66 Science & technology The EconomistJune 27th 2020
21told, the world’s mammals and birds play
host to between 700,000 and 2.6m as yet
unknown species from families of viruses
that have shown the potential to cause zoo-
notic disease in humans. Between 350,000
and 1.3m of these unknown viruses, they
argued, could have zoonotic potential.
In 2018 Dr Carroll, Dr Daszak and Jonna
Mazet, an epidemiologist at the University
of California, Davis, put forward a proposal
aimed at turning those statistical esti-
mates into genetic sequences. The Global
Virome Project is conceived of as a decade-
long effort to scour the entire world for its
millions of unknown viruses and then read
out all their genomes. The cost, the three
researchers reckoned, would be $4bn. A
scaled-back version—one that concentrat-
ed on the highest-risk countries, the
groups of people most vulnerable to out-
breaks within those countries and the spe-
cies, particularly mammals and water
birds, most likely to be sources of spill-
over—might get 70% of the data for a quar-
ter of the money. But that was still five
times the cost of predict, the most ambi-
tious such project to date. No funding bo-
dies have yet taken the bait.
The fact that viruses with zoonotic po-
tential outnumber those that have actually
made the jump to people by something like
a thousand to one reinforces the idea that,
for any given virus, getting into humans
and staying there is not that easy. The final
products of predict, which are now wend-
ing their way to publication, try to tease out
the factors that help the jump to happen.
Among other things, having a registry of
such risks might make it possible to identi-
fy hotspots where an unhealthy number of
the conditions for zoonoses coexist. The
predict programme’s risk registry in-
cludes virological, ecological and sociolog-
ical factors. Viruses which store their genes
as rna, for example, are categorised asmore risky than dnaviruses, because of
their increased ability to mutate. Viruses
already found in more than one host are
also flagged up. They clearly have an adap-
tive knack. And being adapted to a species
reasonably close to Homo sapiensmatters
too. A virus able to reproduce in the cells of
one species will, other things being equal,
have a better chance of adapting to life in a
related species than an unrelated one. siv
did not have to change all that much to be-
come hiv. Reptile viruses, by contrast, are
less of a threat.The boys who cried “Goose!”
On the human side of the equation the
presence of people who are using an envi-
ronment in new ways is a palpable risk.
Proximity and frequency of contact are im-
portant as well. Farmers who work with
lots of animals day-in and day-out are the
most threatened, especially where this
happens in the presence of wild animals,
too. And the sheer number of viruses a spe-
cies has to offer is also significant (see
chart on next page). Bats are a particularly
rich source of emerging infections. Thelarge groups in which many bat species
live, sometimes numbering in the mil-
lions, give viruses a huge arena in which to
mix, evolve and develop the kinds of char-
acteristics that might make them capable
of spilling over into people.
Besides being the original reservoirs of
sars-covand sars-cov-2, bats also har-
bour another coronavirus, mers-cov,
which causes Middle Eastern respiratory
syndrome, an illness first detected in 2012.
They are also the source of the virus which
causes Ebola and of the hendra and nipah
viruses which, over the past three decades,
have led to small outbreaks of deadly respi-
ratory and brain infections in Australia and
South-East Asia.
The viruses in question do not always
travel directly from bats to people, as the
civet-related example of sars-covshows.
Ebola seems to have done so. But hendra
and nipah arrived via horses and pigs re-
spectively that bats had defecated onto. In
the case of mers, the intermediaries were
camels. Pangolins have been suggested as a
conduit for sars-cov-2.
Dr Daszak and his colleagues at the Eco-
Health Alliance have collaborated with
Chinese researchers, including some at the
Wuhan Institute of Virology, which estab-
lished the chiropteran links with both sars
and covid-19, to gather samples from thou-
sands of bats and other mammals across
southern China. In a recent paper posted to
bioRxiv, a preprint server, they published
the genetic sequences of 781 coronaviruses
found in bats, including more than 50 close
relatives of sars-cov. In a paper in the
March edition of Biosafety and HealthDr
Daszak describes how some of these virus-
es have been shown to bind to human cells.
In mice with genetically engineered “hu-
manised” cells in their lungs, some of them
cause a disease similar to sarsthat is not
responsive to therapies and vaccines de-
veloped against sars-cov.
On top of this, several groups studying
blood samples from the parts of China
where the new coronaviruses were found
have seen antibodies suggesting that peo-
ple there were exposed regularly to some of
these viruses between the emergence of
sars-cov in 2002 and of sars-cov-2 in- “Together,” Dr Daszak wrote in the pa-
per “these data mark wildlife-origin coro-
naviruses as a ‘clear and present danger’.
They also highlight exactly the issue of key
concern in the current [covid-19] out-
break—that there is a large diversity of viral
strains in wildlife in China with significant
potential for emergence in people.”
If something like the Global Virome
Project were to identify markers for most of
the world’s potentially zoonotic viruses,
keeping an eye out for one of them crop-
ping up in a species that human beings
routinely mix with would be easy—espe-
cially as genetic sequencing is now a hun-
Animal origins
Discoveries of emerging infectious diseases
BydecadeSource:EcoHealthAlliance *To 20041501209060300
1940s 50s 60s 70s 80s 90 2000s*sZoonotic Not zoonotic