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When a virus jumps: of man, microbes and pandemics

Climate change effects or complex man-animal interactions—why are zoonotic disease outbreaks like covid-19 becoming more frequent?

An electron microscopic image of the SARS-CoV-2 virus
An electron microscopic image of the SARS-CoV-2 virus

Inger Andersen, executive director of the UN Environment Programme, said last month that nature was sending humankind a “message" through the coronavirus pandemic and climate crisis. In an interview with The Guardian, the Danish environmentalist said there were too many different stress points on the natural system at the same time. Something, she said, had to give.

Even as lockdowns around the world continue, including in India, the number of novel coronavirus cases has crossed 2.7 million. The virus, which started spreading from Wuhan, China, in December, has so far claimed more than 190,000 lives globally. In India, the number of covid-19 cases has crossed 23,000, with 720-plus deaths so far.

But this is not the only major outbreak the world is dealing with right now.

February saw parts of Africa reeling under an Ebola outbreak, while Saudi Arabia had a resurgence of cases of MERS, or the Middle East respiratory syndrome. China had a few cases of the rare but deadly hantavirus disease in March.

The common thread? They are all zoonoses—diseases that crossed the species barrier from animal hosts before infecting human beings.

According to the World Health Organization (WHO), the world sees an estimated one billion cases of illness and millions of deaths every year from zoonotic diseases. Around 60% of the emerging infectious diseases, or EIDs, are zoonotic. A February 2008 paper, published in the scientific journal Nature, analysed a database of 335 EID “events" between 1940-2004. It concluded EID events had risen significantly over time and are dominated by zoonoses, with the majority originating in wildlife.

But why is this happening? Why are viruses making the jump from animals to humans more frequently?

“Each time you have a disease, it is a sign that deep down the relationship between man and microbe has changed in some fundamental way. Something has changed," says Thomas Abraham, a former editor at the South China Morning Post and adjunct associate professor at The University of Hong Kong’s Journalism and Media Studies Centre. Abraham also authored the 2004 book Twenty-first Century Plague: The Story Of SARS.

Speaking from Bengaluru, he explains that human populations have grown at an unprecedented rate since the 1900s. “We have expanded sixfold at least," he says. People have ventured into areas they had never lived in. “Forests are being cut, lakes are being drained, the environment is changing, and we are also coming into contact with new forms of animal life. As we come in closer contact, the pathogens that these animals have, which are probably harmless to them, get an opportunity to pass on to man," he adds.

Graphic: Ahmed Raza Khan/Mint
Graphic: Ahmed Raza Khan/Mint

Another reason is our ability to detect and classify these diseases better. “When you identify the causes, what you thought was one big disease is actually probably 10 different diseases," he adds.


Before covid-19, the most recent flare-up of a zoonotic disease in India was the Nipah virus outbreak in Kerala two years ago. In 1957, the western and central districts of Karnataka suffered the Kyasanur Forest disease, caused by a virus of the same name, for the first time. Also known as monkey fever, the virus’ original carriers are hard ticks that infect other animals as well as humans. Human cases become more frequent when a person comes in contact with infected animals that might also carry the ticks as parasites.

“The ticks existed in deep forest areas where humans didn’t go. That’s why the disease didn’t get transmitted. I correlate this with the covid-19 virus. It supposedly originated from bats because humans have come in much closer contact with them," says Yogesh Gokhale, area convenor, Centre for Forest Management & Governance at The Energy and Resources Institute, a Delhi-based think tank.

Seasonality plays a big role in the spread of this disease, with more cases reported in dry periods. Worryingly, climate change effects have a constant influence on man-animal interface and zoonotic diseases. While some disease vectors are sensitive to temperature changes, extreme changes in temperature also impact transmission patterns. So rising global temperatures become a key factor in the prevalence and re-emergence of zoonotic diseases.

“Across the world, warmer temperatures and the combination of warmer temperatures and high humidity widens the transmission window for vector-borne diseases. This is something that India has also been concerned about for some time," says Arunabha Ghosh, founder-CEO of the policy research institution Council on Energy, Environment and Water. “Higher temperatures increase the activity, reproduction and frequency of the so-called blood meals of these vectors. These pathogens, harboured by mosquitoes, for instance, also mature faster, resulting in more rapid expansion and increased intensity of disease," he says during a video call.

The notion that warmer temperatures could have an adverse effect on the covid-19 virus was debunked earlier this month when Harvard researchers warned in a study that the virus might not fade away in warmer weather.

