The proportion of rats testing positive for viruses jumped substantially between the start and end of Vietnam’s supply chain
Asia’s booming wildlife trade is fuelling the spread of coronaviruses by providing the ideal opportunity for animals to infect each other and potentially humans, two studies have found.
Across Asia, wild animals including rodents, pangolins and bats are transported, often illegally, thousands of miles in crowded and chaotic conditions for use in restaurants and traditional medicines.
Experts have long thought this amplifies the transmission of coronaviruses, making the possibility of a jump to humans more likely.
In one new study, published as a preprint without peer review, researchers analysed oral swabs from more than 2,000 field rats in three provinces in southern Vietnam. They found that the animals smuggled across the Mekong River Delta, from traders to restaurants, tested positive for six different coronaviruses. More significantly, the incidence of infection increased significantly along their journey.
Roughly 20 per cent of wild rats caught by traders tested positive for at least one coronavirus, rising to 32 per cent of rodents in large markets. In restaurants, the final step in the chain, 55 per cent of rats were infected.
“The observed viral amplification along the wildlife trade supply chain for human consumption likely resulted from the mixing and close confinement of stressed live animals,” the researchers wrote.
They added that the results demonstrate that human behaviour that is driving spillover events, where viruses jump from animals to people, and said added disease surveillance systems are needed.
A wet market in Guangzhou, China
A wet market in Guangzhou, China CREDIT: K. Y. Cheng/South China Morning Post
“To minimise the public health risks… we recommend precautionary measures that restrict the killing, commercial breeding, transport, buying, selling, storage, processing and consuming of wild animals,” researchers said.
Rodents are a popular item on the menu across southeast Asia and China, where they are viewed as a healthy and nutritious food source. Rats are regularly bought and sold in wet markets along the Mekong Delta in southern Vietnam, where the study took place.
Little concrete data is available on the scale and scope of the current market, but in the early 2000s between 3,300 and 3,600 tons of live rats were estimated to be processed in the Vietnamese field rodent trade. The study notes that the market was then valued at $2 million.
The data in the latest report was collected between 2013 and 2014, long before Covid-19 emerged, in a study led by the Wildlife Conservation Society.
Researchers also took fecal samples from animals on 28 wildlife farms – six per cent of both Malayan porcupines and of bamboo rats tested positive for coronaviruses, while 75 per cent of bats on bat ‘farms’ were infected. The flying mammals are generally considered the natural reservoir of coronaviruses.
In a separate study, also published as a preprint on the website bioRxiv, researchers took samples from nearly 300 pangolins seized in Malaysia over a 10-year period. The critically endangered animal is one of the most trafficked mammals in the world – their scales are used in traditional medicines, while pangolin meat is deemed an expensive delicacy.
A pangolin displayed among other wildlife at a wet market
A pangolin displayed among other wildlife at a wet market CREDIT: AKINTUNDE AKINLEYE/EPA-EFE/Shutterstock
But while scientists detected Sars-Cov-2 in pangolins in February, none of the animals in this study, led by the Eco Health Alliance, tested positive. The report suggests this contrast is a result of the point in the supply chain at which samples were taken.
“Our samples were drawn from an ‘upstream’ cohort of animals yet to enter or just entering the illegal trade network, whereas all others were drawn from ‘downstream’ cohorts confiscated at their destination in China,” the authors wrote.
“We therefore conclude that the detections of Sars-CoV-2 related viruses in pangolins are more plausibly a result of their exposure to infected people, wildlife or other animals after they entered the trade network.”
Experts say that the results of these studies strengthen theories that the new coronavirus was in circulation before it emerged at the Wuhan wet market in December 2019.
“I think we underestimate the sheer size of that [wildlife supply chain] network and the dynamics within it – it’s enough to drive outbreaks,” said Dr Peter Daszak, president of EcoHealth Alliance.
He added that the latest research on pangolins was “one more small piece in [the] Covid emergence jigsaw.”