The Bat Edition

Since I originally wrote this in the early days of the pandemic, there have been a lot of good conversations about the origins of the virus. Whether it was a lab leak or specie spillover, everyone seems to agree that eventually, this thing traces back to bats. - Noah (NRB)

Noah here. As a general rule, there’s a relationship between the size of an animal and its lifespan. The larger it is, the longer it lives. A mouse has a life expectancy of a few years at most, while a blue whale can live to almost 100. (Bowhead whales are believed to be able to live to over 200.) There are a few exceptions to that rule, most notably humans, who live remarkably long—about 4.5 times longer than our mass would suggest—despite our diminutive size in the eyes of elephants and whales. In fact, there are only 19 species of mammal that live longer than humans when you take into account their size: the naked mole-rat (which is miraculous for a host of other reasons as well) and eighteen species of bats.

From Compton’s Encyclopedia, via Laughing Squid

Although we—or at least I—don’t think of bats much, they actually account for around 20% of mammal species and can be found just about everywhere except the earth’s very coldest climates. They have obviously been in the news as many scientists believe they are responsible for the current coronavirus. In a prescient paragraph, here’s Princeton epidemiologist Andrew Dobson in 2005 describing the role of bats in the original SARS outbreak:

Three species of horseshoe bats (Rhinolophus spp.) have now been officially recorded as the natural reservoir host of the coronavirus that causes severe acute respiratory syndrome (SARS). The emergence of this pathogen (SARS-CoV) in southern China in 2002-2005 almost brought the burgeoning economy of Southeast Asia to its knees. Bats are now known to be natural reservoir hosts to several and Hendra viruses and potentially Ebola to “older” and more well-known pathogens, such as rabies virus, which frequently resurge into human populations or domestic livestock. Fieldwork on SARS illustrates not only the crucial role that conservation organizations play in frontline research on emergent diseases, but also the shortcomings in our understanding of the etiology of these diseases.

Why is this interesting?

The age data clearly shows that bats are unique, but what makes them so prevalent as a host of diseases that are harmful to humans? Well, for one thing, as mentioned, they make up a large proportion of mammal species, so there’s just a lot of them to get us sick. There are two other things that make them unique, though. First, they are one of the only mammals other than humans to live in highly dense populations (“one cave in Texas houses twenty million bats,” according to Viral). Like densely populated humans, this means viruses can move around fast. 

But they’re also unique as the only flying mammal and it’s this ability that makes bats excellent vehicles for incubating human threats. Here’s Andy Dobson again, but this time from his recent Santa Fe Institute Transmission essay:

Bats have very different immune systems from other mammals, likely as a consequence of their ability to fly. [6,7] Humans and other non-volant mammals produce the B-cells of their immune system in their bone marrow. Because bats fly, they have hollow bones; the only place they have bone marrow is in their pelvises, so they produce B-cells at much lower rates. Similarly, active flight raises their body temperature to levels akin to fever in non-volant mammals, possibly constraining viral growth. Bats also do not store fat, as it compromises their aerodynamic ability. Instead, they can enter torpor to get through periods of limited food resources. These all act as constraints on viral pathogens that disappear when the pathogen finds itself in a novel host whose immune response may interact with that pathogen in ways that are detrimental to both the host and the pathogen. [8] 

At the risk of being repetitive, hollow bones allow them to fly, but also mean less marrow, which produces fewer antibodies. So why don’t they get sick? For one, as Dobson points out, they maintain a very high temperature as a result of that flying, which may keep the viruses contained. In a recent paper (source #8 in the Dobson quote above), Cara Brook et. al point out that bats also have a special “antiviral immune response called the interferon pathway perpetually switched on.” While this would be a problem for most mammals (we are seeing the dangers of over-active immune responses in COVID patients), bats “have adapted anti-inflammatory traits that protect them from such harm.” 

It’s that peculiar set of evolutionary circumstances that may be responsible for the heightened danger to humans of bat-borne disease. Here’s Brook and team again:

Additionally, we demonstrated that the antiviral state induced by the interferon pathway protects live cells from mortality in tissue culture, resulting in in vitro epidemics of extended duration that enhance the probability of establishing a long-term persistent infection. Our findings suggest that viruses evolved in bat reservoirs possessing enhanced IFN capabilities could achieve more rapid within-host transmission rates without causing pathology to their hosts. Such rapidly-reproducing viruses would likely generate extreme virulence upon spillover to hosts lacking similar immune capacities to bats.

In other words, when you put all this stuff together it creates an environment particularly conducive to a really nasty virus. No matter how modern and urbanized the world may get, we can’t escape nature. (NRB)

Update: I asked Andy Dobson on Twitter whether bats actually have hollow bones (per a comment on the original WITI) and here was his response: “Hi, bats are not hollow, they mainly have hollow bones, as do birds. Their pelvis is the only one with bone marrow, so immune cell production in bone marrow is reduced and has to be compensated for in another system.”

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