A Perspective on Bats Implicated in COVID-19
Published on 8 March 2020
The novel coronavirus, SARS-CoV-2, responsible for the current viral outbreak of COVID-19, is similar to bat coronaviruses. It likely originated in chrysanthemum bats, intermediate horseshoe bats (Rhinolophus affinis), or greater horseshoe bats (Rhinolophus ferrumequinum). The progenitor virus circulates harmlessly in bats. But a stressed animal is known to shed its viruses.
The International Union for Conservation of Nature (IUCN) traces the geographic distribution of Rhinolophus ferrumequinum from Europe to Japan. According to the India Biodiversity Portal, this species has a wide range in Europe, Africa, South Asia, China, Korea, Japan and Australia.
Bat – A Unique Mammal
The bat is a flying mammal, belonging to the Order Chiroptera. It harbors many disease-causing viruses. It is responsible for a higher proportion of zoonoses among all mammals. Zoonoses are diseases that spillover from animals to humans. Bats are natural reservoir hosts for henipaviruses (Nipah and Hendra), filoviruses (Ebola, Marburg) and coronaviruses (SARS-CoV and MERS-CoV).
Bats can live up to 41 years compared to the house mice, which has an average life span of two years. One bat can be a host to many viruses. They can devour a ton of disease-carrying insects. Ticks live on bats and feed on their blood. Bat ticks are also capable of carrying viruses and transmitting them to humans. Rodents, primates and birds also carry diseases which jump to humans. Bats account for a quarter of all mammalian species and rodents, 50 percent. They are wide-ranging and are found everywhere but the poles and a few Oceanic islands.
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Bats can be nectivorous feeding on nectar of flowers, frugivorous feeding on fruits, seeds and flower pollen, and insectivorous feeding on insects like beetles, moths, mosquitoes and agricultural pests. There are bats that eat fish, frogs, lizards, birds and even other bats! Vampire bats drink blood of animals like cows, sheep and horses. Bats cannot see and can only hear. They fly with the help of echolocation. They lead nocturnal lives, hunt and forage high in the canopy, and roost in hollow trees and caves.
Horseshoe bat is a microbat, well-represented in the temperate zone and extends throughout the old world tropics of Africa, Asia and Oceana. Chinese horseshoe bats are insectivorous. Cave nectar bat is a megabat, found in human-dominated habitats of northern India, southern China and Singapore. It can fly long distances at night in search of nectar of flowering trees and shrubs. It is an important pollinator of fruit trees, such as durian and seeks the nectar of flowers of trees like banana, Golden Penda and Sea Apple. The fruits which bats feed on have anti-inflammatory and antioxidant properties.
Even when the viral load is extremely heavy in bats, they do not fall sick, as they have a fierce immune system. In the case of a virus-induced inflammation, they are able to quickly launch antiviral and anti-inflammatory responses. A dampened NLRP3 protein due to impaired production of mRNA is responsible for this adaptive mechanism. Similar infectivity and virulence can wreak havoc in humans. When the virus enters the human body, the human immune system is activated and there is an inflammatory response against the infection. With chronic inflammation, even healthy human cells get damaged and disease sets in.
Bats spread zoonotic diseases. Four of the eight human afflictions prioritized by WHO are linked to bats. This is pinned to the fact that bats, which are good hosts for disease-causing agents, are the only mammals capable of sustained flight. It requires huge amounts of energy which can typically lead to extensive oxidative damage, but the bats have a mechanism in place to repair the damage. Viruses they host are kept in check by their efficient immune system, but stressors due to lack of food can make the animal shed viruses into its surrounding through saliva, urine and feces. As they typically live in large roosts at close proximity, they can share microbes between them. Compared to other mammals, they are long-lived while being small-sized. The researchers feel that the bats’ ability to host pathogens is tied to effectively managing oxidative damage, repairing impaired DNA, and avoiding senescence.
What Connects Bats and Humans
Bats live in close proximity to humans, mostly in huge colonies in caves and trees. The viruses previously restricted to animals, have become prevalent in humans due to their increased contact with wild animals. Even sea mammals like Beluga whales carry coronaviruses. When viruses spillover to humans from a primary host like bats, they become virulent. The SARS virus jumped to humans through Asian palm civets, MERS virus via camels, Ebola virus via gorillas and chimpanzees, Nipah virus via pigs, Hendra virus via horses, and Marburg virus via African green monkeys.
