What's the Deal with Bats?

What's the deal with bats?  They are reservoir hosts for some very deadly zoonotic viruses, including Ebola, Hendra, Marburg (causes hemorrhagic fever in Africa, case fatality rate of 24-88%), Nipah, (causes acute respiratory infection, very high fatality rate of 40-75% and one the WHO monitos closely) and SARS.  

Bats are unusual mammals.  They are the only mammals that fly, which means they have much higher metabolic demands than other mammals.  In most mammals, such a high metabolic demand causes stressed mitochondria to produce a lot of free radicals that can damage DNA, which in turn can lead to high levels of inflammation and lead to a shortened life span.  Bats seem to have evolved a workaround.  

Bats have a mutation that dampens their inflammation response to these free radicals, but also to viruses.  Many viruses are deadly to animals because they cause an uncontrollable release of interferons and other inflammation-inducing molecules that overwhelming the immune system.  Because bats have a mutation that dampens this interferon pathway, it stops their immune systems from going into overdrive.  This allows the bats to remain relatively healthy despite high viral loads.  

Bats also transmit viruses effectively within and between species because many species aggregate together in crowded roosts.

Bats Aren't Evil

Bats have gotten a bad reputation - you know vampires, Halloween, death, evil. But bats are rarely directly dangerous to humans, and actually provide really essential services to ecosystems.  They are involved in seed dispersal, helping disperse fruit trees following logging in the tropics.

They pollinate plants, like the Saguaros of the desert southwest.

They are major predators of insect pests like mosquitoes that themselves can transmit disease, and of agricultural pests too.   In Texas, bats found to consume 140 to 147 tons of insects, nearly 300,000 pounds of bugs every night during the growing season.  Just in the cotton fields of Texas, Mexican free-tailed bats save farmers an annual average of $724,000 in pest control costs and losses from insect-related damages. It is estimated that bats are peform $23 billion in pest suppression services in the US each year.

Finally, their guano, which is rich in nitrogen, is used as biological fertilizer.  You can buy bat poop for your garden on Amazon, seriously!!  (Maybe a new microgreens experiment...?)

Bats perform incredible services and rarely are a direct danger to humans.  Their danger as reservoirs, however, is of concern to human health, and as we'll see later in this module, our own behaviors may be amplifying the indirect risk posed to humans by their harboring of disease.

Optional Resources

Below are some optional resources for those of you that would like more information on bats' ability to harbor high viral loads.

• Discover Magazine: Why Bats Are Breeding Grounds for Deadly Diseases Like Ebola and SARS
• The Scientist: The bat version of the STING protein helps dampen the mammals' immune response to infection, researchers have found.
• Nature: Bat–man disease transmission: zoonotic pathogens from wildlife reservoirs to human populations