A study of the wildlife trade in three provinces in southern Vietnam produced startlingly clear confirmation for one of the underlying objections to the wildlife trade in Asia — the trading offers an ideal opportunity for viruses in one animal to infect another.
In field rats, a highly popular animal to eat in Vietnam and neighboring countries, the percentage that tested positive for at least one of six different coronaviruses jumped significantly. It increased from 20 percent of wild-caught rats sold by traders, to slightly more than 30 percent at large markets, the next step in the supply chain, to 55 percent of rats sold in restaurants that tested positive.
In other words, the odds were about even that a field rat on the plate was infected with some kind of coronavirus.
The data for this study has taken a long time to process. The tests were done in 2013 and 2014, for common coronaviruses, long before the emergence of the treacherous novel virus that has caused the current pandemic. The results show unequivocally how viruses spread from animal to animal as they are transported in crowded conditions to market.
Sarah H. Olson, an epidemiologist with the Wildlife Conservation Society who directed the research, said she expected some increase in infections, because many animals are shipped together in close proximity in the wildlife trade, putting them under high stress. “It’s classic disease ecology,” she said.
But she didn’t expect the degree of rising infections. The numbers floored her. “We saw this huge step-by-step increase,” she said. “I kept going back to check the data.”
She and a team of scientists posted a report of their research, which has not yet been peer reviewed, but has been submitted to a scientific journal, on a website for unpublished research, bioRxiv.
Edward C. Holmes, an infectious disease expert at the University of Sydney, in Australia, who was not involved with the research, said the paper’s data clearly laid out the risk posed by the wildlife trade. “It is striking that the number of infected animals increases as you up the supply chain toward consumption,” he said.
Data like that provided in this paper has been scarce. Neither Dr. Holmes nor Dr. Olson was aware of similar studies, although both acknowledged that so much research is published that it’s impossible to know all of it.
The paper paints a vivid picture of the wildlife trade and the passion for wild meat in urban restaurants and markets. “In the early 2000s,” the authors write, “the Vietnamese field rat trade was estimated to process 3,300-3,600 tons of live rats annually for consumption.”
The rats are considered healthy and nutritious food. “Field to fork” consumption, as Dr. Olson described it, has increased greatly in recent years, both in Vietnam and neighboring countries.
The research effort in testing for coronavirus RNA had several prongs. One was the sampling of field rats, of six species common to Vietnam, at different points on the supply chain. Another was sampling wildlife farms, and a third, bat guano farms. Small farmers build artificial bat roosts so they can collect the guano for fertilizer to use themselves or sell. Other animals and children often go under the roosts.
The researchers were spurred to look at coronaviruses because of the SARS epidemic and how common the viruses are in bats and other wildlife. Most cause minor or no illness in humans, but SARS showed how dangerous they could be.
They sampled 28 farms that raised Malayan porcupines and bamboo rats, a different creature entirely from field rats, which are more closely related to the rats familiar to Westerners. The farms were not specialized, however, they also were homes to “dogs, cattle, pigs, chickens, ducks, pigeons, geese, common pheasant, monitor lizards, wild boar, fish, python, crocodiles, deer, civets” and other creatures, such as pet monkeys and free ranging rats.
The researchers used oral and anal swabs of carcasses along the field rat supply chain. But at the farms they tested fecal samples. They found coronaviruses in all of the field rats at wildlife farms, in six percent of Malayan porcupines and about six percent of bamboo rats.
At the bat farms, 74 percent of the animals were infected with the virus.
Dr. Olson said the Wildlife Conservation Society is not against all wildlife consumption. “We definitely have to maintain local access for sustainable use.” But, she said, the booming trade in wild meat has little to do with people who survive on eating wildlife.
Wild animals are popular dishes in urban areas.
“These preferences are now creating what we’re seeing with Covid-19 across the world,” she said. While China has banned the wildlife trade amid the pandemic, whether the prohibition holds in the long term is, as she said, “the three trillion dollar question.”