Humans are infected when they come into close contact with the body fluids of horses carrying Hendra virus. The natural host for Hendra virus is the flying fox. Horses may be infected by eating food contaminated by flying fox urine, saliva or birth products. There is no evidence that the virus can spread directly from flying foxes to humans or through the feces of flying foxes to humans. People who care for bats and have frequent contact with flying foxes have shown no evidence of exposure to the viruses they carry.
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A study in October 2019 by researchers from Singapore, India, US, and China noted that different types of viruses could colonise a bat and spillover to humans, as in the case of filoviruses from cave nectar bats. The bats contained antibodies to three different types of filoviruses. The bat hunters, who lived near the bats, were exposed to bat saliva, blood and excreta.
In 2017, there was a study by a team of researchers from US, India and China on humans living in close vicinity to bats. In many remote parts of the world, as in Nagaland in India and Uganda in Africa, humans live in close contact with the bats, but it seems they never develop life-threatening diseases.
So much so, a village in Tamil Nadu, called Kazhuperumpakkam, near Vanur in Villupuram District, has an unusual practice of not bursting crackers to celebrate the festival of Deepavali, as loud sounds disturb bat colonies, which roost in the village banyan tree. Bat-hunting is also banned. During the Nipah virus disease outbreak in India, the health department inspected and cleared the village of any threat from the bats. This gesture of safeguarding bat habitats is common to many villages in India. In the state of Bihar too, the bats are revered, not reviled.
Bats are a source of food and even a delicacy for humans in some African, Asian and Pacific Rim countries. In China, wholesale markets dealing with seafoods sell live, farmed, exotic and wild animals. In a tribal community called Yimchungru in Nagaland, India, it is an ancient traditional practice, going back several generations, to hunt and eat bats. The Narikuravar, a hunter-gatherer, non-pastoral community of nomads in Tamil Nadu, India, also feed on bats among other animals and plants. It is difficult to change behaviours of culturally held beliefs about food which go back thousands of years, just as it is difficult to get communities to reduce contact as they live near the bat caves.
Stressed Bats Shed More Viruses
The Chinese rufous horseshoe bat (Rhinolophus sinicus) was identified as a natural reservoir of SARS coronavirus. In the case of Nipah virus, it is found in fruit bats or flying foxes (Pteropus medius). Most bats in the genus Pteropus carry antibodies for Nipah or other closely related henipaviruses, which means they harbour live viruses in their tissues. Nipah virus emerged from bats due to intense pig farming in bat-rich biodiversity hotspots. The fruit bats with viruses were feeding on mango trees overhanging the pig sty and dropping the fruits into them. This passed the infection to pigs, and from pigs to farm workers. A host-switch took place and the virus then began the human-to-human transmission. While the pigs died from acute respiratory distress, the people died from encephalitis, a deadly brain inflammation. To contain the virus, a million pigs were slaughtered, which crippled the pork industry in Malaysia, where the outbreak first began in 1999. Nipah virus re-emerged later in India and Bangladesh, which did not have large-scale pig farms, ruling out the presence of an intermediate host. Of note, there is an overlap between bats and people because both eat fruits.
In 1997-1998, there were El-Nino-related droughts and anthropogenic fires in Indonesia. Driven by the lack of food source, as the forests burned down, bats, the natural reservoir hosts of Nipah virus, migrated closer to cultivated orchards and pig farms in Malaysia. The initial transmission of the virus from bats to pigs took place through the contamination of the pig swill by bat excretion. In some cases, due to the lack of primary forest and reliable food, bats develop a taste for the closest food source. This was seen during the Nipah outbreak in Bangladesh, where the bats started feeding on date palm sap. The people who had fallen ill also had a habit of drinking the sap. The route of disease transmission was established.
