Earth’s deadliest enemy: Zoonotic diseases that jump from animals to humans pose as big a threat to humanity as nuclear war or climate change, argues JOHN VIDAL
No one who watched the first episode of It’s A Sin, Channel 4’s new drama about the spread of Aids in the early 1980s, can have failed to register the sense of freedom and optimism that permeated an era when gay men could express their sexuality without fear for pretty much the first time in British history.
As the series progresses, of course, the story becomes much darker. It is a chilling reminder of the last time our world was confronted by a mysterious and lethal virus trailing thousands of deaths and devastation in its wake.
Of course, the Aids virus, HIV, and the Covid-19 virus are very different, not least in terms of transmission and their impact on the body. And yet there is a fundamental link between the two that has potentially catastrophic implications for the future health and survival of our ever-expanding global population — even after this present crisis is over.
That’s because both HIV and the Covid virus, along with a host of other deadly microbes, originated in animals and jumped the species barrier into humans. They are known as ‘zoonotic’ diseases.
They did so because, for a host of reasons, mankind has changed its relationship with both wild and farmed animals, destroying their habitats and crowding them together — and the process, as we shall see, is only accelerating.
This trend presents us with two vital questions: when will we really wake up to the greatest new threat of our age?
And when will we understand that — like the war on terror, the threat of nuclear war, or the climate change crisis — our battle to defeat it must become one of the globe’s top priorities?
If we fail to appreciate the seriousness of the situation, this present pandemic may be only a precursor to something far graver still.
Let us return to HIV: it was in 1981 that previously healthy young men in New York and California began presenting at hospitals with fever, flu-like symptoms and a range of rare infections and conditions. It wasn’t long before cases were being reported in London and other major European cities of a baffling disease that attacked the immune system and seemed to target gay men, drug users and haemophiliacs.
Cameroonian rainforest and became infected via contact with the animal’s blood or from eating the ‘bush meat’
Panic escalated when it became clear that heterosexual men and women were also vulnerable to infection.
Scientists united in a global effort to identify the virus — soon to be named HIV/Aids — and its source.
Just as with Covid-19 today, there were denialists and conspiracy theorists.
Aids, it was claimed, had been created in a U.S. government laboratory and spread by the CIA as a ‘weapon’ to reduce black populations. Another theory suggested it was the result of a polio vaccine programme that had gone wrong.
Forty years on, genetic sequencing and historical records now confirm that the virus that causes Aids originated in a harmless chimpanzee virus that mutated and crossed into humans, probably when a hunter butchered an animal deep in the Cameroonian rainforest and became infected via contact with the animal’s blood or from eating the ‘bush meat’.
Slowly, this virus spread in humans over decades, continuing to mutate until one strain evolved that was infectious enough to be passed from person to person via bodily fluids.
Ecologists at University College London (UCL) report that, since 1940, 335 new and potentially fatal diseases have emerged globally
Travelling from village to village down remote rivers in central Africa, the virus eventually reached Kinshasa, the teeming capital of the Congo (now the Democratic Republic of the Congo), perhaps sometime in the 1960s or even earlier.
There, in one of Africa’s fastest-growing cities, it began to establish itself in the population and spread further as it hitched a ride on boats and planes around Africa and the rest of the world, courtesy of those it infected.
In the 1970s, increasing global travel, mass movement of people and the sexual revolution accelerated its spread. To date, HIV has killed 32 million people and infected more than 75 million.
No one knows how it arrived in the U.S. or Europe, but scientists researching medical records have, over the years, cited candidates including a Norwegian sailor, a Canadian flight attendant and — most recently — a World War I soldier as among the first Western victims.
With hindsight, we can see the emergence of HIV/Aids as a milestone in a new era of pandemics and serious dis-eases linked to mankind’s increasing domination of the planet, the destruction of the natural world and disregard for wildlife.
Ecologists at University College London (UCL) report that, since 1940, 335 new and potentially fatal diseases have emerged globally, of which more than 200 are zoonoses — viruses, bacteria, parasites, fungi and prions that occur naturally in wild and domesticated animals but which are turning up in humans.
They include some of the most deadly diseases ever encountered: HIV, of course, Ebola, Lassa Fever, Marburg and Simian foamy virus in Africa; Sars and now Covid-19 in China; Chagas, Machupo and Hantavirus in Latin America; Hendra in Australia, and Mers in Saudi Arabia.
