The study, published on open-access preprint repository bioRxiv this month, conducted coronavirus testing at several sites involved in the sale and processing of live rodents for human consumption, bat guano farms and natural bat roosts close to human dwellings in the southern provinces of Dong Thap, Soc Trang and Dong Nai from January 2013 to March 2014.

Coronaviruses are a group of disease-causing RNA viruses found on birds and mammals, including humans. The 2002-2004 SARS outbreak, MERS outbreak and ongoing Covid-19 pandemic are all caused by coronaviruses.

By analyzing fecal, urinal and tissue samples of 1,506 bat and rodent individuals, including 1,131 rodents and 375 bats, researchers found 33.5 percent tested positive for different coronaviruses. The study however did not detect SARS-CoV-2, the specific strain that causes Covid-19, among the samples.

Researchers noted the increasing proportion of positives found along the rodent trade supply chain from capture sites to markets then restaurants. The transit of multiple animal species through the supply chain, overcrowding and close confinement likely led to altered immune functions and more opportunities for inter- and intra-species mixing, resulting in greater chances of these viruses infecting traders, butchers, cooks and consumers in zoonotic transmissions, according to the study, funded by the United States Agency for International Development (USAID) Emerging Pandemic Threats PREDICT project.

To minimize the public health risks of viral disease emergence from such sites, the study recommends several precautionary measures that restrict the killing, commercial breeding, transport, buying, selling, storage, processing and consuming of wild animals.

The first reported cases of the Covid-19 pandemic, which has now infected over eight million people and killed around 439,000, were in China's Wuhan City in December last year. Several reports, including from WHO, had pointed to a wet market in the city as a potential, unverified source for the initial outbreak.

Vietnam, where wet markets with several live animals being kept in close proximity are prevalent, is also a major hub for wildlife trafficking, usually located in a strategically convenient location to be both a transit point and destination market for wildlife products trafficked from and to its neighbors like Laos, Cambodia and China, an Environment Investigation Agency report revealed in 2018.

The country’s field rat trade in the early 2000s was estimated to process 3,300-3,600 tons of live rats annually for consumption, a market valued at $2 million, the study said, noting that while rats are still commonly traded in wet markets and sold live for consumption across Mekong Delta in southern Vietnam, no recent published data on the scale and scope of the trade is readily available.