Everything you need to know about zoonosis
For decades now, scientists have been warning of the risks of a major pandemic emerging from the animal kingdom and crossing over to humans. Despite several serious warnings, we’re still acting too late. How did we get to where we are now, what can we learn from the past, and what do we need to change?
“How many more wake-up calls do we need?” Professor Andrew Cunningham exclaims.
“SARS was a big wake-up call. Zika was a big wake-up call. The Ebola epidemic in West Africa was a huge wake-up call. How many more of these wake-up calls do we need before governments are actually going to start putting money into prevention rather than into shutting the stable door after the horse has bolted?”
The Zoological Society of London’s deputy director of science seems genuinely exasperated. You would be too – if you had long warned of the potential for a global pandemic to take hold, but the world hadn’t listened, and now, again, it was too late.
At the time of writing, more than 200,000 cases of COVID-19 – or ”coronavirus disease” as it has officially been named – have been reported to the World Health Organisation (WHO). Over 8,000 people have tragically lost their lives to it.
An illness that impacts on the lungs and airways, COVID-19 stems from a large family of viruses called coronaviruses, named after the spikes on their surfaces which look like little crowns. Seven types can infect humans, and indeed, four of them cause between 15 and 30 per cent of common colds.
But COVID-19 is a new strain. Thought to have originated from bats, it is believed to have passed to humans in a wet market in Wuhan, China, where it had likely infected one of the wild animals on sale there. Coronavirus disease is an example of a zoonosis – a type of illness we must steadily prepare ourselves to see much more of in the coming years. And here’s why.
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A zoonosis is an infectious disease that can pass from animals to humans. Caused by pathogens – like bacteria, viruses and parasites – zoonoses originate in host species, or “reservoirs”, and pass either to humans directly or via what’s known as an intermediate species or a vector.
Any type of animal can be a reservoir: from a chimpanzee, to a chicken, to a bat. Indeed, any type of animal can be a vector too. While some will succumb to the diseases they carry, others – on account of immunity – may experience no symptoms at all.
This may be the first you have heard of “zoonosis”, however, it’s likely you’ll be familiar with some of the illnesses the term encompasses. You may even have experienced one yourself. Take salmonellosis, for instance.
Commonly, and incorrectly, known as “salmonella” – which is the offending bacteria, not the disease – salmonellosis is a form of food poisoning. Yes, you know the one: it’s likely to leave you puking your guts out and/or spending an inordinate amount of time on the lavatory.
It stems from the salmonella bacteria, which lives in animal intestines and is commonly shed through faeces. Human infection most often occurs when we eat raw or undercooked poultry, meat or seafood contaminated with – to put it bluntly – infected sh*t, most likely at the butchering stage or in the case of seafood, through contaminated water.
Other zoonoses you may know of include lyme disease (passed from animals via ticks), rabies (spread through animal saliva), malaria and zika (both contracted from mosquitoes), and plague (found in rodents, passed to humans via fleas). Then there is HIV.
HIV originated from two strains of a zoonotic disease, called “simian immunodeficiency virus” (SIV). One of these strains came from chimpanzees, while the other, from sooty mangabey monkeys, and both would have been transmitted to humans through exposure to infected blood or secretions – most likely when animals were hunted and consumed as bushmeat, or kept as pets, in the early 20th century. On entering the human body, they mutated to form two types of human immunodeficiency virus – HIV‑1 (which came from chimps) and HIV‑2 (from sooty mangabeys).
Because of the mutation process however, HIV is arguably not a zoonosis – more, a human disease with zoonotic origins.
As we can see, the world is no stranger to zoonoses – and we are almost certain to become more familiar. As a recent Frontiers report by the United Nations Environment Program revealed, on average a new infectious disease emerges in humans every four months – and zoonoses account for 60 per cent of all human infectious diseases, and 75 per cent of all emerging infectious ones too.
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Common suggestions are that our immune systems are getting weaker, or that viruses are getting more aggressive. In truth, neither of these hypotheses are correct. The rise of zoonotic diseases is believed to largely come down to – simply, and infuriatingly – human nature. That is, us and our treatment, or mistreatment, of the natural world.
“It appears that zoonoses are on the rise, particularly from wild animals, and we think there are a couple of reasons [for it],” Professor Andrew Cunningham explains.
“In the last few decades, people have been interacting far more with wildlife and encroaching far more into natural wildlife habitats than they had done for quite some time previously, particularly at a global scale.
