By Hannah Tyler
Covid-19, the virus that has shown complete disregard for our lives, has been traced back to one place: the Huanan Seafood Wholesale Market in Wuhan. Twenty seven initial Covid-19 patients can be traced there in early December 2019. But how did the virus get there?
Viruses aren’t the only microorganisms that cause disease—there are several other pathogens capable of doing so. Yersinia pestis is a type of bacteria that caused (and occasionally still causes) the Black Plague. Ringworm can be caused by around 40 different types of fungi. Chagas disease is caused by trypanosoma cruzi—a protist. (Protists are the least well known of the groups and aren’t particularly well loved by scientists either—they don’t have much in common but got thrown together in a group because of what they aren’t: plants, animals or fungi.) You can catch helminths, or parasitic worms including hookworm, by walking in soil contaminated with cat or dog feces. Mad cow disease is caused by a prion—a type of protein which has morphed into something harmful.
Zoonotic diseases make up over 60% of human infectious diseases. They originate from animals and ‘jump’ to humans. These pathogens can spread in a variety of ways, from direct contact (like bodily fluids or scratches) to consuming infected food or water, and even coming into contact with surfaces or soil that have been contaminated. Many diseases that still affect us today have their origins in animal diseases—smallpox has been found in humans for over 2,000 years. While the CDC currently advises that smallpox can’t be spread between humans and animals, genetic tests have found that the virus most likely originated in rodents.
The problem is that we humans tend to think of ourselves as clean and of animals as creating disease.
Recently a tiger, Nadia, caught coronavirus at the Bronx Zoo. It was an alarming headline, but zoo animals are routinely vaccinated to prevent diseases jumping from wild populations. Most zoo exhibits, especially of large animals, can’t be completely sealed off from rodents, insects, or even zookeepers themselves. Disease is not just a one-way street of wild animals giving us new pathogens; great apes such as gorillas can catch the measles from humans. We vaccinate and treat our pets not only because we can catch things from them, but sometimes they need to be protected from us. The problem is that we humans tend to think of ourselves as clean and of animals as creating disease, instead of seeing ourselves as a part of an ecosystem that we contribute to with our actions—and suffer the consequences of.
Despite all the internet jokes about people eating bats, the process of a pathogen morphing into something that transmits between humans is usually quite complex. Often the pathogen exists in a reservoir—such as an animal species who are usually unaffected by that virus, or the environment. Anthrax, a bacterium, naturally exists in the soil. Often, we catch a disease through a vector—usually another species of animal, though there are exceptions. Sin Nombre, a type of hantavirus, jumps straight from deer mice to humans through particles from urine or droppings that spread into the air. Yersinia pestis lives happily in rats but doesn’t jump to humans—until the fleas on the rats start to bite humans. Hendra jumps from bats to humans though horses. Ebola jumps from what believe to be bats to humans through other forest animals in West Africa. SARS jumps from bats to humans though the masked palm civet.
I’m sure you see the pattern.
As the reservoirs of a host of viruses, bats have acquired a rather bad reputation. The working theory for Covid-19 is that the virus evolved in bats, with the beautiful, beleaguered pangolin (the most trafficked wild mammal in the world) acting as a vector. This hasn’t yet been proven, but even discounting the novel coronavirus, bats account for a large number of zoonotic diseases. Why?
The UK’s Bat Conservation Trust answer the following FAQs on their website: Can I catch Ebola from UK bats? Can I catch coronavirus from UK bats? The answer is, of course, no and no. The Ebola reservoir is present somewhere in a group of (most likely) West African bats. The disease only lives within this group, with no impact to humans—unless, say, someone touches the carcass of a dead chimp who’s eaten a plant covered in bat urine, and then touches their face. Or taken a wild animal to eat as bushmeat. The virus must then jump to other humans to cause a spillover, or an outbreak. Ebola first became known to humans in 1976, and outbreaks mostly affected a few hundred people at most, until the 2014 outbreak when over 11,000 people died. There are so many factors that contribute to whether and outbreak fizzles out or becomes a global pandemic.
When the media talks about bushmeat and wet markets the meaning is clear -—‘we’ wouldn’t eat this ‘unclean’ food. In America and the UK, eating wild animals is simply known as game. The idea that the animals categorised as bushmeat are ‘exotic’ comes from people not having those animals in their own backyards—things are only exotic if they aren’t your normal. The West’s idea of what is normal to eat is a culturally constructed idea, as much as anyone else’s. Rodents are almost as notorious for carrying disease as bats, but it is not uncommon to see rabbits in your local butchers window.
Besides, in many cases within China wild animals are farmed, not caught, and supermarkets and wet markets often buy food from the same wholesale suppliers. The markets are generally open markets that sell food, an alternative to supermarkets. Many older people in China grew up without refrigerators and prefer meat from a market where they can assess its freshness easily. The term wet market comes from the fact that the ground was hosed down at intervals throughout the day. They are a social thing as much as an economic one. Wildlife trafficking is an issue, and a small amount of animals for sale at wet markets are those that are poached from the wild. It is one that is important to tackle for the preservation of a great many species, but a greater issue is our constant encroachment into animal territories.
Bats populate every continent except Antarctica. There are 962 species of bat, making them the second largest group of mammals behind rodents. While it would be tempting to say that their prevalence and the frequency with which they come into contact with humans (or with other animals that come in to contact with humans) is the reason for the outbreaks, this doesn’t account for their biology.
Bats have been found to carry over 130 types of viruses. 60 of these pose a threat to humans. Since the outbreak of SARS-Cov in 2002-2003 there has been a number of studies of bats as carriers of viruses, and in particular coronaviruses—though we still don’t know much about it, and the area has historically been underfunded.
Bats are the only mammals that fly. Their order, Chiroptera, means hand-wing because their wings evolved from hands. It’s theorised that the fact that they fly is what makes them such an effective reservoir for disease. The constant activity required for flight increases their body temperature and metabolic rate and puts them into almost a constant state of fever.
They have higher levels of interferons, which are produced by their (and your!) body to fight a virus. And you may recognise the term cytokine from media coverage. A cytokine storm is what many with Covid-19 die from. Cytokines signal sites of infection to immune cells. A cytokine storm is when the body overestimates its response, and essentially attacks itself. Bats have lower levels of inflammatory cytokines.
Bats in the UK are classed as an indicator species, if you see bats around, it probably means the environment in the vicinity will support a range of wildlife. Their absence also points to problems – If insect population numbers go down, bat numbers go down. They are a brilliant natural control for agriculture – the insectivores keep insects that are pests down. Generally smaller bats that are insectivorous are the ones that use echolocation. Fruit bats are the larger species that are actually, not as blind as a bat, and eat fruit. Some eat nectar from flowers and even pollinate flowers that come out at night. Bats fertilise plants such as bananas, mango, durian and agave. You can thank them for your margaritas.
Many wild species of animals play a role in our food systems, and those that we we use for agriculture (chickens with bird flu, pigs and swine flu) can pass us terrible diseases. But we mitigate that risk as part of a life that needs animals to function – we vaccinate livestock against anthrax, for example. So why don’t we mitigate wild animal diseases in the same way? Bats are an essential part of our lives and ecosystems as well as beautiful animals. We just need to work around them.
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