In the recent Australian bushfires, which raged from November 2019 to February 2020, alongside nearly half a billion endemic animals, the native bats too have been decimated. Once the animal is stressed, as it has lost its niche and food source, it migrates long distances in search of food. In extreme heat, the stressed adult bats often abandon their pups. The fires have also destroyed millions of trees fruit-eating bats rely on. Disrupting bat habitat stresses the animal and makes them shed more viruses through their saliva, urine and feces, which in turn infect other animals. Bats also defecate and urinate while in flight. Environmental threats to bats increase zoonosis.
For instance, durian fruit, although native to Malaysia, is farmed in Sri Lanka, southern India, southern China and Thailand. There is an insatiable appetite for this exotic fruit and it enjoys high popularity in China and Hong Kong. In 2016, Thailand was the lead exporter of durian to China. Frozen durians were imported from Malaysia. By 2017, Malaysia overtook Thailand to export fresh durians to China. As a result of which vast durian orchards swallowed up tribal lands and ancestral forests in Malaysia. Due to high demand of the exotic fruit and its skyrocketing prices, the Chinese started to grow durian trees themselves. Fruit bats, which feed on flowers of durian, are its vital pollinators. Not just these megabats, even the microbats, civets and slow loris were capable of pollinating durians. Farmers kill bats as they are considered pests of fruit crops, but in the case of durian, they are its pollinators!
Bat feces can contaminate domestic swimming pools. Health risks arising from such contamination can be effectively managed by disinfecting pools, maintaining free chlorine levels of 2 milligrams per litre and keeping the pH between 7.2 to 7.8.
From Biggest Allies to Biggest Threats
Flowers of bat-loving plants are modified to attract the nocturnal visitors and even secrete amino acids crucial to bat health. Migratory bats pollinate a variety of plants as they travel and the plants tend to cater to this need by flowering during their migratory season. There are more than 500 species of plants, including mango, banana, guava, durian, agave, avocado and date palms that rely on bats to pollinate their flowers.
Bats eat mosquitoes, which are disease-carriers themselves. Bats feed on agricultural pests of rice and maize. The movement of bats is influenced by the availability of food and weather conditions. They move in response to the irregular blossoming of plant species. They are important in the balance of insect population and control of insect pests. They are keystone pollinators, seed-spreaders and migrate in directions that is beneficial for them.
This intricate balance is disturbed when humans invade bat territory, re-organize their source of food, trade them in livestock markets and even consume them. The consequence can be devastating. A study in 2018 by American and Chinese researches reiterated the importance of virus surveillance in natural reservoirs, as people living near bat caves could be in danger of a potential spillover. A 2019 study by researchers warned that the next coronavirus epidemic in China would be near caves, as hotspots for spillover. Genetic diversity of bat coronaviruses in China was extensively studied.
Bats cannot be blamed for viral epidemics. There is a need to monitor them routinely for the sake of public health. For this reason, EcoHealth Alliance has created a database on mammals and the viruses they host.
With climate change here to stay, natural catastrophes and calamities might be the new normal. It will be an ecological nightmare should the bats go extinct. Deforestation is threatening the bat population, as well as, a lack of food, parasitism, climate change affecting roosting sites and predator risks. Trigger-happy, gun-trotting humans hunt bats for target practice as well as for consumption. Man-made environmental mismanagement can have far-reaching effects.
Web of Life
In the web of life, everything is interconnected. Insect apocalypse will have a cascading effect as the insects serve as food for the higher level organisms in the food chain. For instance, frogs and amphibian eggs are food for snakes. When a fungal pathogen wiped out frogs in Central America, a snake species became thinner and starved to death due to lack of food. The snake community became less diverse and the number of snakes declined due to this mass die-off.
Similarly in Assam, India, when the leopards dwindled in number due to hunting and changing landscape, the rhesus macaque population increased. This led to these Old World monkeys feeding on chickens and raiding the village grain stores and kitchens. The landscape preferred by leopards, like the sugarcane fields and fruit tree patches, were replaced by tea gardens and commercial crops.
In case of plants, non-native ornamental species are grown in most tropical cities to beautify urban areas. Native birds feed on the seeds and the fruit bats feed on the fruits and flower nectar of these exotic species, which provide for the entire year. Competition between invasive and indigenous species ensues.