“While local people have interacted locally with wild animals, there hasn’t been this global commoditisation of wildlife and wildlife products in the past, which there is now. For example, the supply chain for the wet markets in China – before they were closed down following COVID-19 outbreak – was a global supply chain; animals were being brought in from all over the world to those markets.”
If any of those animals happened to be infected with a zoonosis, well, we only need to look at Wuhan to see the potential for disaster.
But it’s not just through meat consumption that zoonoses can spread; it is also possible to contract them through mere contact with animals, which can occur when we plunder their environments for resources. Indeed, a study published in Mammal Review revealed a prominent connection exists between deforestation and outbreaks of the zoonosis Ebola.
Then there is the rise of global connectedness, which Professor Cunningham believes also plays a part. “If there was a zoonotic spillover somewhere in the world in the past, it would likely have fizzled out quite quickly – because people would have fought off the infection and got better, or would have died before they could have got to other people to infect them.”
But with an increased human population and greater connectedness – due to motorised vehicles and air travel – an infected person could be anywhere in the world within a few days, having infected a number of others en route. Then factor in a healthcare system which pools infected people together in centralised environments.
“All these things combined means that if there is a zoonotic spillover event, the chances of it fizzling out locally are much reduced,” he warns. “And the chances of it becoming more of a localised epidemic, or even as we’ve seen with COVID-19, a pandemic, are increased.”
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While some zoonoses – like the aforementioned salmonellosis – can mean no more than a day off work, as we are now more than well-aware of, others can be severe and potentially deadly. In fact, of the major pandemics and epidemics in recent years, a high number have been caused by zoonoses.
COVID-19 may be the coronavirus causing worldwide terror at the moment, but in 2003 and 2012 it was two other types of coronavirus terrifying many parts of the world: Severe acute respiratory syndrome (SARS-CoV, known as SARS) and Middle East respiratory syndrome (MERS-CoV or MERS).
SARS is an airborne virus, spread from person-to-person via infected saliva droplets that have been launched into the air through coughs and sneezes. Initial symptoms can be much like those of the flu – fever, severe tiredness and muscle pain – and these can be followed by a dry cough and trouble breathing. For some, symptoms may prove fatal.
Like all zoonotic diseases, SARS is transferred to humans through animals, and in 2003, it spread so vehemently it caused a pandemic. But how did it emerge?
Skip forward to 2017 and the answer finally presented itself. For fifteen years, scientists had been searching for the source of the SARS crisis which had killed more than 740 people in 26 countries around the world. They knew it had originated in south China, and suspected civet cats as the intermediate species – which are mongoose-esque creatures, often sold as medicinal food in Chinese wet markets. But how these fruit-eating species contracted the disease, nobody knew.
On entering Shitou Cave in the Chinese province of Yunnan, all became clear. A team of scientists discovered a species of animal that carried a strain of coronavirus with the same genetic make-up of that which had caused SARS. The species in question was the horseshoe bat.
Swine flu, 2009
An acute respiratory infection, swine flu was responsible for the first pandemic of the 21st century, in 2009, spreading to more than 170 countries. Also known by its shortened (scientific) name “H1N1“, swine flu originates from pigs, and is thought to have first appeared in 1918. The 2009 pandemic is alleged to have come from a four-year-old boy named Edgar Hernandez, who lived in the village of La Gloria in Mexico.
Scientists believe the virus – which causes symptoms similar to common flu – had actually been active in the area for a decade before the emergent strain, capable of infecting humans, developed. It is not known how Hernandez came into contact with it, however it may have been via one of the American-owned pig facilities in the region.
From Mexico, it traversed the globe, and went on to infect an estimated 20 per cent of the world’s population, most of which were children and young adults. It is estimated that 200,000 people lost their lives to the disease.
The medical world, however, was quick to respond and by September that same year, vaccines had been developed. By 2010, WHO director general Dr. Margaret Chan announced the pandemic was effectively over, saying it had “largely run its course”.
A bronze statue of Edgar Hernandez – who survived his illness – was later erected in La Gloria.
In April 2012, a viral respiratory disease began to sweep through the Middle East. Originating from Saudi Arabia, like SARS, it was a coronavirus and as such produced similar symptoms. It became known as Middle East respiratory syndrome (MERS) – a dangerous illness with a fatality rate of between 36 and 40 per cent.
Since that first outbreak in 2012, MERS has spread to 27 countries in Asia, Africa, Europe and North America, and while it peaked in 2014, there have continued to be cases ever since. So far it has killed more than 900 people.
