The recent pandemic has made us pay more attention to zoonotic viruses, those whose natural reservoir is an animal species other than humans. Specifically, bats are famous reservoir animals for many types of viruses in addition to the well-known coronavirus. However, these animals, which act as hosts for certain viruses or bacteria, do not get sick from their presence. How is it possible to be a carrier of a virus, but not get sick from it?
What’s so special about bats?
Today, on Bat Appreciation Day, we want to tell you some interesting things about these interesting mammals . It is noteworthy that bats are the only mammals capable of flight, they are found in a great diversity of climates around the world, they have different types of feeding and are famous for their mechanism of echolocation and magnetoreception (the abilities to use sound to locate oneself in space and differentiate the south pole from the north pole). Despite the many features that make them special, we are going to focus on your immune system.
Many scientists have tried to understand the keys to the close relationship between viruses and the animals they use as reservoirs, looking for clues in the communication mechanisms. If they manage to understand these keys, they could be applied in humans and thus fight infections or acquire tolerance. A recent analysis collects the most recent evidence on the immune system of bats and gives clues about the possible responsible mechanisms .
The key: a delicate balance
This article, published in 2021 in the prestigious journal Nature , collects the results of genetic tests and infection models in laboratories with bats. The studies focused on monitoring different markers and sensors of the immune response caused by the infection. The conclusion they have reached is that the bat presents an almost perfect balance between the immune response and tolerance against the pathogen , in this case, the virus.
When our body detects the presence of the coronavirus, our immune system recognizes it as a threat and begins to send out danger signals. These signals trigger a strong immune response to try to get rid of the virus, releasing cytokines and recruiting many types of cells to the site of infection to destroy the intruder. An entire orchestra of responses with the purpose of containing and eliminating the threat quickly and efficiently . And it is precisely this set of processes that causes many of the symptoms that we have observed in SARS-CoV-2 infection: malaise, fever, cellular infiltration that causes pneumonia, cytokine storm, etc. Our immune response is triggered by the presence of the virus and if it is not curbed, it can result in very serious symptoms.
However, the bat presents a much more measured response to the presence of the virus in its body . This does not mean that your immune system is deficient or that the virus is capable of hiding from it, but rather that the response triggered does not put the entire body on alert. A series of self-regulated signals follow one another to ensure that the response does not overflow, but without allowing the virus to spread excessively within your body.
And why might it be interesting to study the immune response of bats to the viruses they carry? Because the immune system of bats and humans share many of the cells, mechanisms and elements involved. This is why it is so helpful to understand how these signals can be combined to achieve this delicate balance. Perhaps these peculiar animals contain some key that can help us develop a therapy against autoimmune diseases, where the body attacks its own cells in a runaway manner . For these diseases, a more specific immunosuppressive treatment could be very useful.
Studying the presence of bacteria that use the bat as a reservoir can also help control possible outbreaks of infections . If a bacterial strain is detected in increasingly higher values in bat populations, it can be analyzed and action taken if it poses a risk to humans. On the other hand, some bacteria present problems when trying to investigate their characteristics in laboratories, because they do not behave as they do in nature. In this way, bats can be a good animal model for the study of viral or bacterial infections since they present a natural but contained interaction, since it would not lead to a disease for the animals.
Immunologists have studied the human immune system for years, but other animals may have developed different evolutionary strategies that help protect them from viruses and other pathogens . Who knows which animal will hold the ultimate clue to a future virus treatment?
Irving et al. 2021. Lessons from the host defences of bats, a unique viral reservoir. Nature. doi: 10.1038/s41586-020-03128-0.Veikkolainen et al. 2014. Bats as Reservoir Hosts of Human Bacterial Pathogen, Bartonella mayotimonensis. Emerg Infect Dis. doi: 10.3201/eid2006.130956