1064 6 MARCH 2020 • VOL 367 ISSUE 6482 sciencemag.org SCIENCE
CREDITS: (PHOTO) WORLDFISH/FLICKR/CC BY-NC-ND; (GRAPHIC) D. MALAKOFF/SCIENCE;(DATA) FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONSA
bout a decade ago, farm-raised tilapia
in Israel began to die mysteriously.
The fish had ulcerated skin and inter-
nal hemorrhages; sometimes ponds
full of fish were wiped out. In 2014,
researchers identified the culprit: a
previously unknown virus they named tila-
pia lake virus. Since then, the virus has been
detected on farms in Asia, Africa, and the
Americas. There’s no cure and no vaccine,
and the virus is likely spreading, threaten-
ing one the world’s most important farmed
fish. “It’s a major global problem,” says John
Benzie, a geneticist at WorldFish, an interna-
tional publicly funded research center.
New findings, however, are providing
hope that Nile tilapia (Oreochromis niloti-
cus), the most common kind of farmed ti-
lapia, could be bred to resist the virus. In a
lucky break, in 2018 the virus struck a pond
at WorldFish holding numerous tilapia pro-
duced for breeding experiments, and some
of the fish proved completely resistant to
the virus, Benzie and colleagues reported
last month in Aquaculture. “It’s good news
for the tilapia sector,” says Morten Rye,
a fish geneticist at Benchmark Genetics,
a company that breeds tilapia and other
aquacultural species.
Tilapia is the second most popular fish
in aquaculture, after carp, with farmers in
more than 120 countries now harvesting
about 6 million tons per year. It’s especially
important in developing nations, many of
which rely on a productive strain first de-
veloped in the 1990s at WorldFish.
The emerging virus affects several types
of farmed tilapia, and was probably caus-
ing problems for at least several years be-
fore its discovery. Although researchers
know some regions have been hit hard, the
overall distribution and impact of the virus
are not clear. Because the World Organisa-
tion for Animal Health has not certified adiagnostic test that works in all situations,
countries are only required to report major
outbreaks. More than a dozen countries
do voluntarily report additional data, but,
“We really need descriptive and analytical
epidemiological studies to help us under-
stand the situation in the field,” says Mona
Dverdal Jansen, a veterinary epidemiologist
at the Norwegian Veterinary Institute.
In the meantime, researchers at World-
Fish are working with the University of
Edinburgh’s Roslin Institute to breed Nile
tilapia that resist the virus. The 2018 out-
break in WorldFish’s pond in Malaysia
is helping them identify promising ge-
netic variants. In all, the pond contained
1821 individually tagged tilapia belonging
to 124 groups of siblings. Each group had
different pairs of parents. Nearly 40% of all
the fish died; that was enough to allow the
researchers to pick out which groups of sib-
lings had the highest survival rates.
Just as encouraging, the WorldFish out-
break enabled researchers to show that
about 50% of the variation in survival was
due to genetics. “That bodes well for fu-
ture breeding to improve resistance to the
virus,” says co-author Ross Houston, a fish
geneticist at the Roslin Institute. The re-
searchers also found that bigger—and more
valuable—fish were just as likely to resist
the disease as the smaller fish. That find-
ing suggests breeders won’t have to sacrifice
yield to boost resistance, which would be a
problem for growers.
Still, getting resistant tilapia into the
world’s ponds could take years. Research-
ers need a genomic test that would al-
low breeders to quickly identify fish with
desirable genes, and they need a reliable
and realistic way to infect the fish to find
out whether they really are resistant. Rye
cautions that even if breeders develop a
winning strain, mass-producing the fish
in hatcheries and distributing them—
especially in the developing world—will be
a tall order. “It’s not changing everything
overnight,” he says.
Some companies are pursuing a differ-
ent approach: developing a vaccine against
tilapia lake virus. But viable candidates are
still far off, and even a highly effective vac-
cine might not be cheap enough to be eco-
nomically viable, especially because tilapia
is a relatively low-value fish often grown by
poor farmers.
Given such issues—and the threat that
the virus is continuing to spread—creating
hardier tilapia breeds has become “a matter
of urgency,” Benzie says. “We’re going hell
for leather on this.” jUnplanned experiment could
help save a key farmed fish
Random outbreak at research center points to tilapia
variants that resist the deadly emerging tilapia lake virus
ANIMAL HEALTH
Farm-raised tilapia at a market in Egypt, where
growers have had outbreaks of a damaging fish virus.By Erik Stokstad
1990 1995 2000 2005 2010 201501223Million tonsAsia Africa AmericasA threatened bounty
Production of pond-raised Nile tilapia has been
booming in Asia and Africa, helping the species
become the second most popular farmed finfish
in the world, behind carp.Published by AAAS