Rainfall patterns have similar effects on certain diseases. Dengue fever, chikungunya, Zika and Rift Valley fever are known to spread during periods of heavy rain and flooding, which result in greater vector capacity. “The point is to keep looking at the non-linear risk of climate change because there is no flattening of the curve here," adds Ghosh. “Warm climate keeps becoming warmer and that’s what we have to prepare ourselves for."


In the past, researchers have traced the origins of some of the deadliest animal-borne disease outbreaks (Marburg virus, Ebolavirus, SARS virus, Nipah, novel coronavirus, or SARS-CoV-2) to one source: bats. But how do bats survive so many viruses in their bodies?

A study published in February in the scientific journal eLife by researchers at the University of California, Berkeley explains how fierce immune systems in bats drive viruses to higher virulence, making them deadlier in humans who have a relatively “tamer" immune system.

Fruit bats roosting on a tree on Janpath, Delhi.
Fruit bats roosting on a tree on Janpath, Delhi.

Rohit Chakravarty, a bats researcher and doctoral candidate at the Leibniz Institute for Zoo and Wildlife Research in Germany, explains how viruses mutate among bat species. “Bats are really diverse. There are more than a thousand species with their own natural histories, lifestyles. India alone has about 128 bat species," he says. “Sometimes, a lot of the species roost together in a cave. When all these species, with different diets and exposure to different viruses, come together and group in one cave (or other natural settings), it is very easy for viruses to keep mutating," he adds.

In April, an Indian Council of Medical Research study found coronaviruses in two local bat species—the Indian flying fox and Rousettus—based on swab samples from 25 bats from these two species in Kerala, Tamil Nadu, Puducherry and Himachal Pradesh. It noted there was no relation between these coronaviruses and the covid-19 virus, or any evidence that these viruses could be transmitted to humans. Coronaviruses are a large family of hundreds of viruses that circulate among certain animals but sometimes jump to humans.

Aaron T. Irving, a senior research fellow at the Duke-NUS Medical School in Singapore, says much of the problem is the human immune system: It’s “overreacting and killing us". The SARS-CoV-2 virus causes fatal inflammatory responses and acute lung injury in humans. “Bat immune systems have a very strong primary antiviral response but they seem to prevent excessive inflammation. They block cytokine storms that are commonly seen in humans and prevent a massive recruitment of immune cells to where the virus is. This means bats tightly regulate their immune system to prevent it going into overdrive," adds Irving, who focuses on bat immunology.

Another hypothesis explains how the ability to fly protects bats: in other words, “flight as fever". Their metabolic rates increase in flight and the rigorous physical activity results in a rapid rise in body temperature, similar to the fever we experience when our immune system is trying to ward off an infection.

Yet Chakravarty believes it is important not to forget the role of anthropogenic pressures. When animals are stressed and driven out of their natural comfort zones, their immune systems stop functioning efficiently, which means they are more likely to get infected and then pass on viruses to humans.

Arinjay Banerjee, a postdoctoral researcher at McMaster University’s Institute for Infectious Disease Research in Canada, says it would not be fair to describe bats as reservoirs of deadly viruses. Banerjee is part of a Canadian research team that last month successfully isolated and cultured the covid-19 virus in a high containment laboratory—a big step towards understanding more about the virus’ biology and developing a vaccine.

Banerjee believes this outbreak stresses the need to understand disease ecology and transmission of pathogens from animals. “The unfortunate thing about this coronavirus is that it is more easily transmissible. SARS and MERS have higher mortality rates but they never infected as many people as SARS-CoV-2," he says. “We need to be prepared for future outbreaks once we have dealt with covid-19."

Initiatives like the Global Virome Project are already working on understanding how and where the next viral outbreak might come from (GVP is a 10-year scientific effort of public, private and philanthropic organizations to discover zoonotic viral threats). “Only by understanding virus diversity and the associated transmission risks may we move from constantly reacting and responding to epidemics to preventing epidemics," says Jonna Mazet, a member and implementation director of the GVP leadership board.

On email, Mazet explains that in seeking out viruses, the project is also strengthening country capabilities to detect and control viral spillover, by improving surveillance, biosecurity and laboratory capacity.

History offers an example. In the 1960s, researchers described the first human coronaviruses— HCoV-229E and HCoV-OC43, which were studied extensively till the mid-1980s. They caused a worrying infection that is known to us today as the common cold.

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