Imagine you are a smoker who drives for a living. Every day you throw burning cigarette butts from the car. It’s happened many thousands of times without incident. The smouldering stub lands, bounces and dies. Occasionally one may singe a clump of grass but nothing more. You drive on in blissful ignorance.
This analogy, say virologists, is a good way of thinking about zoonotic spillover events like the one which sparked the Sars-Cov-2 pandemic. They are happening all the time but are seldom noticed. Only very rarely, when environmental conditions are just right, do they spark a violent wildfire which spreads exponentially.
“These things happen every day around the world, and we miss them because we don’t focus enough on supporting surveillance of unusual illnesses in under-served and often distant rural communities”, says Dr Peter Daszak, the veteran virus hunter and one of the world’s foremost experts on coronaviruses.
Working backwards to find the origin of Covid-19 is necessary but misses the main point, adds Dr Daszak. If you are very lucky you might find the smoker who sparked the wildfire, but this discovery won't stop the next one unless something more fundamental changes.
“The key problem is that too many ill-informed people are working outside of their own experience to try to ‘trace back’ from ground zero”, says Dr Daszak. “What we need to do is look at it the other way round: from the bats to the people”.
The idea that zoonotic spillover events are much more widespread than we realise is not just a theory but a fact, and variants of the Sar-Cov-2 virus are a case in point.
A study published in 2018 by Dr Daszak and Zheng-Li Shi - Wuhan’s famous ‘batwoman’ (below) - documented how they took blood from 218 people who live in close proximity to bat caves in Yunnan province, China. Most were farmers and 97 per cent had a history of exposure to or contact with live-stock or wild animals.
In total six of those tested (2.7 per cent) were found to have antibodies to a Sars-Cov variant carried by local bats. The virus did not appear to have caused harm but even if it had we may never have known. In the 12 months prior to the sampling date, only one of those infected had travelled outside of Yunnan. Several of the others had never left their village. This particular spark - like the vast majority of them - briefly smouldered, then died.
But that’s not always the case. Sometimes the conditions are right for things to catch fire. And on occasion, the blaze can be vast: one recent outbreak has cost 30 million lives to date and is still burning.
Take Hendra virus. Its traits mean it has never triggered a major outbreak - it isn’t contagious enough - but every so often, it still sparks a lethal blaze. The first reported outbreak was in 1994 in Hendra, a small suburb in Brisbane, Australia when a racehorse called Drama Series fell ill. She was feverish, frantic and frothing from the mouth. Within 24 hours she was dead.
Drama Series was the first of 21 racehorses to die in that outbreak. “At the height of the crisis, seven animals succumbed to their agonies or required euthaniasia within twelve hours,” David Quammen writes in his book, ‘Spillover’. “Seven horses dead in twelve hours - that’s carnage, even for a case hardened veterinarian.”
The disease also jumped to two humans, killing one - a horse trainer called Vic Rail. After a week in intensive care he died in hospital after his organs failed and he could no longer breathe.
It took years to find where the Hendra virus came from but in the end it was traced to fruit bats, which are common in east Australia. Since 1994, it has crossed from bats to horses in 52 separate spillover incidents and infected a total of seven people.
A bigger viral blaze was sparked in Yambuku, a village with a Belgian missionary outpost deep within the Congolese forest, in 1976. A mysterious haemorrhagic fever killed well over 200 people in just three weeks before apparently petering out.
The virus, later named after the nearby Ebola river, has now sparked more than 28 different outbreaks (each a separate spillover event) and killed more than 22,000 people over the last 40 years. The worst occurred in West Africa between 2013 and 2016, when the first spark fell near a major road, enabling it’s spread. At least 11,323 people died.
Even today as Covid spreads, the World Health Organisation - now armed with a vaccine - is fighting a new Ebola outbreak in the north west of the Democratic Republic of the Congo. In just four weeks, there have been 56 confirmed cases and at least 20 deaths.
Experts suspect the “natural reservoir” for Ebola is also bats - but they are far from the only animal to carry zoonotic viruses. The Spanish Flu of 1918 is thought to have started in North American poultry. Middle East respiratory syndrome (Mers), another coronavirus, comes from camels and has killed 858 people since it was first discovered in Jordan in 2012.
The case numbers reported for these and other zoonotic diseases are almost certainly just the tip of the iceberg. The vast majority of spillover events go unreported, say experts.
“We’re continually exchanging viruses with animals, that's what happens”, says Dr David Redding, from the Centre for Biodiversity and Environment Research at UCL. “These conspiracy theories about labs misunderstand the basics of virology. We know that all species are sharing pathogens all the time. It is through this process that viruses naturally mutate and evolve.”
It is HIV that best illustrates the point, a virus now known to date back to the early 1900s. Its simian version (SIV) is thought to have jumped from monkeys to humans through hunters and butchers in Africa. Cities like the former Belgian colony Leopoldville, which were rife with prostitution and the ulcerating venereal disease syphilis, are thought to have provided the ideal environmental conditions for the virus to mutate and adapt to humans.
Dr Daszak describes HIV, which has killed an estimated 32 million people, as the “ultimate” example of spillover. After many decades of repeated small scale flare ups (all unnoticed at the time) it exploded as a pandemic in the early 1980s. What had changed was not so much the virus itself - the spark - but the society it landed in. The population boom in Africa, the globalisation of air travel, the sexual revolution in the west - they all played a part.
“Changes to human behavior increase the transmission of viruses between people, for example sexual contact or injected drug use,” says Dr Daszak. “These changes alter the ‘R’ or reproduction rate of a virus and may assist in driving their emergence.”
Sars-Cov-2 may also have been circulating longer than thought. The virus has mutated very little since first being discovered in humans. This may be because it is a stable virus which faces little pressure to adapt. But it may also be because it has already adapted.
“The evolution of this virus to become a human pathogen may have already happened and we missed it,” Angela Rasmussen, a virologist at Columbia University told Science magazine last week.
Prof Linfa Wang of the Duke-NUS Medical School in Singapore also thinks a version of the virus may have circulated earlier in humans in southern Asia. “If it happens in a small or remote village, even with some people dying, nobody is going to know there’s a spillover,” he said.
Today there are roughly 300 animal pathogens from 25 “high risk” viral families which are known to infect people. That more will emerge is inevitable. Researchers estimate that as many as 1.7 million viruses from these same families exist in the wild, including some 700,000 with “zoonotic potential”. That’s a lot of sparks.
There is temptation to think that major events like pandemics are once-in-a-century events but the logic of spillover may suggest the opposite. Just as we have seen wildfires proliferate around the globe in recent years because of a change in climatic conditions, there are environmental changes that make it more likely that viral jumps will also burn bright. The growth in the world’s population, the development of previously undeveloped nations and the growth in the international trade in wildlife are just a few of them.
“We need to think about things like supply chains,” says Dr Redding. “Demand for palm oil and soya [in the west], for instance, are driving huge changes in the way people interact with biodiversity in the locations in which they’re farmed.
“This is all a bit hidden at the moment. We don’t take responsibility for that. But international trade is driving changes to land use across the world. This is far bigger than wet markets.”
Protect yourself and your family by learning more about Global Health Security