The emergence of SARS-CoV2 and the COVID-19 pandemic have given bats very bad press. Media coverage has made many more people aware that bats can pass diseases to other animals and humans in a process known as zoonosis.
Zoonotic diseases come from a variety of infectious agents, such as viruses, bacteria, parasites (vectors), fungi, and even prions. But it is zoonotic viruses that have the greatest potential to cause pandemics. In recent decades, bats have been the source of most of the major zoonotic viral healthcare crises.
I’ve worked in the field of zoonotic diseases, specialising in sarbecoviruses originating in bats for almost 30 years. We’re seeing new emergent viruses more frequently today than when I began my career.
The majority are sarbecoviruses which originate in bats. Bats have a unique immune system that allows them to sustain high virus loads without disease, making them ideal “natural reservoirs” for viruses.
However, while bats might be the reservoirs of the viruses, it is nearly always humans that create the circumstances that lead to zoonotic events.
In real life bats don’t act like they do in horror movies. They don’t seek out humans to infect them, preferring to have as little contact with us as possible. Unfortunately, they can’t really avoid us as our towns and farmlands are increasingly encroaching on their natural habitats. In some Asian and African cultures, people even hunt them and other wild animals for food, increasing their contact with humans and their domesticated animals, which in turn causes a rise in emergent zoonotic diseases.
The drivers for emergences are complex. They vary from virus to virus, but one thing is certain. They are largely human induced and revolve around economic growth, encroachment into wilderness areas, a desire for wildlife meat and the resultant live animal trading.
With SARS-CoV1, we know that bats harboured the ancestral virus, and civets — popular as bushmeat — played an important role as intermediate hosts before it was transmitted to humans, followed by human-to-human transmission in the wider population. My 30 years of experience in this field leads me to think SARS-CoV2 probably followed a very similar route of emergence. The major driver for the emergence of the most recent Ebola virus outbreak in West Africa was habitat encroachment and bushmeat trading.
Over the past century the northward spreading suburbs of Sydney in Australia crept closer to the territory of the bat reservoirs for the Hendra virus. Under pressure from climate change and habitat encroachment by humans, the bats extended their range 1,000 kilometres southwards. Eventually human and bat habitats overlapped, allowing the Hendra virus to jump from bats to humans, through horses, the intermediate host.
The emergence of the Nipah virus in Malaysia was triggered by a combination of unusual farming practices — forest clearance for land palm plantations, forest fires, and factory pig farming. Bats are very sensitive to smoke and forest fires which cause their stress and virus levels to rise. Driven by smoke and the loss of habitats, they can travel long distances looking for new territory, eventually coming in contact with domesticated pigs and humans, resulting in transmission via pigs to humans.
While the specific drivers of zoonotic emergence may vary, habitat incursion and increased contact with wildlife — particularly of bats — remain the main factors.
Reverse zoonosis
At a keynote lecture at the Special Ministerial Conference for ASEAN Digital Public Health last October, I emphasised reverse zoonosis could be even more dangerous than zoonosis.
Zoonosis typically refers to animal-to-human transmission but the traffic is not always one-way. Humans can also transmit diseases to animals in a process called reverse zoonosis. If that happens, it could create a new class of reservoirs, which I call the “unnatural wildlife reservoir”.
In zoonosis we start with a “natural reservoir”, often bats. For example, SARS-COVID-2 is believed by most scientists to exist in bats, probably in Asia. It passed through Animal X (an intermediate-amplifying host), possibly pangolins or civets, and then transmitted to humans, followed by massive human-to-human transmission.
To the surprise and concern of the scientific community, this zoonotic virus seems to find going from humans back to animals much more easily than previous viruses.
Perhaps the best documented human to animal transmission has been to minks, which happened in Denmark and the United States. The virus passed from human handler to mink, replicated, mutated, and then passed back again to humans. This presents a dangerous challenge because the virus is no longer the same virus that humans gave to minks.
As it occurred on farms, it was relatively easy to contain. But what would happen if humans or minks gave the virus to a novel wild host, such as bats on the American continents, creating an “unnatural reservoir”? We would have the virus mutating and being passed to animals previously unexposed, with the potential that the virus could mutate further in animals X, Y and Z before spilling over again into humans as a more virulent SARS-CoV3. This is why it is of utmost importance that we ensure that kind of reverse zoonosis does not happen.
As the scientific community has been warning for decades, it is not a matter of if we have another pandemic but when. The two most likely candidates are an influenza virus or another bat-borne coronavirus.
If it is an influenza, there is not much risk of human to animal transmission. But if it is another bat-borne coronavirus, there are two key predictors for reverse zoonosis. First is transmissibility, or how frequently the virus can transmit among humans. Second is species tropism, or how many different species the virus can infect. If it is highly transmissible and can affect many different species, it would be a good candidate for a reverse zoonosis event.
The best way to prevent a reverse zoonosis event is to prevent the initial animal to human zoonosis event but that is easier said than done.
Growing populations and climate change are driving a demand for agricultural land and even more encroachment on the natural habitats of wild animals. Banning wildlife trading has been proposed, but most experts think that may not be feasible; after all it is already illegal in many countries where it still flourishes. Tighter regulations and enforcement might help but changing cultures takes time.
Meanwhile, our best defence needs to be improved surveillance, the regular screening of at-risk populations and international sharing of data. Once an outbreak is detected, a rapid response to isolate and contain it is crucial — every day counts.
Wang Linfa is a professor in emerging infectious diseases at Duke-NUS Medical School, Singapore. He is also executive director of Singapore’s Programme for Research in Epidemic Preparedness and Response, a research scheme launched in December 2020 amid the COVID-19 pandemic.