Scientists think young dromedary camels acted as the vector. Believed to have carried the virus for a number of years – at least as far back as the early 1980s – how these camels came to pass it to humans has not yet been proven, although changes to husbandry and trade practices are thought likely to lie behind it.
Where did the camels get it? Most likely some time ago, via a well-known spreader of disease: bats.
In 1976, two outbreaks of a mysterious illness emerged in Africa over a thousand miles apart. One took place in what is today Nzara in South Sudan. The other, in a village in Yambuku, in, now, the Democratic Republic of Congo (DRC), near a river called Ebola. It is from these waters that this zoonosis took its name.
Ebola virus disease is a hemorrhagic fever which causes damage to the blood vessels and affects the blood’s ability to clot. It often starts with a fever and can lead to internal bleeding and bleeding from the eyes, ears, nose and mouth. Between 30 and 50 per cent of cases result in death.
Again, it is widely believed that bats acted as the Ebola reservoir. Humans likely contracted the disease when they came into contact with the bat’s blood or bodily fluids, or those of a vector – which could have happened merely from coming across an infected animal in a rainforest.
The virus spreads from human to human through blood or bodily fluids and can be passed from mothers to babies via breast milk, and can even spread at funerals if contact is made with the infected body of the deceased.
Almost 40 years after it first emerged, Ebola inflicted devastation upon west Africa. On 26th December 2013, in a village in south-eastern Guinea a two-year-old boy contracted the disease and tragically died from it two days later. It is not known how he came across the virus, but an EMBO Molecular Medicine study posited he may have picked it up playing in a hollow tree which had been occupied by a large colony of free-tailed bats.
From Guinea, Ebola spread rapidly to nearby countries Sierra Leone and Liberia, and in August 2014, the WHO was forced to declare an international public health emergency.
Over the next two years, Ebola claimed the lives of over 11,000 people. At its peak, whole communities in west Africa were quarantined. Funeral traditions stopped, schools closed, and people even refrained from basic human contact: the famous Liberian “finger snap” greeting, for instance, became a thing of days past.
And yet, as communities took heed of accurate information on how to stave off the disease, and burials began to be conducted safely, the epidemic started to subside… But it was soon to rear its head again, this time, in the DRC in 2017.
Three years on, and with more than 3,400 cases, it counts as the second largest Ebola epidemic on record. As of 6th March this year, figures released by the DRC’s Ministry of Health indicate more than 2,260 people have died.
Yet there may be a light at the end of the tunnel. Two Ebola vaccines are now in development. The rVSV-ZEBOV vaccine, designed during the 2014 – 2016 epidemic, may remain in clinical study phases – and without a license – but as of mid-November 2019 it had been administered to over 250,000 people.
A second vaccine is currently being put through clinical trials.
(The same year also saw the start of a less deadly outbreak of Zika Virus in Oceania: A mosquito-borne virus which causes flu-like symptoms along with conjunctivitis and the potential to cause birth-defects in pregnant women. This also prefaced a wider outbreak in 2015 – 2016 across the Americas.)
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It won’t have escaped your attention that one creature in particular seems to be largely “responsible” for devastating the world with zoonoses: the not-so-humble bat.
This might seem unusual – rodents, after all, vastly outweigh bats in number and are more often considered in the “carrier of pathogen” category. However, bats exist in vast numbers, live on every continent on Earth bar Antarctica, and make up a quarter of the globe’s mammals. Able to fly, they can travel long distances and unfortunately – for us humans – their faeces can spread disease.
“The pathogens that live in bats without causing disease in [them] have evolved to withstand flight,” explains Professor Cunningham. “When bats fly, their metabolic rate increases hugely and their body temperatures also increase, which almost mimics fever.”
In non-bat mammals, if you get an infection one of the body’s first responses is to develop a temperature. Most pathogens cannot survive them, so they naturally die as a result.
“But the pathogens that have evolved to live in bats can withstand fever, because they’ve been able to withstand the high body temperatures bats get when they’re flying,” Cunningham says.
“So it’s quite likely that the pathogens that we get from bats… [will be able to] withstand the fever that our body produces as a first line immune defence. That’s what we think could be one of the reasons at least, why bat pathogens in particular are causing problems in people in recent decades.”
So we know that bats are a problem. The question is, what should be done about them?
The answer? Nothing at all.
