The Zika virus and birth defects: the evidence builds
A growing number of studies are drawing the conclusion that the virus is the culprit.
The sight of rows of Brazilian mothers nursing babies with small misshapen heads, shocked the world into awareness of the Zika virus early this year. The World Health Organisation declared Zika a public health emergency.
The virus exploded into the Brazilian population last September, mostly causing mild symptoms – fever or rash. Its impact on unsuspecting pregnant mothers was a tragic sting in the tail. Until now, health authorities were unaware that Zika could cause birth defects ranging from mild microcephaly – small heads but normal brain capacity – to severe brain damage.
Part of the surprise is that Zika, a flavivirus transmitted by Aedes mosquitos (and in rare cases through sexual transmission), is closely related to other mosquito-borne viruses like dengue, yellow fever and West Nile fever. But none of these cause microcephaly. “There was no precedent,” says Cameron Simmons, a dengue expert at the University of Melbourne.
The virus was first identified in rhesus monkeys in Zika Forest, Uganda in 1947. Apart from isolated cases, it took until 2007 for it to emerge in a human population, with a large outbreak in the Yap islands in Micronesia.
By 2013 other Pacific Island countries were reporting cases. Most infected people recovered fully, though after an outbreak in French Polynesia, the virus was linked to cases of Guillain-barré Syndrome where the immune system, triggered by a virus, goes on the rampage against peripheral nerves. The result can be paralysis and death.
By 2014 Zika made its way to the Americas, reaching epidemic proportions in Brazil in September 2015. But Brazil, with its huge, close-packed, mosquitoridden population, was set for a perfect storm. By the end of the year the microcephaly epidemic was obvious: the number of affected infants spiked to around 40 times that previously reported.
But nailing Zika as the culprit has been controversial.
For one thing Colombia, where more than 2,000 pregnant women have been infected with Zika, has not reported an increase in microcephaly. Last February a group of Argentinian physicians suggested the cause might be an antimosquito larvae agent, pyriproxyfen, which had been introduced into Brazil’s drinking water supplies in 2014.
“That argument doesn’t fly,” says Simmons, pointing out pyriproxyfen has been used all around the world without raising the risk of birth defects.
March saw a rapid-fire build up in the case against Zika. On 4 March, the New England Journal of Medicine (NEJM) published the first study of 88 pregnant women in Rio de Janeiro. All of them had
BRAZIL, WITH ITS CLOSE- PACKED, MOSQUITO- RIDDEN POPULATION, WAS SET FOR A PERFECT STORM.
recently reported a rash and 82% tested positive for Zika virus.
Of 42 infected women who consented to have their foetuses scanned by ultrasound, 29% showed microcephaly or other foetal abnormalities. No abnormalities were found in 16 women who were not infected.
On 10 March another study, in the same journal, reported that a European aid worker who had contracted the virus in north-eastern Brazil aborted a foetus at 29 weeks that had been diagnosed with microcephaly during an ultrasound in a Slovenian clinic. Zika virus was detected in the brain tissue of the foetus.
As far as nailing Zika to brain defects, “that’s unequivocal evidence”, says Simmons. “What remains is working out the true rate at which these defects occur.” That was partly answered on 15 March. The Lancet published a retrospective study of microcephaly in the French Polynesian epidemic of 2013-2014. Looking back, they found that the cases of microcephaly had risen dramatically. But the estimated risk of microcephaly was much lower than suggested by the Rio study: one in 100 for women infected with Zika virus during the first trimester of pregnancy.
That shows it is still very early days for measuring the risk from Zika.
WE DON’T YET HAVE ENOUGH INFORMATION TO COUNSEL WOMEN – THAT’S THE GREY AREA.
“I think most people believe there’s an association,” says Michelle Giles, an infectious diseases expert at Melbourne’s Alfred hospital.
“We don’t yet have enough information to counsel women – that’s the grey area.”
With no anti-viral treatment for Zika and no vaccine, the key efforts have been targeted at mosquitoes, including the release of genetically engineered, sterile
Aedes mosquitos. As the March 10 editorial in NEJM put it: “Once again, an outbreak is going to challenge our public health infrastructure and require a substantial response.”