by J. Bruce Neill, Ph.D.
Some diseases caused by infectious agents can ‘jump’ from one species of animal to another, these are called zoonotic diseases, or zoonoses. Not surprisingly, most research on zoonoses has been on those that infect humans. Viruses attack just about all living things hijacking their cellular machinery to create copies of new viruses, and, we are learning, many viruses are zoonotic.
In the United States, we tend to think of zoonotic diseases as originating in other, less civilized parts of the world, but in fact, they are very common, both here and abroad. Research indicates that 60% of known human infectious diseases originate in non-human animals, and 75% of new or emerging human infectious diseases are zoonotic.
We are fairly certain that CoVID-19, the pandemic disease caused by SARS-CoV-2 is a zoonotic disease, that made its ‘jump’ in southeast Asia; most likely coming to humans from bats. Bats are known to carry many viruses that cause serious diseases in people, including Rabies, Ebola, Nipah, SARS, and now CoVID-19. And here’s the really odd thing – bats rarely get sick from those viruses.
There are many ways that bats could infect humans with a virus. We likely will never know the exact pathway for the CoVID-19 transmission. We seem to jump to the conclusion that someone ate an undercooked, infected bat, but the chances are that the transmission pathway was much more mundane, and similar to the human-human transmission we are trying to avoid. Regardless of the pathway, it’s pretty likely that as human population continues to expand into more direct contact with shrinking wildlife habitats, the incidence of zoonoses will increase.
In many regards, bats are interesting animals. They are mammals, with fur, bearing live young and nursing their offspring. They are the only flying mammals, and most feed on flying insects – although some are frugivorous (fruit eaters). There are about 1,200 described bat species (20% of all mammal species are bats); divided into two major categories, big bats and small bats. The smallest of the small bats is just over an inch-long with a 6-inch wingspan, the largest of the large bats has a wingspan of over 5.5 feet. In general, big bats tend to be the frugivores and little ones, the insectivores.
Two fascinating aspects about bats is that they are very long-lived, and they seem to tolerate viral infections. Recent research indicates that these two things may be related.
On average, mammalian longevity is related to body size; larger mammal species tend to live longer than smaller species. There are 19 species of mammals that live longer than humans (adjusting for body size differences) and 18 of these are bats. It is still unclear (as you might guess, funding sources for basic bat research used to be pretty limited – but I’m thinking that may change in the future) but their longevity may be related to their ability to tolerate viral infections so well.
In humans, an infectious agent is met by a complex and massive defensive counter-attack known as an immune response. A part of the immune response is inflammation, inflammation among other things, facilitates our having a fever in response to infectious agents.
Bats don’t commonly exhibit inflammation. There are two realms of explanation that could account for this lack of inflammatory immune response. The first is that something bats already do makes them not need inflammation, the second is that they have developed a way to deal with viruses that is independent of inflammation.
As the only flying mammals, it was initially thought that the very high energetic demands of flying may account for why bats rarely exhibit inflammation. The mitochondria (the powerhouse of the cell) work very hard to meet the vast energetic demands required for flying, so that essentially, flight is (acting as) fever, inhibiting the impact of viral infections.
However, recent research indicates that the answer is likely not so simple. A lack of inflammation appears to be rooted in the chemical and cellular pathway of the immune response of bats. They have adapted anti-inflammatory immune responses – their immune response tends to not include inflammation. This lack of inflammation appears to be the result of the loss of certain genes that normally initiate a typical inflammatory response. This is fascinating because when we think of change over time, adaptation, we typically think of the addition of new traits; this seems to be adaptation through the deletion of genes and traits.
And, in a complex way, this lack of inflammation seems to help confer increased longevity. Bats have minimal immune responses to viral infections, carry fairly heavy viral loads with seemingly little negative impact, and they seem to live longer because they don’t have the massive counter-attacks to invading viral particles.
That we can ponder the fascination of other animals we share this life with is nothing short of a miracle. I hope we can all find joy and peace in knowing how lucky we are. Enjoy the small things. Wash your hands thoroughly.
Dr. Neill is the Director of Education at Sanibel Sea School. Part of the SCCF (Sanibel-Captiva Conservation Foundation) Family, Sanibel Sea School’s mission is to improve the ocean’s future, one person at a time.