The Golden Penda tree, frequented by the bats and native to Australia, is grown as an exotic ornamental tree, lining the streets of Singapore. Such instances of growing native trees in non-native habitats is very common around the world and it can have a cascading effect on the web of life. Efforts are being made to introduce native species back where they belong.
One Health Approach
In 2017, the World Health Organization (WHO) initiated ‘One Health’ an approach to work towards better public health through implementing research, programs, policies and legislation. High on its priority are food safety, combating antibiotic resistance and controlling zoonoses.
This ‘One Health’ approach is also adopted by EcoHealth Alliance, the international nonprofit dedicated to protecting the health of people, animals and the environment from pandemics in global hotspots and to promote conservation.
To study spillover of bat-borne diseases and the heightened environmental threats to bats, which may in turn add to the threat of zoonosis, a bat monitoring program called ‘Bat One Health’ is currently underway in Madagascar, Bangladesh, Ghana and Australia. The project explores the link between loss of bat habitat and the spillover of bat viruses into other animals and humans.
The world is extremely vulnerable to epidemics and pandemics. An independent body called the Global Preparedness Monitoring Board (GPMB) tracks the progress and identifies the gaps. According to GPMB, public health systems should be adequately funded and supported even in the absence of disease outbreaks.
China, the second largest economy in the world, faces acute ecological and environmental problems due to its rapid industrialization and urbanization. The concept of ‘One Health’ in the context of zoonoses at the intersection of human-animal-environment interface has been investigated widely in China.
After COVID-19, the Chinese government has introduced stringent rules on the trade and consumption of live wild animals for food. Early sufferers of the coronavirus were employees of the wet market, where wildlife animal products are sold for food, fur and for use in traditional medicine. Exotic animal meat is in high demand not just in China, but in many other countries as well.
It Pays to Pay Heed
Nearly one million plant and animal species are facing extinction, and among them, half account for insects. Animals need insects to survive. Sharp drop in bird population is blamed on the insect collapse due to pesticide overuse. It could be catastrophic for humans, warn scientists.
After the SARS outbreak, civets which were its intermediate hosts were culled. Similar fate might befall the pangolins, should they be confirmed as the intermediate hosts of COVID-19. Pangolins have already made it to the endangered list of wild animals. With bats being viewed as disease-carriers, they might be culled too.
Only a fifth of the 5.5 million insect species have been identified. The services provided by insects like pollination, nutrient cycling and biological pest control are irreplaceable.
Image Source: Pixabay
With the insect population vanishing rapidly, most plant pollinators might disappear altogether and humans will end up hand-pollinating the plants in the future. Also, it is much cheaper to get people to hand-pollinate, as human labor is cheaper than renting bees to pollinate crops!
Bats in general are known for long hibernation, elaborate grooming rituals, complex temperature regulation, bio-sonar communication and long-distance migration. They have even helped advance vaccine research! It is also important to remember that they are a nonintrusive form of life and are capable of complex long-term social relationships, just like humans. The problem is never the animals, but the humans who encroach into the animal territory. There is always a price to pay for the invisible ripple effect of animal extinction and land degradation. The social and economic impacts will follow.
January 2021 – A novel SARS-CoV-2 related coronavirus in bats from Cambodia
April 2021, Science – Scientists scour the Amazon for pathogens that could spark the next pandemic: “The findings fit a widely debated hypothesis known as the dilution effect, which holds that in forests with greater biodiversity, mosquitoes and other vectors have more targets and end up biting animals not capable of incubating a given virus, thereby slowing its spread. Reducing biodiversity by clearing land can do the opposite, and it also pushes humans into closer proximity to wildlife. Bats are a particular concern, because they often roost in buildings.”
31 May 2021, Nature Food – Land-use change and the livestock revolution increase the risk of zoonotic coronavirus transmission from rhinolophid bats: “……human–livestock–wildlife interactions in China may form hotspots with the potential to increase SARS-related coronavirus transmission from animals to humans. Mitigation of SARSr-CoV emergence can be enhanced by reducing livestock or human density, respectively, thereby reversing ongoing dietary and urbanization trends. Thus, environmental health is tightly connected to both animal and human health, as recently stressed by planetary and ‘one health’ discourses, which advocate for more holistic views of global health, encompassing environment, animals and people, and the interactions among these factors.”