Coronavirus and Bats

If you were in Wuhan, China and wanted some nice fresh seafood, the place to go was the Huanan Seafood Market. Indeed, freshness is important to most Chinese and even small stores have aquariums where people can pick out dinner while it’s still swimming.

A thorough walk-through of the Huanan Seafood Market, however, makes it obvious that there’s a lot more than fish for sale. If you know where to look, you can find a nice fat beaver or porcupine; snakes are available as well as a plethora of other creatures. Cuteness is not a defense: wolf pups and koalas are also on the menu. Pick one out, see it slaughtered. That’s why local slang for these establishments is “live” or “warm” markets. They’re also known as “wet” markets, because of the large volumes of water needed to wash out floors littered with butchered animal products.

In addition to the exotic animals listed, there are also…bats. Bats are natural reservoirs for the coronavirus that are used in traditional Chinese medicine.  It’s believed that a bat or some “middle-man” animal from the Huanan market started the entire pandemic.  The SARS outbreak in 2003 was traced to civets (an animal with a cat’s body and a ferret’s face) sold in a live animal market where horseshoe bats were also available.

(Note: This article concentrates on an animal origin for the COVID-19 epidemic, but the theory exists that a lab accident from the Biosafety Level 4 lab in Wuhan, China is responsible for the epidemic. While I have no knowledge that such an event happened, there is no evidence disproving the possibility.)

Some studies suggest that the intermediate host was a snake: The Journal of Medical Virology suggests that snakes were susceptible to harbor 2019-nCoV. For an animal without legs, a snake makes an unlikely “jumping-off” point; another animal, the pangolin (looks like an anteater with scaly armor), has been suggested as a middle-man. To date, only educated guesses exist.

Bats, however, have been proven to harbor coronaviruses in past studies. Why do some many deadly viruses, like SARS, MERS, Ebola, Marburg and perhaps COVID-19 seem to originate in them?

A new study out of UC-Berkeley reports that bats have an impressive immune response to viruses. This protects the animals but could drive viruses to reproduce more rapidly. If the viruses mutate in a way that allows them to jump to other mammals with an average immune system, such as humans, they run amok.

Why do bats have a special propensity to mount defenses against viruses? Viral infection activates a mechanism that prevents the virus from penetrating cell walls. This makes bats a unique reservoir loaded with rapidly reproducing and highly transmissible viruses. While the bats themselves remain healthy, they act as a repository for rapidly-multiplying viruses. If a virus mutates and moves into another animal, the new virus can quickly overwhelm their new host.

 Perhaps it’s the unique ability of the bat to fly.  High metabolic rates appear to be required for this activity to occur in mammals. The effect of such a metabolic demand normally would be higher tissue damage due to an accumulation of molecules called free radicals. Although bats require a high metabolism to fly, they have somehow managed to clean up these destructive molecules that cause inflammation. This might explain why bats live so long compared to other small mammals. A mouse or rat the size of a bat might live two years or so; many bats can live to the ripe old age of 40.

An antiviral immune response in humans involves inflammation, but bats seem to dampen this effect. This may be due to a substance produced by bats that warns other cells to activate defenses before a virus invades. One study showed that when a bat cell line was exposed to a virus, it successfully walled itself off from infection. When exposed to the same virus, a monkey cell line was quickly overwhelmed.

Bats may have evolved to protect themselves against viruses, which is good for them. If those viruses mutate to infect other species, however, it’s not so good for us. It may never be proven that a particular live market meat purchase started the COVID-19 epidemic, but it’s true that coronavirus can be found in many animals and is prone to mutate. It hasn’t been proven to cause coronavirus to mutate into a strain that infects humans, but it hasn’t been disproven either.

Joe Alton MD

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