Almost all have spread internationally and there are vaccines for only one or two. In addition, three new deadly pandemic influenza A strains have surfaced since 1957, and bird and swine flus have crossed to humans.
Princess Diana talking to AIDS patient with Kaposis Sarcoma on his legs during the opening of the Rodney Porter Ward for AIDS Patients at St Marys Hospital
Any one of them can mutate and potentially devastate human populations.
So how and why are these zoonoses making the transition? Over centuries, the close proximity of humans and the animals kept for food in primitive dwellings was a factor — and remains so in parts of Asia, especially when it comes to poultry.
Today, however, zoonoses are increasingly linked to deforestation, land clearance, the encroachment of urban dwellings into previously wild areas, and the hunting and sale of animals for food in markets.
Others, like Dengue and Chikungunya, both mosquito-borne, are spreading fast as the world warms because of climate change, allowing disease-carrying insects to flourish in new areas. Some years ago I travelled to the heart of the Congo basin rainforest in Gabon where there had been an outbreak of Ebola — named after a river in the Congo — several years earlier.
Accompanied by local guides, it took me several days by car on some of the worst roads in Africa, and then by small boat down the Ivindo River to reach the small village of Mayibout 2.
Drenched in sweat in the intense heat, under constant attack by insects and fearful of catching malaria, I met the village chief, Bob Lucien.
The Ebola outbreak was over but nothing would be the same again for the community, he said. He, like others, had fled the village when the disease struck, but it had killed two of his family and many of his friends.
Over a glass of palm wine, I was told that it had started when a group of youths, hunting with dogs, killed a chimpanzee and brought it back to the village to eat.
Several families had prepared and shared the chimp meat and 26 people were infected, said Lucien.
The first signs of Ebola are as innocuous as a sore throat or fever but within days these would escalate to violent vomiting and diarrhoea. As the virus spreads through the body, it damages the immune system and organs and ultimately destroys cells responsible for blood-clotting, resulting in severe, uncontrollable bleeding. In the case of the Mayibout outbreak, most of those infected died.
Bob told me that the villagers were still traumatised and refused to eat anything from the forest now. Ebola for them was a curse and they feared its return.
He added that when the village was small and the forest pristine, there had been very little illness. But as roads were built, the population grew and the village expanded into the forest — even as hunters, loggers and gold-miners moved in and deforestation commenced — that changed.
There was nothing to prove conclusively that environmental change was the cause of the Ebola outbreak, but researchers have since studied many other incidents of this and other diseases and found almost all have started in places where deforestation had recently taken place.
It is thought that disrupting the normal forest habitat of bats, often a reservoir of viruses that may ultimately endanger humans, forces them to find other places to live, presenting new opportunities for primates to become infected — and the viruses can then be passed via primates to humans.
The growing demand for bushmeat — African wild animals killed for food — and the popularity of ‘wet markets’ in Asia and elsewhere selling fresh meat, often from exotic animals, is another factor in the growth of zoo- notic illnesses.
According to researchers at Brown University in the U.S., the past three decades has seen an increasing number of infectious disease outbreaks from an increasing number of sources. Some are unexpected.
In 1997, fires set by palm oil companies to clear vast tracts of Indonesian forests for plantations, killed thousands of orang-utans and other animals.
Months later, a new disease broke out near a town called Sungai Nipah hundreds of miles away in Malaysia, killing hundreds of people.
It took scientists years to establish that fruit bats, which usually foraged on trees in the Indonesian forests, had been forced by the fires to seek new food sources.
Some had flown to Sungai Nipah where they had roosted in the fruit trees and dropped pieces of half-eaten fruit into the pig pens below.
The pigs ate the fruit, which was contaminated by bat urine, and then passed the disease, now called Nipah, to humans (the movie Contagion is modelled on the evolution of Nipah).
Not all diseases that have spilled over from animals to humans in the past 50 years come from tropical hotspot areas, however. Lyme disease, caused by tick bites, has become an epidemic in the northern U.S. because developers are building houses in recently-cleared forest land on the edge of cities.
Bats that colonise Australian suburbs can infect horses with a disease called Hendra and they can then pass it on to humans.
