COVID-19 is the latest in a long line of devastating zoonoses. Unless we radically alter our behaviour, it won’t be the last.
SARS-CoV-2, the virus that causes COVID-19, is one of a family of coronaviruses, named for the wreaths that ‘coronate’ them. As of March 16, SARS-CoV-2 has infected 171,000 and killed 6,500. SARS-CoV-2, first reported in China, is from the same family as 2003’s SARS virus, also from China, that killed 775; and MERS, from Saudi Arabia, that has killed 282 people 2012-2019. These three diseases have more in common besides their kinship. They are all zoonoses: infectious diseases that came from nonhuman animals. SARS-CoV-2 came probably from pangolins; 2003’s SARS from civets; and MERS from dromedaries. All three coronaviruses jumped from nonhuman animals to humans. Throughout history, this has been the norm for the origin of human infectious disease.
Zoonoses are the norm
In 1962, infectious disease was declared a problem largely solved: thanks to vaccines, antibiotics, and public sanitation. Now, “emerging and re-emerging infections have become a significant worldwide problem,” declares PS Brachman, in the Journal of Epidemiology (2003). Infections like tuberculosis, endemic in Eurasia for centuries, have staged a comeback in drug-resistant forms. And new infections have emerged. Ebola. Zika. Swine flu. These are all zoonoses.
Like most organisms, pathogens specialise: as a hummingbird specialises in extracting nectar, so a pathogen specialises in exploiting one species/genus. Yet most major diseases of human history came from nonhuman animals: up to 75%. How does a pathogen of nonhuman animals evolve to infect humans?
Wolfe, Dunavan, and Diamond (2012) list the stages. At Stage One, a pathogen specialises in animals and cannot infect humans (barring blood transfusion etc.): this includes most species of malarial plasmodia, each of which specialises in one species of nonhuman mammal/bird/reptile. At Stage Two, a pathogen travelling from animal to human can sicken the human, but this infected human cannot spread the infection further (anthrax, rabies). At Stage Five, the pathogen has evolved to specialise in humans. For various reasons, many pathogens stay “stuck” at earlier stages. After millennia of close canine-human contact, the rabies virus remains at Stage Two. Whereas perhaps just a few centuries of close contact between humans and chimpanzees sufficed for SIV (S for ‘simian) to evolve into HIV.
Most major human pathogens originated in either (i) livestock species: cows alone ‘gave us’ smallpox, tuberculosis, and measles; or (ii) wild animals, generally encountered in hunting: HIV came from chimpanzees hunted in the Democratic Republic of Congo in the 1920s; Ebola in West Africa came from bats or nonhuman primates; 2003’s SARS came from bats via masked palm civets. A third route of transmission is indirect: (iii) a domesticated animal becomes a disease intermediary between a wild animal and a human. Influenza A, which kills 290,000-650,000 annually, came to humans via poultry, who got it from wild birds.
Differential ‘access’ to zoonoses shaped human history
Polymath scientist Jared Diamond’s 1997 bestseller Guns, Germs, And Steel traces the origin of major historical zoonoses to the Neolithic Revolution: when hunter-gatherers settled down to tend to crops and livestock. Large populations, crowded living, and close human-animal proximity encouraged zoonoses to specialise for human hosts; zoonoses became endemic (like most tropical diseases) or broke out in periodic pandemics (most temperate diseases). For biogeographical reasons, it was in Eurasia that most of the domesticable animals lived. It was primarily in Eurasia that these zoonotic ‘crowd diseases’ developed: periodically killing millions, but also conferring immunity to survivors. Survivors who were carriers. This accident of biogeography was to have profound historical impact: altering whole continents’ demographics and culture.
Zoonoses were the secret weapon that Europeans carried to the New World. This is the ‘germs’ part of the thesis of Guns, Germs, And Steel. Criticised on ideological grounds, the book holds up to scientific scrutiny: it was the advantage Eurasians had in advanced technology, central organisation, and an agricultural package that equipped them with deadly germs – that allowed the Old World to dominate the New. Guns helped: it was guns that allowed Spaniard Francisco Pizarro’s 168 men to defeat an Incan army 80,000 strong. But vastly more effective were the Europeans’ germs, notably smallpox of bovine origin: decimating, in many places, 65-90% of the New World’s population. Europeans also deliberately weaponised zoonoses against each other, and against Native Americans.
Zoonoses shaped human history and culture in other ways. Tropical zoonoses (malaria, sleeping sickness) shaped how Africans lived; these novel illnesses impeded Europeans’ progress through Africa and Asia; but Europeans also spread them to the Americas. Within Europe, zoonotic epidemics including the plague (from rats, attracted to agricultural communities’ surplus food and poor sanitation) increased the wages and bargaining power of surviving workers; they also slowed urbanisation.
The ubiquitous threat of infection has shaped, also, human psychology. It equipped us with the behavioural immune system: which makes us over-vigilant to signs of infection, motivates xenophobia, enforces conformity to social norms, and biases our mating preferences.
