6 March 2021 | New Scientist | 43How diseases
jump to humans
The pathogens other animals host
include bacteria, viruses, fungi and
other assorted parasites. They can
spread to humans by direct contact,
for example by eating the animal.
Often, though, another host plays
the role of an intermediary. Genetic
analysis has suggested – although
it remains far from certain – that
SARS-CoV-2, the virus behind the
covid-19 pandemic, may have
spread to humans from an original
bat host via anteater-like animals
called pangolins.
These intermediate hosts can
provide an opportunity for a potential
pathogen to acquire the genetic traits
it needs to infect people. The closest
known relative to SARS-CoV-2, a
coronavirus designated RaTG13
that infects the Rhinolophus affinis
horseshoe bat, doesn’t appear to
have a spike protein suited to bind
to human receptors.
If a virus does spread to a
human, the danger comes when
it successfully replicates in them.
Fortunately, most animal viruses
can’t do this. To have the potential
to unleash an epidemic, a pathogen
must acquire the ability to transmit
from human to human, which is
what SARS-CoV-2 did.
Finally, once it has the ability to
replicate in humans, a pathogen
can evolve, as SARS-CoV-2 has
with its new variants – or as HIV
(human immunodeficiency virus)
did, eventually becoming, as its
name suggests, a human-only virus.and cloven-hoofed animals such as sheep,
goats, cows and camels. Camels were the
source of infection for the coronavirus behind
Middle East respiratory syndrome, or MERS,
which has seen sporadic outbreaks since its
first appearance in 2012.
Why it is these five groups is still being
explored. With primates, it is probably because
they are close relatives to us and the same
pathogens can more easily infect us. For
carnivores and hoofed animals, it may be due
to our proximity to them. With rats and bats, it
could partly be because of their large number.
Blaming any one animal or group of
animals for a zoonosis can be difficult. Take
bats, often mentioned as the ultimate source
of SARS-CoV-2. “You’ve been told a story about
bats being the most dangerous group,” says
Keesing. Although another virus in horseshoe
bats is 96 per cent identical to SARS-CoV-2’s
genome, the missing 4 per cent is a reminder
that the “vast majority of pathogens come
from lots of hosts”. More often than not,
a pathogen has passed through, and mutated
in, more than one animal before it arrives innumbers of them too.
Redding hasn’t yet been able to quantify
what that means for the risk of people
becoming infected. Kimberly Fornace at the
London School of Hygiene & Tropical Medicine
is involved in one effort to find out, using
GPS collars on macaques and GPS tracking
on human volunteers to map the emergence
of a zoonotic malaria in Malaysian Borneo.
“We’ve found exposure to infected
mosquitoes is often higher at forest edges,
where mosquito, macaque and human
habitats overlap,” she says.
But we should be wary of a simplistic
idea that biodiverse habitats are a hotbed
of pathogens all waiting to jump to us.
“The problem is that story doesn’t wash, it’s
wrong,” says Felicia Keesing at Bard College
in Annandale, New York. “It assumes all
the elements of biodiversity are equally
dangerous to us. They are not.”
Five groups of animals are the most likely to
carry zoonotic diseases that spill over into
humans, says Keesing. They are rats, bats,
primates, carnivores such as cats and dogs, >“ More often than not, a pathogen has
passed through more than one animal”
Left: the Chinese
pangolin (Manis
pentadactyla)
may have helped
transfer the
SARS-CoV-2
virus to humans.
Right: a Middle
East respiratory
syndrome (MERS)
clinic in Malaysia
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