Because the underlying issue is not that bats can spread disease. The underlying issue is that we interfere with them. “The reason that we think we’re getting more zoonotic spillover from bats in recent years, is partly because of our increased contact with bats – encroaching into their habitats – and partly because people are increasingly hunting bats as bushmeat,” Professor Cunningham reveals.
“Whereas in the past, we’d have concentrated on much larger animals, those much larger animals are either extinct – locally or globally – or are no longer in the abundance that makes it worthwhile spending time trying to track them down and hunt them.”
He adds: “We’ve sort of moved on to an ever-decreasing size of animal, until we’re now down to bats.”
None of this is the fault of the animal. As David Quammen, author of Spillover: Animal Infections and the Next Human Pandemic, opined in The New York Times, “We disrupt ecosystems, and we shake viruses loose from their natural hosts. When that happens, they need a new host. Often, we are it.”
Bats are integral to a number of natural ecosystems, indeed they are viewed as “keystone species”. They can pollinate plants – providing fruits for other animals to eat and for us humans to trade – while some dispense seeds through their droppings so environments such as rainforests may be replenished. Taking out these crucial species is simply not an option. It is our way of life that needs to change.
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Scientists have been predicting zoonotic pandemics, like the one we are in the midst of, for years. In only January last year, Chinese virologist Shi Zhengli co-authored an article in journal Viruses which stated: “…it is highly likely that future SARS- or MERS-like coronavirus outbreaks will originate from bats, and there is an increased probability that this will occur in China.”
That Shi called the current crisis is not surprising, in view of her expertise. Nicknamed China’s “bat woman”, she was part of the team that entered Shitou Cave in Yunnan to discover the link between SARS and bats. Her work was mentioned in the Hollywood thriller Contagion (2011). And she helped identify COVID-19.
“The Wuhan outbreak is a wake-up call,” she recently told Scientific American.
But as Professor Cunningham asks, how many more of them do we need? We can’t seem to move past the main problem: that largely, the global response to zoonoses is the same – reactive. Many scientists believe we need a change of tack: there has to be greater focus on prevention.
How might we go about this? One method advocated by both Shi Zhengli and Professor Cunningham is the “One Health” approach. A term coined by the American Veterinary Medical Association, it refers to “the collaborative effort of multiple disciplines – working locally, nationally, and globally – to attain optimal health for people, animals and our environment”.
What this means is that if future zoonotic outbreaks are to be preempted – and blocked – a 360-degree understanding of what leads to transmission must be obtained. All factors have to be considered – not just the epidemiology of a disease itself, but how the spread of it is influenced by the environment, by human behaviour, and beyond that, by government policy and the global economy.
“Look at this COVID-19 response. Huge response. Huge amount of money going into possible cures, treatments and vaccinations,” Professor Cunningham says.
“That’s all focused at the medical end of the spectrum, and that’s obviously necessary once we’ve got to the situation that it is today.
“But if we took a One Health approach, we could have foreseen these sorts of diseases, like SARS and COVID-19, spilling over. We could have foreseen their emergence. Because if you understand the ecology of diseases, and the ecology of their natural hosts, you can then predict where human behaviours may be a high risk for zoonotic spillover, and you can then mitigate those, usually by changing human behaviour.”
He continues, “We argue that when there are big infrastructure projects, there should be a One Health assessment of what impact that may have and how it may lead to disease issues for people or for livestock or for other species. Because when we perturb habitats, we perturb the interactions of different species in ways we may not even be able to predict.”
He cites the example of the rabbit virus, myxomatosis. Who could have predicted, he asks, that when it was introduced into Britain in the 1950s, it would directly link to the extinction of a butterfly – the Large Blue – 30 years later?
But there are things we can predict – such as, the higher number of close interactions we have with wildlife (especially those species that we have not had close interactions with historically), the more likely we are to see disease spread over into people.
We can predict, for instance, that by bringing together different species that would not naturally mix, more outbreaks of zoonotic disease may occur.
We can also predict that in packing large numbers of animals tightly next to one another and transporting them halfway around the world to trade, there is a greater risk of spreading disease.
In an article, co-authored by Professor Cunningham for the Royal Society’s Philosophical Transactions B journal, it describes how a One Health approach can provide solutions to the current zoonotic problems we face – through, for instance, educating communities on the dangers of wildlife consumption, and prioritising the identification of new pathogens with zoonotic potential before they have a chance to wreak havoc.
There is unquestionably a lot of work to be done. Will governments now, finally, take heed?
“I hope so,” Professor Cunningham replies. “I really hope so.”