In the Western Ghat mountains of India, a rare but devastating disease known as Kyasanur forest disease (KFD) has long been known — but in the past decade has spread far and wide as cattle farmers have felled more forests.
‘The more we destroy or change nature, the more likely we are to see fearsome diseases like Covid-19 emerging,’ says Kate Jones, professor of ecology at UCL.
The coincidence of the appearance of new diseases with the simultaneous expansion of human dwelling and destruction of the natural environment is highly significant, she argues.
Peter Daszak, the British-born ecologist now in Wuhan as part of the World Health Organisation team investigating the Covid-19 outbreak, agrees.
‘Rampant deforestation, uncontrolled expansion of agriculture, intensive farming, mining and infrastructure development, as well as the exploitation of wild species, have created a “perfect storm” for the spill-over of diseases,’ he says.
One of the side-effects is a growth in the number of interactions between humans and nature at its most red in tooth and claw.
‘Intensive farming increases the frequency of contact between humans and wildlife and exposes us to diseases never encountered before,’ says Sean O’Brien, president of NatureServe, a U.S. group of scientists working with conservation organisations.
‘We are bringing together wildlife that would never naturally encounter each other in nature, creating bizarre links in a chain that can allow a disease to jump from one species to humans via another species.’
While hundreds of millions of lives and the economies of many countries have been devastated by Covid-19, it may be that with this pandemic, the world has actually dodged a bullet.
Covid is highly contagious — as shown by its rapid spread around the globe — but so far it has not proved nearly as dangerous as other new diseases, killing ‘only’ around two per cent of the 105 million people it has so far infected.
Many new influenzas and zoonotic diseases are far more dangerous but not nearly so easily spread.
Ebola, for example, kills more than 60 per cent of the people it infects; Mers 34 per cent; and many other diseases more than 20 per cent.
The nightmare scenario that governments are having to face up to is the emergence of a new disease — or a new strain of an older one — which is as contagious as, say, measles, and as deadly as Ebola.
Then humanity could face a far worse pandemic than Covid-19, possibly on the scale of the Black Death, which killed up to one in three people in Europe in the Middle Ages.
Because of air links and global trade, a virus could be spread round the world in a few weeks by people unaware that they are infected, killing tens of millions of people before borders could be closed.
‘Covid-19 may be only the beginning. Future pandemics are likely to happen more frequently, spread more rapidly, have greater economic impact and kill more people if we are not extremely careful about the possible impacts of the choices we make today,’ warns Josef Settele, biologist and co-author of a new UN-level study on future pandemics.
And he is no solitary voice. ‘I think we will get a wave of new zoonotic diseases emerging, a mixture of old and new ones,’ says Delia Grace Randolph, co-leader of animal and human health at the International Livestock Research Institute in Nairobi, Kenya.
‘Animals have thousands of viruses. Some of them are now familiar, like Ebola and Marburg and avian flu.
‘But some will emerge which we did not know about. The worst may be yet to come.What we are doing is destroying the natural world, constantly chipping away at wildlife habitats.
‘When you put animals in situations where they have to eke out resources in a human situation you get diseases. The trouble is, we don’t know what else is out there.’
Despite a global pandemic of a new respiratory illness or virulent influenza strain being one of the highest priorities on the risk registers of most developed countries, our experience of Covid-19 this past 12 months has shown just how unprepared we were for that reality.
Surely now is the time for politicians to do more than deliver platitudes about population growth and urban expansion, environmental destruction, hunting and the lucrative trade in wild animals around the globe.
Yes, we must maintain plentiful supplies of PPE (personal protective equipment), be able to expand intensive care space rapidly at short notice and establish an international network of well-funded vaccine labs that are constantly vigilant and primed to react instantly if a new virus emerges.
But, above all, we cannot continue on the path of wilful disregard of our duties as custodians of the natural world, and we must take global measures to defeat the threat our destruction of the environment poses.
We may have chosen to ignore those facts before — but we know now that our lives depend on it.
n John Vidal is writing a book on the links between nature and disease.
Battle: (From left) Princess Diana in an HIV hospital ward in London in 1989, doctors in PPE gear treat a patient in Cambridge last year, and a boy with a monkey in Gabon, Africa
Pictures: rex/shutterstock/getty/afp
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