By Katherine McFerrin
Amid the COVID-19 pandemic one might wonder how this virus came to be. While it’s well known that the virus came from a live-animal wet market in Wuhan, China, there’s more to the background story than that. It’s been proposed that the virus was transferred to humans from pangolins that were sold at the market or perhaps snakes. Whether pangolin or snake, that infected animal, known as the intermediate host, a vector from which the virus spread to humans, presumably got the virus from an even earlier host. These initial hosts, called reservoir hosts, likely would’ve been bats.
Bats already get a bad rap for carrying rabies and fungi that causes histoplasmosis, a type of lung infection, among other diseases. However, now as their tendency to spread infectious diseases to humans is evident once again, a valid question to ask is why is it so often bats that carry these diseases? “These diseases”, ones carried by animals that can be transferred from animal to animal as well as animal to human are called zoonotic diseases. When such diseases are transferred from one species to another it’s called a spillover.
Bats are a prime host for zoonotic diseases because bats prefer to live close to one another in large groups. Colonies range from a few dozen to several hundreds of bats. Bats not only give the diseases plenty of opportunities to spread within the colony, but they are a convenient mode of transportation for diseases. Like airplanes carrying and delivering precious cargo, bats carry these diseases and then export them via feces, known as guano, as well as scratching and biting.
Despite carrying such infectious diseases, bats aren’t too affected by the diseases. This may have to do with the bats’ immune systems and body physiology that allows them to fly. Organisms have an antiviral immune pathway that deals with invading pathogens. In bats this pathway has evolved to be not as vigorous as in other mammals. At first thought, the bats’ immune system set up sounds counter-intuitive. However, it allows the bats to have just enough defenses against the diseases, but not enough defenses to send their immune system into overdrive. In humans and other mammals, a large immune system defense can cause severe illnesses in response to viruses such as the symptoms of COVID-19 and even autoimmune diseases. Bats don’t have the energy or time for this.
Flying is taxing on a bat, requiring lots of energy. When they fly, bats increase their body temperature to around 104℉. At high temperatures such as inside a flying bat, viruses aren’t able to replicate as easily. With their modified immune systems and high body temperature during flight, bats are able to carry zoonotic diseases while not being affected.
A human fever is part of the immune system. It is the body trying to inhibit the virus’s ability to replicate while destroying some of its proteins. Because of this, a virus that came from a bat may be unaffected by the human immune system.
Although bats are ideal hosts for zoonotic diseases with their large group living and body physiology, the question of why bats could be further pondered. Why some bats but not others? Say for example, why are the bats in China carrying COVID-19 but not bats in West Virginia? Home to 14 bat species, WV has more than enough caves and forested hills for the bats to reside. So why not our bats? Maybe we’re just lucky, but more likely, it’s due to other influences than just the bats that led to the outbreak in Wuhan.
In West Virginia, there are a multitude of reasons why our bats haven’t spread COVID-19. Because bats don’t necessarily harbor the same diseases, the ones here might not carry COVID-19. The conditions simply might not be in line for a spillover event to occur. Humans or other animals would have to be in close contact with infected bats that have the virus. We also don’t have markets with lots of different animal species close to one another. For those reasons, we can consider ourselves lucky.
While bats do carry and spread infectious diseases, it isn’t their fault for causing massive human disease outbreaks. The diseases jump from bats to humans (or bats to other animals and then humans) when humans encroach on bats’ habitats. Habitat invasion such as building animal farms in woods with bats or razing bat populated trees to build a suburb increases the probability that humans or animals will come into contact with a bat or its feces that are carrying diseases. Whether an unfortunate pangolin happened to be walking near an infected bat colony at the wrong time or the mixture of animals at the wet-market in Wuhan gave the virus its chance to spread, the human closeness to bats or other infected animals is to blame.
Fears of bats and the diseases they carry should not overshadow the bats’ importance to us. Bats are pollinators. As they fly from tree to tree and drink nectar from flowers, they distribute pollen that has been picked up on their fur to other plants. Pollination by bats is called chiropterophily. Fruit bats help disperse seeds from the fruit they eat by expelling the seeds in their guano which acts as a fertilizer. Bats are also pest controllers. Eating thousands of insects a night, bats keep insect populations in check. This helps decrease agricultural pests and diseases spread by insects. Bats even act as indicators of biodiversity since their presence indicates an abundance of insects and habitats such as woodlands, wetlands or caves that they prefer to roost in. Their essential role in an ecosystem outweighs their unfortunately high hospitality for hosting infectious diseases (unfortunate for the humans- fortunate for the pathogens).
While knowing the exact origins of COVID-19 doesn’t necessarily help with treating infected people or finding a vaccine, it does draw attention to the interconnectedness of humans and animals within the ecosystem of the world. It’s also a reminder of our Darwinian roots. Sharing common ancestors, animals and humans are linked together in sickness and in health. Knowing that animals play such a big role in transmitting diseases to humans highlights the importance of giving the animals the respect they are entitled to whether it be applauding their intriguing immune systems or simply giving them the space they deserve. After all, we humans may be better off for doing just that.
Katherine McFerrin is a sophomore at Carleton College in Northfield, Minnesota, where she enjoys cross country skiing and running across snow covered trails. She studies biology with an interest in molecular biology, zoonotic diseases, and learning words such as chiropterophily. She is currently socially distancing with her family in Morgantown, WV, as she completes this trimester online.