They are the training ground for viruses — a partner in crime.
A study published in Nature in 2017 agrees that “bats harbor a significantly higher proportion of zoonotic viruses than all other mammalian orders.” Bats are reservoirs for many lethal viruses that have affected humans —such as Ebola, SARS, Marburg, Nipah, rabies, MERS, and the latest SARS-Cov-2.
Who knows what else in the future.
Wildlife trade greatly facilitates this zoonotic transmission of viruses from bats to humans. An estimate of 72% of all zoonotic diseases originates from wildlife. No wonder China recently banned all wildlife trade in the market — shattering a multi-billion industry — in response to the rapidly spreading SARS-Cov-2.
Not to mention the fact that bats are the only mammal capable of prolonged flight — enabling them to easily shed viruses to other mammals in many parts of the world.
But coming back to the main point of this article: What makes bats the ideal “incubator” or “breeding ground” for viruses?
2012 Insights: DNA Repair and Interferons
A 2012 study printed in the world’s top academic journal, Science, performed comparative genomic analyses of bats and other mammalian species. Although bats have similar genomic content to other mammals, they found that it’s their level of gene expression that makes them unique.
Bats have a higher expression of genes that code for proteins involved in DNA repair. And, indeed, viruses are known to damage DNA as part of their virulence mechanism. This also explains why bats don't get cancer — whose main culprit is unrepaired DNA mutation.
In other words, bats don’t get sick because they can quickly repair the DNA damage done by viruses.
The 2012 comparative genomics also revealed that bats have a higher expression of antiviral genes that sustain indefinitely. It means that they have a robust antiviral defense in place that lasts almost forever, unlike other mammals whose antiviral system needs rest. This antiviral component is the interferons, which are immune cells that “interfere” with virus replication.
2014 Insights: Flight and Fever
One purpose of fever in mammals is to increase the metabolic rate of immune reactions to control infections. During flights, bats’ metabolism increases by 15–16-fold, with the heart beating >1000 times per minute and core body temperature rises to ~40°C. “For most land mammals, these are signals that would trigger death,” comments Linfa Wang from Duke-NUS Medical School in Singapore who studies bats-associated viruses.
“Higher metabolic rates during flight in bats may enhance, facilitate, or perhaps subsidize any inherent cost of raising metabolism to activate an immune response,” the 2014 authors wrote.
Put it simply, flying that naturally increases metabolism might give the bats’ antiviral immunity a free boost by coincidence or by convenience.
Concerningly, this also implies that such viruses can survive under such harsh conditions of a metabolically active immune system driven by fever. Imagine what these viruses can do to a host with a more feeble immune system.
“We don’t have that sort of immune system,” comments Angela Luis, a disease ecologist at the University of Montana and author of the 2014 fever-flight study.
2020 Insights: Interferons
A 2020 computational modeling study utilizing modern informatics and molecular techniques also found that bats’ interferons are unusually high.
This enables viruses to multiply without inflicting any significant damage to the bats owing to its robust antiviral defenses.
“That causes cells… to go into an anti-viral state, restricting viral entry into those neighboring cells and halting the progression of the infection,” explains Cara Brook, one of the study’s lead authors.
Viruses can, therefore, multiply as much as they want inside the bats that wouldn’t die no matter how much the viruses replicate — as the computer simulation showed. This is because interferons only “interfere” with virus replication; they don’t eradicate viruses.
Obviously, this is the golden opportunity for viruses to assemble their army before a war with other mammals. And when the virus leak to other mammals without an equally potent antiviral immunity, all hell breaks loose.
As Brook and colleagues wrote: “Such rapidly-reproducing viruses would likely generate extreme virulence upon spillover to hosts lacking similar immune capacities to bats.”
Putting it Together
“The bottom line is that bats are potentially special when it comes to hosting viruses,” says Mike Boots, a disease ecologist from UC Berkeley. “It is not random that a lot of these viruses are coming from bats.”
In my previous article, I touched on the concept of the “evolutionary arms race” between viruses and their host where both compete for a ‘life or death game of chess’ — as Dr. Rich Berry from Monash Biomedicine Discovery Institute puts it. Each player — viruses and the host’s immune system — deploy strategies to checkmate the king.
Basically, viruses evolve new mechanisms to overcome the host’s defenses. And the host’s immune system, in turn, evolves ways to fight the viruses. “This continued in a back-and-forth, ever-escalating way,” says Dr. Peter Kerr who studies co-evolution of rabbits and viruses.
So bats have a spectacular antiviral immunity, which trains viruses to evolve and withstand challenging conditions. Bats are indeed the perfect training ground and partner in crime for viruses.
This article was originally published in Microbial Instincts.