28 November 2020 | New Scientist | 47eventually finding their way into the human
population. As well as bats, SARS-CoV-2 has
been found to infect a wide ranage of animals
including pet cats and dogs, lions and tigers
at a New York zoo, farmed mink and wild
ferrets, hamsters and macaques – although
the risk of a person catching covid-19 from
these animals is believed to be low. So viruses
that can spread from animal to animal are a
priority for scientists looking for potential
human threats.
Take Hendra virus, which is usually found
in bats but can be passed to horses, and from
them into people, causing fever, coughs and
sometimes meningitis and coma. Dozens
of outbreaks and four deaths have been
reported in north-eastern Australia since- Hendra outbreaks are linked to periods
of heavy rainfall, says Raina Plowright, an
infectious disease ecologist at Montana
State University. In wet conditions, trees such
as the eucalyptus produce fewer flowers for
infected bats to eat. As a result, they are more
likely to search for food on farms, where they
may come into contact with horses. Hungry
bats are also stressed and so tend to shed
higher quantities of the virus.
researchers say their discovery proves that
this type of screening is an effective way to
catch emerging zoonotic infections.
Even when viruses do get inside host
cells, they still face the challenge of
replicating without alerting the immune
system. And we don’t fully understand
what changes allow a virus to flourish in
one species and struggle in another.
A successful zoonotic virus – including
SARS-CoV-2, the cause of covid-19 – tends
to be a generalist, says Mike Farzan, an
immunologist at Scripps Research in Florida.
This means that “most of its proteins work in
most cells from other species”. Zoonotic
pathogens with many different host species
have a higher chance of survival, and ofLA
UREN
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Y^ IMAGES>500,000
Estimated number of animal viruses
capable of jumping to humanshuman cells in lab tests. Most viruses from
animals can’t do this because they lack the
proteins needed to bind to our cell receptors.
For example, the PREDICT team found a
coronavirus in a bat in Uganda in 2017 that
looked genetically similar to the MERS virus,
which is found in camels and can infect
people. But there were differences in the
spike protein, which viruses use to penetrate
would-be host cells. Lab tests confirmed that
this made it harmless to humans.
In 2013, the team identified a coronavirus
similar to the one that causes SARS. It could
infect human cells, but there was no evidence
it had actually infected people. And in 2018,
the project identified another novel virus in
a bat, this time in Sierra Leone. Called
Bombali virus, it is from the same family as
Ebola. Lab experiments confirmed that it
could infect human cells, yet, again, there
is no evidence that anyone has actually
caught it or that it could be spread from
person to person. It isn’t even clear whether
the Bas-Congo virus is a human pathogen.
It was found in blood samples taken from
a nurse who treated two children who died
of a haemorrhagic fever in the Democratic
Republic of the Congo in 2009. But the
virus hasn’t been found in anybody else
since, and something else could have killed
the children.
Another programme to detect emerging
viral threats took place in Vietnam, and
focused on people who live and work closely
with animals at high risk for zoonotic
pathogen transfer. From 2013 to 2016, the
VIZIONS project followed almost 600 farmers,
animal-health workers and people involved
with animal slaughter or who sell rats for
meat. Whenever one of them reported a
cough, sore throat or fever, local doctors would
visit within 48 hours to take samples and
record details about their exposure to animals.
About two-thirds of the workers developed
a respiratory infection over the three-year
period. Analysis of the samples found mostly
common pathogens such as the influenza A
virus, but also some novel viruses. It isn’t
clear where these came from or if they
might pose a clinical risk, but the VIZIONS
A bat’s toenails are
painted to help identify
it after sampling its
bodily fluids