Why do so many deadly viruses come from bats?
From Rabies and Ebola to SARS and COVID-19, bats have caused some of the deadliest diseases in human history. Rabies carries a 100% death rate without treatment, Ebola killed over 10,000 people between 2014-16, and we are now in the midst of the largest pandemic in recent times due to COVID-19.
So the question is- what is it about bats that make them produce such devastating viruses?
To best answer this, it's helpful to know a bit more about bats and their biology.
Do bats transmit more viruses to humans than other animals?
Bats make up over 20% of all mammal species. In total, there are over 900 different bat species and they are found on every continent except Antarctica.1 The fact that bats exist in such large numbers may be an important clue as to why there are so many bat viruses that infect humans.
An analysis of viruses that have spread from animals to humans (known as zoonoses) suggests that the reason so many viruses come from bats is that there are lots of different bat species. There seems to be a clear relationship between the number of species in an animal group and the number of viruses that come from that group. In other words, we are equally likely to catch viruses from all mammal species, it just happens that there are a lot of bat species, so there are a lot of bat viruses.2
Whilst that may explain the high number of bat viruses, it doesn't explain their high death rates. Rodent species outnumber bat species 2:1, but they have produced far fewer deadly viruses. To find out what makes bat viruses uniquely destructive, let’s look at how flying and a long lifespan might influence how bats fight off infection.
Is flight the key to a longer life?
Bats have unusually long lives for animals of their size. For most mammals, it’s proportional. Larger mammals, like elephants, live longer whereas small animals, like mice, tend to die younger. It has been suggested that this is linked to metabolic rate which is higher in smaller animals.3 Generally speaking, the smaller the animal, the higher the metabolic rate- leading to faster tissue damage and a shorter lifespan.3
Flying animals- including bats and birds- buck the trend when it comes to lifespan. For example, the small brown bat weighs only 7g but lives for up to 40 years (roughly 10x longer than a mouse that is double its weight!)4.
So how do they manage this? Well, bats and birds have developed complex adaptations to help them survive during flight. Flight requires lots of energy and this releases lots of harmful molecules which can damage tissue and DNA . To counteract this, bats have higher levels of natural antioxidants compared to land animals like sheep, rats, and mice. These antioxidants help to ‘mop up’ the damaging molecules, possibly helping bats achieve a longer lifespan.5
Linking flight, lifespan and the immune system
So how does a longer lifespan relate to viruses? It is thought that those complex adaptations developed in flying animals may also influence their response to viruses.5 Viral infections also cause damage to tissue and DNA, so the same mechanisms that are used for damage limitation during flight may help bats control viruses.6
As well as this, viruses generally prefer cooler temperatures and struggle to survive in the heat. This is thought to be one of the reasons that animals (including humans) develop a fever during infections, as it helps to kill off the invading viruses. When bats fly, it causes their body temperature to increase to around 40 degrees celsius (similar to a fever).7 Bat viruses have therefore developed the ability to survive in feverish temperatures- which may explain why they are able to cause such extreme diseases in other animals.
Did COVID-19 come from bats?
The exact origin of SARS-CoV-2 (the virus responsible for COVID-19) remains uncertain, but evidence to date strongly suggests that it started in bats. The genome of COVID-19 has been found to be 96% identical to another bat coronavirus isolated from bats in China.8 However, the SARS-CoV-2 virus itself has not yet been found in a bat.
How did COVID-19 move from bats to humans?
How humans acquired COVID-19 is not yet clear. The virus may have been transmitted directly from bats to humans or it may have come through a different animal (known as an intermediate host). This was shown to be the case with the virus that caused the first SARS outbreak, where the virus was found in palm civets and raccoon dogs in a market in China.1
One animal that has been suggested as a potential intermediate host is the pangolin. This is because a similar coronavirus has been found in pangolins which is 92% similar to COVID-19.9 However, this degree of similarity is not enough to draw any conclusions at this stage. The WHO recently announced that it is launching an investigation into the origins of COVID-19 so more information will hopefully come to light soon.
Battling the bat-lash
For decades, people have been attempting to cull bats to reduce the spread of bat infections. However, this method is ineffective, and in many cases, counterproductive. For the last 30 years in Peru, vampire bats have been culled in an attempt to stop the spread of rabies. A study into these bat colonies revealed that culling led to an increase in the proportion of bats exposed to rabies.10
Culling bats also has a significant environmental impact as they are important pollinators, pest controllers, and seed dispersers. Over 500 plant species rely on bats for pollination, and as insectivores bats reduce the need for pesticides.
Going forward, researchers are looking into more effective and environmentally friendly methods of controlling bat viruses
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