Zoonoses are emerging and reemerging
To some extent, these historical infectious diseases have been contained (many diseases of temperate origin have been virtually eradicated; tropical diseases have become treatable, but still impose huge costs). In recent decades, novel zoonoses have caused several pandemics: H1N1, originating in Mexico’s large-scale pig pens, killed 150,000-575,000 in the first year; since it was recognised in 1982, chimpanzee-derived HIV has killed 35 million. And older diseases have reemerged: malaria, tuberculosis. As we struggle to contain COVID-19, let’s not forget the huge costs of established zoonoses: tuberculosis killed 1.5 million Indians in 2018 alone.
New patterns of human behaviour are driving both reemerging (tuberculosis) and emerging (COVID-19) zoonoses. These include: rapid growth of population and urbanisation (many zoonoses are ‘crowd diseases’); contact with wildlife via hunting, transporting wild animals live, and keeping wild animals as pets; frequent long-range travel; and poor sanitation and nutrition in many densely-populated regions. The reemergence of old diseases has been fuelled by antibiotic resistance: a major challenge for the medical community, and one that has also empowered pathogens once mostly harmless. Antibiotic resistance also occurs in livestock, partly fuelled by the practice of administering chronic subclinical doses of antibiotics: a cheap way to fatten livestock. Livestock antibiotic-resistance is a particular problem in emerging economies, including India: affecting not just livestock, but the people who eat them or live around them. Whether in humans or livestock, antibiotic resistance is not just an economic problem: it is a crisis of global public health.
Highly infectious emerging and reemerging zoonoses have one thing in common: they transcend social boundaries, forcing us to acknowledge our common humanity. Today, young people, at low risk of severe COVID-19, are being beseeched to take precautions: for the sake of their elderly contacts, if not their own. (Some governments initially claimed that asymptomatic individuals couldn’t transmit the disease; this is now in question). Of course, like everything else, zoonoses disproportionately affect those already disadvantaged; influencers are beseeching us to stay home to ‘flatten the curve’ so that everyone who needs care can get it, also to keep in mind what illness and shutdowns may do to those of us who can’t afford to stay home. (Washington Post’s interactive COVID-19 simulator demonstrates transmission dynamics.)
How should we act to contain, not only COVID-19, but all zoonoses – and, more broadly, all infectious disease? Clearly, not by killing animals. After 2003’s SARS outbreak, palm civets were decimated: a tragedy for the cat, whose only fault was being hunted by humans. Nor will the international blame game do any good: zoonoses have come from everywhere, and are a risk anywhere humans come into close contact with any nonhuman animal.
What we must do is to ban, or strictly limit and regulate, the international wildlife trade. This trade, driven largely by Chinese demand, includes wild rhino horns; ivory; and bones, skin, and genitalia from farmed or poached tigers. The origins of SARS-CoV2 are still being investigated; it probably came from pangolins in a live wildlife market in Wuhan. (Wuhan was also the origin of 2003’s SARS outbreak.) Chinese traditional medicine encourages consuming wild animal parts: though there’s no evidence for their benefit, and though synthetic substitutes are often available.
It was in the 1970s, when the Chinese government responded to famine by opening up agriculture to private industry, that China’s longstanding appetite for wildlife began to be met. Today, China’s farming and trade of wildlife is a massive market: in 2016, the food-related sector alone was worth 125 billion yuan, and employed 6.3 million people.
The Chinese government has sent mixed messages about wildlife trade. South China Morning Post reports: “The Chinese government has long encouraged the commercial use of wild animals.” 2003’s post-SARS ban on wildlife trade lasted only six months before lobbyists got it reversed. Even when laws do exist – e.g. banning the trade of specific animals – China has enforced them poorly. Pangolins, the probable source of SARS-CoV-2, are an endangered animal, and were illegal to trade in China: but were nonetheless found at Wuhan’s market, suggesting that what laws do exist are poorly enforced. On the other hand, in 2018, China legalised sale of rhino and tiger parts: though both animals are critically endangered.
Clearly, so massive an industry, and so deeprooted a cultural demand, cannot be erased overnight; a premature law would only drive the business underground, increasing its potential for harm. But a series of zoonotic outbreaks – and now this pandemic – forces us to confront that, besides ethics and conservation, public health is threatened by the wildlife trade.
Polls indicate overwhelming Chinese support for a permanent ban of the wildlife trade; many consumers simply don’t know the costs of their consumption, and are horrified to learn it. Since the COVID-19 outbreak, China has already imposed another ban; as of now, it’s unlikely to be permanent, or to be actually enforced if permanent. The world must pressure China to do better. China houses much of the demand in the wildlife market: but the origin of the goods is often in countries across Asia and Africa, where legal loopholes and poor enforcement allow poaching. We must petition international governments to amend laws and improve enforcement. To protect wildlife and to protect human health.
Further, we need decisive action to improve international cooperation during epidemics, and public health access within nations. World citizens must be educated in the dangers of wildlife trade. Individuals in close contact with wildlife must be monitored. Antibiotic resistance must be addressed. The anti-vaccination movement has already caused an outbreak of measles, which had been virtually eradicated; this baseless and destructive movement must be swiftly dealt with. More than any other advantage of civilisation, vaccination works only if everyone participates. Liberty of thought is important: even the liberty to hold baseless beliefs. But the right to life is more important still.
As citizens and as advocates, we must demand structural reform to reduce the enormous cost of zoonoses. If we don’t, sooner or later business-as-usual will resume, and the clock will start ticking down to the next zoonotic pandemic.
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