Why don’t bats get sick? Because their immune response doesn’t cause much inflammation, because they have proteins that stop the viruses leaving their cells, and because the viruses have evolved to live with them.
Bats do get sick and some viruses are lethal to them. They also carry a lot of viruses that can jump to other animals. It is thought that Covid 19 jumped from bats to humans somewhere in China. There are also other viruses that make it to humans by going from bats via other animals. A Hendra virus made its way to humans by going via horses. Researchers have found over 130 different kinds of viruses living inside bats. Bats live in large colonies on the roofs of caves or in other dark places, and it is very easy for viruses to jump between them. Then the vats pass these viruses on in bodily fluids.
So, if bats are hosts to so many viruses, why don’t they get sick? Well, they do. Rabies, for example, can kill bats. A rabies virus can go through a colony of bats and wipe out over half of them. However, the majority of viruses that are dangerous to us have no effect on bats. Why is that? There appear to be three reasons and the first reason is that bats are the only mammals that can fly. There are a few types of squirrels that can glide from tree to tree, and this could be the first step on the evolutionary ladder to flying, but bats are the only mammals that have sustained and controllable flight. They don’t fly in the same way as birds because their wing is a membrane that stretches over the five long fingers they have on their hand, while birds have a long arm and one finger. This wing formation gives them a lot more mobility than birds, but they have evolved to fly and can’t really walk. When they fly, they use a lot of muscle power because they have to flap about ten times a second to keep airborne. This causes their heart rate to go up to 1,000 beats per minute and their body temperature rises to 41℃. This high heart rate and high body temperature protect them against viruses. When we have a viral infection, our body raises our internal temperature, triggering a fever, to make our bodies inhospitable to the virus. This happens every time the bat flies. They have also evolved to keep their immune system from reacting. Because they fly so fast, they have a very high metabolism and that causes damage to their cells. When our cells are damaged, the body initiates an immune response and triggers inflammation in the cells. This is one of the things that make us sick. Bats cannot afford to have inflammation every time they fly, so they have evolved not to. Their immune response doesn’t trigger inflammation as readily as ours does, which means viruses don’t cause a harmful immune response, which means they don’t get sick.
The second reason is that their cells produce proteins that stop the viruses from getting away. This protein is called tetherin and we produce it as well. It can stop viruses from entering a cell, it can stop viruses from replicating themselves, and it can stop the virus copy from leaving the cells. Our bodies produce two different types of tetherin proteins, which are able to stop a few different types of virus. Bats produce fifteen different types of tetherin proteins, which means they can stop and control a much larger number of viruses.
The third reason, which may be somewhat conjecture, is that viruses have evolved to be less harmful to bats because they can use bats as a host and as a way to move to other species. The single goal of a virus is to reproduce its DNA. That is it. If a virus cannot reproduce, it will die out and disappear. If a virus can reproduce, it will spread and spread. That is evolution. It is counterproductive for a virus to kill its host because it becomes more difficult for that virus to get to a new host. This is true with bats as well. It makes far more sense for a virus to evolve to be able to survive in a bat but not make the bat sick, so that the bat can fly around and spread the virus to other species. And this is what I learned today.
Photo by Erick Arce: https://www.pexels.com/photo/close-up-of-bat-feeding-on-flower-15915605/
Sources
https://www.bats.org.uk/about-bats/bats-and-disease/bats-and-viruses
https://howthingsfly.si.edu/ask-an-explainer/how-do-bats-fly-differently-birds