Zika fever
Virus chequers
A newly emerging
disease is threatening the Americas
Jan 23rd 2016 | SÃO
PAULO | From the print edition
Timekeeper
ZIKA, a
mosquito-borne virus that arrived in Brazil last May, is an avid
traveller—and an increasingly feared guest. It has since found its
way into 17 other countries in the Americas. Until October, Zika was
not thought much of a threat: only a fifth of infected people fall
ill, usually with just mild fever, rash, joint aches and red eyes.
Since then, though, evidence has been piling up that it may cause
birth defects in children and neurological problems in adults. On
January 15th America’s Centres for Disease Control and Prevention
(CDC) advised pregnant women not to travel to countries where Zika is
circulating.
The virus was first
isolated in 1947, from a monkey in the Zika forest in Uganda. Since
then it has caused small and sporadic outbreaks in parts of Africa
and South-East Asia. In Brazil, for reasons yet unclear, it quickly
flared into an epidemic after its arrival—by official estimates
infecting as many as 1.5m people.
Alarm bells started
ringing in October, when doctors in Pernambuco, one of Brazil’s
north-eastern states, saw a huge increase in babies born with
microcephaly: an abnormally small head, often with consequent brain
damage. In the next four months more than 3,500 cases of microcephaly
were reported in Brazil. That compared with fewer than 200 a year in
the five years before 2015. None of the known causes of the
condition—which include genetic abnormalities, drugs, alcohol,
rubella infection and exposure to some chemicals during
pregnancy—seemed a plausible culprit.
Last week, CDC
scientists announced the best evidence so far that Zika can pass from
mother to fetus: they found the virus in four Brazilian babies with
microcephaly who had died in the womb or shortly after birth.
Previously, Brazilian researchers had found Zika in the amniotic
fluid of women carrying fetuses with microcephaly.
There is another
fear. After Zika arrived in Brazil, and also in El Salvador, both saw
a sharp increase in severe neurological and autoimmune problems,
including Guillain-Barré syndrome, which can lead to paralysis.
These also surged in French Polynesia after Zika broke out there in
2013.
Working out the
extent to which Zika, alone or combined with other things, is to
blame for any of this is tricky. Dengue and
chikungunya—mosquito-borne viruses with similar symptoms—are
common where Zika is making the rounds. According to Scott Weaver of
the University of Texas, tests that spot Zika work only during its
infectious phase, which lasts just a few days. After that, they are
often useless if the patient has had dengue or been vaccinated
against yellow fever. And only laboratories that can do sophisticated
molecular tests are in the game in the first place. All of which
means that most cases of Zika are missed, and many are misdiagnosed.
Bearing these
caveats in mind, researchers are mining the available surveillance
data for answers. More solid results will come from prospective
studies, set up recently, which are tracking pregnant women in
Brazil, looking at whether those who catch Zika are more likely to
have babies with birth defects.
Researchers in
America and other countries have begun work on a vaccine. Unlike the
one for Ebola, though, which had been in the pipeline for a decade
when the epidemic in West Africa began, a Zika vaccine is “at
ground zero”, says Alan Barrett, also of the University of Texas.
That is where potential antiviral drugs are, too.
The spread of Zika
makes attacking disease-carrying mosquitoes all the more important.
Mostly, Zika is transmitted by Aedes aegypti, which is also the
vector of dengue and yellow fever. This insect lives in tropical
climes, but Aedes albopictus, found as far north as New York and
Chicago, and in parts of southern Europe, can also do the job, though
it is not clear how efficiently. A paper published last week in the
Lancet shows where Zika could become endemic (see map). But places
where air-conditioning, screened windows and mosquito control are the
norm are unlikely to see outbreaks flare up.
In December, Brazil
decreed a national public-health emergency. This has removed
bureaucratic hurdles to the purchase of insecticides for mosquito
larvae, equipment for health workers and the like—and prompted
speculation about whether this bureaucracy was necessary in the first
place. It also enabled the deployment of the army to help 310,000
health workers in the mosquito-eradication drive. Brazil was declared
free of A. aegypti in 1958, after a campaign that included regular
fumigation and visits to ensure households got rid of standing water,
where mosquitoes like to breed. Since then, the insect has bounced
back. Might the fear of Zika help finish the job properly this time?
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