Scientists have spent decades studying the diseases lurking in the animal kingdom and they have managed to find very few answers to a whole range of questions.
In late 1976, a doctor at a missionary hospital in the northern Democratic Republic of Congo - then known as Zaire - reported an outbreak of a mysterious illness. Villagers were showing up with symptoms of nosebleeds and bloody diarrhoea. Most of the patients died within days, along with 11 of the 17 doctors and nurses treating them.
That marked the beginning of the Ebola virus, which over the years has resurfaced multiple times on the African continent, killing thousands of people.
Ebola is a zoonotic disease — meaning it originates from wild animals. Researchers have spent four decades collecting and testing samples from mammals, birds and reptiles, yet they have been unable to find the source of virus.
“Two-thirds of all infectious diseases in humans come from animals and three-quarters of all those are from the wildlife,” Professor Kate Jones, a biodiversity scientist at the University College London, told TRT World.
“We don't know much about the sources of viruses and pathogens out there in the wild. This was never really a problem until we started to change the planet by moving around animals and trading them in wet markets.”
As Covid-19, the disease caused by the new coronavirus, spreads around the world, overwhelming hospitals, disrupting daily life and raising fear of a global recession, the urgency of containing zoonotic diseases has come to the fore.
Scientists are still undecided on the number of diseases lurking in the wild. They know little about the viruses that latch onto humans.
“We barely know what biodiversity is out there let alone what pathogens live in those species,” says Jones.
“There have been few papers which have tried to estimate (the number of pathogens in the wild). But these are just kind of guesses and models of what’s going on.”
The Covid-19 or 2019-nCoV has been closely associated with the genetic makeup of a coronavirus found in bats. But that doesn’t mean the virus originates from bats.
An animal can be an intermediary host - a vehicle for the virus to ride on until it infects another species. What scientists are really interested in finding is a reservoir host, the origin of a virus.
A reservoir host is an animal in which a virus can live without killing it. From time to time it spills over into humans and that’s when heads start to turn and alarm bells ring.
Don’t blame it on the bats - yet
In 1994, a mysterious illness started to kill thoroughbred horses in Australia. Soon enough, there were reports of deaths among people who worked in close proximity to the horses. The virus was called Hendra, after a place near Brisbane.
Virologists began looking for the source of the virus. They found antibodies of Hendra in a bat species —the flying fox. That, however, didn’t mean that it was the reservoir. They needed to extract a live virus from a bat to prove it.
“Screening for antibodies is distinct from isolating virus, just as a footprint is distinct from a shoe,” writes David Quammen in his book the Spillover.
Then two years later, a pregnant flying fox got caught up in a wire fence and died. Tests showed that it was carrying a live Hendra virus.
Scientists are not lucky all the time. The animal reservoir behind the outbreak of Severe Acute Respiratory Syndrome (SARS), just like Ebola, is still unknown, according to the World Health Organisation (WHO).
It probably spread to humans from civets, a diminutive nocturnal mammal native to tropical Asia and Africa, which were infected by some other animal. However, some of the most dreaded infectious diseases — Nipah, Marburg and Hendra — have originated from bats.
Jones says bats are a likely suspect because of their sheer numbers. “One in every fifth mammal species is a bat. You would expect them to have a lot of pathogens that can spill over.”
Still it doesn’t explain everything since there are many more rats than bats - one in every three mammals is a rat.
If the answer is not in the numbers then it could be in a bat’s physiology, says Jones, who has closely worked with bats for years.
Bats fly and so they have adapted to live with the pathogens. When an animal flies it burns a lot of energy - roughly estimated 34 times more than compared to a creature that roams the ground. That means bats live with high temperatures.
“So there’s a theory — and this is not a fact yet — about how normal pathogens have to cope with the bat having this really high temperature. Really high temperature usually kills off the viruses but they have adapted to these situations,” says Jones.
Researchers still don’t have answers to a lot of questions and bats continue to surface as culprits in headlines. “I think it's really dangerous to demonise one animal.”
A rare occurrence
Infectious zoonotic diseases such as Covid-19, which spread from human to human, like a wildfire, are very rare.
For instance, rabies disease, caused by an animal bite, is very lethal but it doesn’t get passed on. Ebola, HIV, SARS, MERS and now Covid-19 are just a handful of examples where the viruses have spread due to human interaction.
The scary thing is the fact that these infectious outbreaks have emerged one after another in the past 40 to 50 years - pathogens which have lived in the animal kingdom for thousands of years are only now spilling over to humans.
“What’s really changed in the past 40 years is that the number of humans on this planet has massively increased. If you get these spillovers, it's more likely to go bonkers,” says Jones.
This has also had an impact on demand for wild animals and as a result wet markets have popped up in different countries especially China. Animals in these markets are kept in small cages in close proximity, presenting the chance for different pathogens to mix up.
“So in these markets you have (virus) combinations that nobody has seen in nature before,” says Jones.
“We are moving animals around much more than we ever had and we are coming into contact with them more than ever before. So I think it's a result of human actions on ecosystems.”