2020-11-14NewScientistAustralianEdition

12 | New Scientist | 14 November 2020

prone to jump from one bit of an

RNA strand to another, so if a cell is

simultaneously infected with two

different coronaviruses they can

end up being mashed together.

Bats commonly carry multiple

coronaviruses at the same time

and recombination is plausibly

how SARS-CoV-2 originated in the

first place. It is also possible for

coronaviruses to hybridise with

other, unrelated, viruses.

Reverse spillover requires a

fairly implausible chain of events,

but there is precedent for all of

them. For one thing, there are

documented cases of viruses

naturally spilling over from

humans into bats. In 2013, a group

of researchers including Olival

analysed the overall viral load –

or “virodiversity” – of a species of

fruit bat called the Indian flying

fox. They estimated 58 viruses in

total, two – or possibly three –

of which had a human origin.

One of them was a coronavirus.

This would make getting on top

of the virus that much harder, as

infected bats could reignite under-

control epidemics in the same way

that infected airline passengers

arriving from abroad have done

in New Zealand and elsewhere.

Bats could even become a

crucible for a new deadly virus.

In one possible scenario, SARS-

CoV-2 mutates into SARS-CoV-

as it passes from bat to bat in vast

roosts where many different

species are known to mingle and

swap viruses. In another scenario,

a process called recombination

creates a hybrid of SARS-CoV-2 and

another virus. Recombination

happens all the time in

coronaviruses because their

genome-replicating enzymes are

“Bats could become
a reservoir source of
coronavirus infection
for people”

News Coronavirus

What’s more, laboratory

experiments show that another

coronavirus that circulates in

human populations, the common

cold-causing HCoV-NL63, which

probably also originated in a

bat, is still capable of infecting

lung cells cultured from the

tricolored bat, one of those

North American species.

Calculated risk

Spillback is theoretically possible

(see “How humans and bats

transmit to each other”, left). But is

it likely? “Determining whether or

not it is likely needs to take into

account three main factors,” says

Peel. “One, the covid-19 prevalence

in the human population. Two, the

contact rates between bats and

infected people. Three, whether

novel bats are susceptible to

infection and able to transmit the

infection to other bats.” The latter

factor has the most uncertainty,

she says, but preliminary work

suggests this is possible. “So we

must focus our attention on the

first two factors to avoid exposure

in the first place,” she says.

With all of this in mind, in

April, the US Geological Survey

and the US Fish and Wildlife

Service convened an expert panel,

including Peel, to try to put some

numbers on the risk in the US.

Though working with very

limited data, the panel concluded

that the risk is “non negligible”:

for example, if a research scientist

who was infected and shedding

virus handled a little brown bat

under current fieldwork protocols,

it found there is a roughly 15 per

cent chance of the bat getting

infected. For each such reverse-

spillover event, there is about a

33 per cent chance that the virus

would become established in

wild bat populations.

Using personal protective

Bats and humans

come into close

contact frequently

enough for viruses to

pass between them.

According to

the US Centers for

Disease Control and

Prevention, between

1997 and 2006,

17 people in the US

caught rabies from

bats. Most of these

people reported

handling a bat after

finding it in or around

their house, and two

were bitten. But in

three cases, they had

no recollection of

a bat encounter.

Rabies cannot be

passed from person

to person directly,

but with a highly

transmissible virus

like SARS-CoV-2, it

could take just one

encounter to start

a fresh outbreak.

Humans come into

contact with bats in

many other ways,

including research,

pest control, wildlife

rehabilitation,

wildlife tourism,

recreational

caving and when

investigating disease

outbreaks. Another

contact is during

guano harvesting,

which is an

important source

of income in many

parts of the world.

To pass viruses

from humans to bats,

actual contact may

not be necessary.

Since the virus is

shed in human

faeces and may

remain infective for

days in water, bats

and other wildlife

could pick it up by

drinking sewage-

tainted water.

How humans and bats transmit to each other

equipment such as masks, gloves

and gowns would significantly

reduce the risk, the panel

concluded, though it noted that

PPE for healthcare workers is in

short supply in some places.

In April, the International Union

for Conservation of Nature (IUCN)

recommended a suspension of all

fieldwork involving interactions

with bats. It has since rowed back

from a total moratorium, but does

recommend stringent biosafety

measures, such as wearing PPE,

only handling bats when