Recently I gave the latest update on the origins of SARS-CoV-2 – bottom line, it was very likely a zoonotic spillover event and not a lab leak. Following that I interviewed a researcher, Dr. Andersen, who is an expert on the origins of epidemics/pandemics and has researched this very question. He reinforced the spillover hypothesis, indicated he had considered the lab leak hypothesis early on but it was quickly shown to be highly unlikely.
More important than that, however, was the discussion about why a zoonotic spillover was so likely, and why, if anything, we are more vulnerable to another such event today. Far from learning from the COVID pandemic how to reduce the chance and prepare for the next inevitable such event, COVID has caused “pandemic fatigue” and left us worse prepared than we were. Further, it’s unlikely to be a century until the next such pandemic.
A zoonotic spillover event refers to an infectious agent, usually a virus, that is endemic in an animal population that crosses over into the human population. Viruses evolve and adapt, and therefore a species that is not adapted to humans with a very limited (but non-zero) ability to infect a human can evolve in a human host to become more infectious and virile. When a virus gains the ability to transmit from human to human, we consider that as spilling over into the human population. That is what happened with COVID.
The chances of this happening are proportional to the number of viruses circulating in animals that are in close proximity to humans. The Huanan seafood market in Wuhan China was the perfect setup for this to happen. In this market there is a vast trade in wildlife and farmed animals for human consumption. The supply chain of these animals could not be more risky if it were designed to maximize the spread of viruses.
Every step in the chain – commercial breeding, transport, buying, selling, storage, processing, and killing – puts animals from diverse taxa in close proximity to each other. Often these are animals that would not be in close contact in the wild. The conditions are often unsanitary, the animals are stressed, and they attract rodents that contribute to the circulation of disease. In this environment animals that perhaps had few or no viruses in the wild acquire multiple viruses. This not only magnifies their spread, it allows for recombination – for different viruses to combine, gaining different abilities that expand their ability to infect and spread.
Amanda Fine, WCS Health Program Associate Director, Asia, and a co-author of a study on origins of COVID, said: “Wildlife supply chains, and the conditions the animals experience while in the supply chain, appear to greatly amplify the prevalence of coronaviruses. In addition, we documented exposure of rodents on wildlife farms to both bat and bird coronaviruses. These high prevalence rates and diversity of coronaviruses, added to the species mixing we see in the wildlife trade, creates more opportunities for coronavirus recombination events as well as spillover.”
The same is true not just of coronaviruses, but of many virus types. We then take these now virus-ridden stressed animals, we pack them into a market in the middle of an urban center that we further pack with humans. So if a virus does jump to a human, there will be plenty of other people around to spread the new virus to.
Even though China continues to deny that SARS-CoV-2 originated in the Wuhan market, there have been steps to improve conditions. This includes the proposal of a classification system of wet markets to quantify their risk and to highlight the steps necessary to reduce this risk. The authors write:
“We identify six key risk factors of wet markets that can affect human health: (1) presence of high disease-risk animal taxa, (2) presence of live animals, (3) hygiene conditions, (4) market size, (5) animal density and interspecies mixing, and (6) the length and breadth of animal supply chains.”
In 2020 China quickly closed the Huanan wet market, and it remained closed. They also tightened restrictions and monitoring of other wet markets and officially banned the illegal trade of wild animals. It is too early to tell how much this has helped. The wild animal market was a critical part of the food chain in that part of the world, and cannot instantly be replaced. Also, demand for wild animals is still driven by Traditional Chinese Medicine (TCM). Much of the market has simply shifted to a black market trade, which uses the internet to conceal its activity. There is also a problem of laundering – of selling wild animals products as if they are legal and farmed.
According to the most extensive recent study on the topic:
“The annual value of the illegal trade in wildlife in Southeast Asia has been estimated at USD 8–11 billion and likely exceeds that. The annual wildlife farming industry in China was valued at USD 74 billion.”
This is a critical food source, and you cannot simply take it away over night. It is also a huge industry that will not quietly go away. The best option might be to carefully regulate it to improve conditions and reduce the chance of spillover events. Banning the trade and pushing it underground may actually worsen conditions and make it more difficult to monitor and evaluate them.
At least there was an immediate reaction after COVID to improve conditions in China, but it remains to be seen how the international wild meat market reacts and adapts, and what the net effect will be. These markets are also only part of the problem – humans are increasingly pushing their living spaces into wild spaces, putting them at increasing proximity to wild animals. These animals also have diminishing wild spaces (through deforestation, climate change, urban spread and other factors), and so they are increasingly encroaching into human spaces to find food. And the legal market of domestic animals also often packs many animals into small spaces, making the spread of disease more likely.
The conditions still exist for zoonotic spillover events to happen, despite some efforts to address the worst conditions. In fact, recent studies find the risk is increasing, and historically has been increasing by about 5% per year. A more comprehensive global effort is needed to reverse this trend.
