31 August 2019 | New Scientist | 7Analysis PolioNIGERIA has officially wiped
out wild polio, after three years
without a case caused by the
wild polio virus. This is a
heartening milestone for a
country that nearly derailed
the global drive to eradicate
the disease after some regions
banned vaccination in 2003.
But Faisal Shuaib, head of the
country’s public health agency,
called for “cautious euphoria”.
That’s because Nigeria hasn’t
wiped out polio. As first revealed
by New Scientist in 2000, the
live, weakened polio virus used
in the vaccine responsible for
the breakthrough can spread
between people and mutate to
a form that can paralyse. It has
caused 15 cases of vaccine-
derived virus infection in
Nigeria so far this year.
There are ways to stop this
happening, but they haven’t
been rolled out fast enough,
says Michel Zaffran, head of
polio eradication at the World
Health Organization.
The drive to eradicate polio
was based on a cheap, effective
oral vaccine containing
three strains of live, weakenedpolio virus. The Type 2 strain
replicated faster than the others,
provoking the most immunity.
As a result, wild Type 2 polio has
been eradicated worldwide
since 1999.
The Type 2 vaccine virus is
also the strain most likely to
mutate to a disease-causing
form. In 2018, there were
70 cases of vaccine-derived
polio in seven countries, the
majority Type 2 viruses. So in
2016, everyone shifted to usinga live oral vaccine containing
only Types 1 and 3. Immunity to
those improved, and cases fell.
At the same time, children
were supposed to get an injected
vaccine containing killed
versions of all three strains of
virus, making them immune to
any vaccine-derived virus still
circulating. In this way, India
eradicated all polio in 2014.
But too few children in
poorer nations get routinevaccinations, so “there have
been more outbreaks of Type 2
vaccine-derived virus than we
expected”, says Zaffran.
The only way to stop such
an outbreak spreading is to
give people a live, oral vaccine
containing only weakened
Type 2. This is because while the
injected vaccine will stop people
getting infected, once they are
infected only the live vaccine
will stop them spreading the
virus. In an outbreak, 95 per cent
or more of people infected don’t
have symptoms but spread the
virus, so many people must
be vaccinated.
But the live, Type 2 vaccine
also spawns yet more
potentially dangerous vaccine-
derived virus, which can go on
to cause more infections if it
encounters children who
haven’t been immunised
against Type 2 with the injected
vaccine. Routine vaccination
must improve alongside
outbreak response, but that is a
slow, expensive process and is
hampered in many places by
unrest or conflict, including in
northern Nigeria.
That’s not the only problem.
Only three companies make the
live, Type 2 vaccine, so we could
run out. “We have enough to
cope now, but there could be
a crisis if the outbreaks don’t
improve,” says Zaffran.
Yet there are ever more
people susceptible to vaccine-
derived Type 2 polio, as wild
polio no longer circulates and
immunises people, and too few
receive the injected vaccine.
Polio could roar back worse
than ever if it isn’t contained,
says Zaffran. ❚“ In 2018, there
were 70 cases of
vaccine-derived polio
in seven countries”PIUS^ UTOMI^EK
PEI/AFP/GET
TY
IMAGEThe oral polio vaccine
has helped wipe out
wild polio in NigeriaOrnithology
Jake Buehler
CROWS living in urban areas have
higher blood cholesterol levels than
their rural counterparts. That may
be due to the food we leave behind
for them to feast on.
Crows are “experts at raiding
human trash cans and dumpsters”,
says Andrea Townsend at Hamilton
College in New York. Some of the
food they scavenge is fast food,
which is often high in cholesterol.
Townsend and her colleagues
measured cholesterol levels in
blood samples taken from
140 American crow (Corvus
brachyrhynchos) nestlings in
rural, suburban and urban areas in
California. They also measured the
birds’ body mass and fat reserves,
and tracked their survival rates.
They found that the more urban the
surroundings, the higher the blood
cholesterol of the crow nestlings.
To see if access to the foods
that raise cholesterol in
humans were responsible, the
researchers ran a “cheeseburger
supplementation experiment”
where they left cheeseburgers
near nests in rural New York.
Townsend didn’t have
reservations about leaving behind
burgers for the nestlings as elevated
cholesterol doesn’t appear to
affect all species in the same way,
and has actually been linked to
better body conditions in some
animals, she says.
The burger-fed rural crows had
cholesterol levels that were about
5 per cent higher than nearby crows
that weren’t given fast food. Those
that ate the burgers had cholesterol
levels more similar to crows living
in cities (The Condor: Ornithological
Applications, doi.org/c9r9).
Townsend says these results are
consistent with the handful of other
studies on cholesterol in animals
that live near humans, including
foxes, sparrows and even sea turtles
living near more densely populated
Canary Islands. ❚
City crows may have
high cholesterol
thanks to fast food
Wild polio virus eradicated in
Nigeria, but battle isn’t over yet
Debora MacKenzie