But the fact that these coronavirus RNA fragments can be found in untreated wastewater (known as “influents”) is helpful in tracking outbreaks. In fact, they can predict where an outbreak is likely to occur from a week to ten days before they appear in official figures, the reason being that people clear the coronavirus before symptoms become apparent.
These “pre-symptomatic” people have to get sick enough to get tested, get the results and be admitted to a hospital as an official “case”, hence the week more or less delayed.
As a result, many countries, including Spain, are now monitoring wastewater for traces of coronavirus. In this particular study, wastewater epidemiologists examined frozen samples of influences between January 2018 and December 2019 to see when the virus debuted in the city.
They found evidence of the virus on January 15, 2020, 41 days before the first official case was declared on February 25, 2020. All samples prior to this date were negative, except for one sample from March 12, 2019, which gave a positive result. lead to your coronavirus PCR test. PCR is the standard form of testing to see if someone currently has the disease.
PCR involves obtaining samples of saliva, mucus, frozen wastewater, or anything else that the virus is believed to be hiding, removing everything unnecessary from the sample, and then converting RNA, which is a single strand of genetic material, into DNA. (the famous double-stranded propeller).
The DNA is then “amplified” in successive cycles until the key fragments of genetic material known to exist only in a particular virus are abundant enough to be detected with a fluorescent probe.
Not very specific
In coronavirus tests, scientists often detect more than one gene. In this case, the researchers tested three. They tested positive for the March 2019 sample on one of three genes tested: the RdRp gene. They analyzed two regions of this gene and both were only detected around the 39th amplification cycle. (PCR tests become less “specific” with increasing rounds of amplification. Scientists generally use 40 to 45 rounds of amplification.)
There are several explanations for this positive result. One is that SARS-CoV-2 is present in wastewater at a very low level. Another is that the test reaction was accidentally contaminated with SARS-CoV-2 in the laboratory. This sometimes occurs in laboratories, since positive samples are handled regularly, and it can be difficult to avoid very small traces of positive samples contaminating others.
Another explanation is that there is another RNA or DNA in the sample that closely resembles the target site of the test to give a positive result in the 39th amplification cycle.
Further testing must be done to conclude that the sample contains SARS-CoV-2, and a finding of that magnitude would need to be replicated separately by independent laboratories.
Reasons to be circumspect
A curious thing about this finding is that it does not agree with the epidemiological data on the virus. The authors do not cite reports of an increase in the number of respiratory disease cases in the local population after the sampling date.
Furthermore, we know that SARS-CoV-2 is highly transmissible, at least in its current form. If this result is a true positive, it suggests that the virus was present in the population with an incidence high enough to be detected in an 800 ml wastewater sample, but then was not present with an incidence high enough to be detected for nine months, when there is no control the measurements were in place.
Therefore, until more studies are done, it is best not to draw any definitive conclusions. 
Claire Crossan, researcher, virology, Glasgow Caledonian University.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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