The novel coronavirus was already present in sewage systems of Milan and Turin in northern Italy as early as December, two months before the first Covid-19 cases were detected in the country, a new study showed, suggesting the virus was circulating much earlier than initially thought.

Italy was the first European country to be hard-hit by the virus and the first in the world to impose a nationwide lockdown. The first known case was a patient in the town of Codogno in the Lombardy region in February. The government then designated Codogno a so-called red zone and ordered it shuttered. But the disease may have reached Italy far before that, according to a new analysis.

In a study soon to be published, researchers from Italy’s Institute of Health (ISS) said water from Milan and Turin showed genetic virus traces on 18 December. They looked at 40 sewage samples collected from wastewater treatment plants in northern Italy between last October and February.

While the wastewater samples from October and November were negative, the ones from December were positive in Milan and Turin and the ones from January were positive in Bologna, another Italian city.

The results might help scientists understand better how the virus began spreading in Italy, ISS experts said in a statement.

“The discovery of the virus does not automatically imply that the main transmission chains that led to the development of the epidemic in our country originated from these first cases, but, in perspective, a surveillance network in the area may prove to be valuable to control the epidemic,” said researcher Luca Lucentini.

The findings also confirmed the “strategic importance” of sewage water as an early detection tool, the ISS said, because it can signal the virus’s presence before cases are clinically confirmed. Now, the institute hopes to start a pilot project next month to monitor wastewater at tourist resorts, later expanding it nationally.

Since the beginning of the epidemic, researchers across the world have been tracing the spread of the coronavirus through wastewater and sewage, finding genetic traces. A recent Spanish study found genetic traces in wastewater samples collected in mid-January in Barcelona, about 40 days before the first indigenous case was discovered.

Other studies not in wastewater have also suggested the virus was circulating earlier than previously expected in Europe. In France, a group of researchers discovered that a patient admitted with pneumonia in December might have been suffering from COVID-19 – a month earlier from the first case detected in the country.

“Moving from research to surveillance will be essential to arrive at standardization of methods and sampling,” said Lucentini. “The positivity of the samples is affected by many variables such as the sampling period, any meteorological precipitations, the emission of waste from industrial activities which may affect the results of activities to date conducted by different groups.”