10 | New Scientist | 14 November 2020
ALL 17 million farmed mink in
Denmark were put at risk of being
slaughtered last week after the
discovery that mutant forms of
the coronavirus are spreading
among the animals. The virus has
already spread back to humans.
Some reports suggest that at least
one of the mutations makes the
virus more dangerous, although
the idea is highly disputed, and
claims that it could hamper the
development of a vaccine don’t
yet stand up to scrutiny.
The call for culling has now
been dropped, but the human-
mink-human transmission chain
demonstrates the real and present
danger of what virologists call
“reverse spillover” and “spillback”.
The pandemic began with
spillover of SARS-CoV-2, the virus
that causes covid-19, from wildlife
to humans. Reverse spillover
means transmission from
humans back into animals, both
wild and domesticated. Spillback
completes the circle, with the virus
jumping back into humans again.Threat to wildlife
This cyclical transmission
represents a threat to both wildlife
and humans. “The risk of SARS-
CoV-2 infecting novel wild species
is concerning enough to warrant
preventative measures,” says
Alison Peel of the Griffith Wildlife
Disease Ecology Group at Griffith
University in Queensland,
Australia. Anyone who is at risk
of coming into close contact
with wild animals should take
precautions, she and others say:
wear a mask, wash your hands
and keep your distance.
SARS-CoV-2 is known to be
a highly promiscuous virus.
Research into its origins strongly
suggests that it started life in
horseshoe bats and may have
passed through an intermediateNews Coronavirus
Transmission in animalsMink could be just the start
The fact that the coronavirus can spread back and forth between humans
and animals is bad news for all involved, reports Graham LawtonMink at a farm in
Denmark, where there
have been calls for the
animals to be culledhost, possibly Malayan pangolins,
en route to humans.
The virus’s closest human-
infecting relatives are similarly
unfussy about who they get cosy
with. The original SARS virus,
SARS-CoV, which caused an
epidemic of severe respiratory
disease in 2002 and 2003, is also
thought to have evolved in
horseshoe bats and probably
jumped into another species,
either palm civets or racoondogs, before infecting humans.
The coronavirus behind MERS
also originally probably came
from bats but jumped into
humans from dromedaries.
This jumping between species
makes it impossible to designate
a given coronavirus as, say, a “bat
virus” or “pangolin virus”, says
Samuel Díaz-Muñoz at the
University of California, Davis.
“I have an issue with defining
viruses by their host. As long as
they can move in and do their
thing, viruses don’t care.”
This jumping around doesn’t
stop once the virus has reached
humans. Onward transmission
of SARS-CoV-2 into non-human
animals has already been
documented not just with mink,
but also pet cats and dogs, and
tigers and lions at Bronx Zoo in
New York. Cat-to-cat transmission
has also been confirmed and
while there is no evidence yet of
cat-to-human transmission, it
cannot be ruled out. Even before
the outbreak in mink farms in
Denmark, workers at a Dutch
mink farm were known to have
been infected by the animals.
These animals are in the firing
line due to their close proximity tohumans, but wild animals are
susceptible too. Lab experiments
have shown that the virus can
infect Egyptian fruit bats, as well
as various rodents, monkeys,
marmosets and tree shrews.
Cell culture experiments and
computer models of the molecular
interaction between the virus’s
spike protein, which it uses to gain
entry into cells, and potential host
cells have added dozens more
mammal species to the at-risk list,
at least in theory, as well as a few
birds, reptiles and even fish. Some
animals, however, appear to be
immune and attempts to infect
them have failed, including house
mice, pigs and domesticated birds
such as chickens and ducks.
Unsurprisingly given the central“As long as viruses can
move in and do their
thing, they don’t care
what the host is”