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Mosquitoes, including <i>Aedes aegypti</i>, spread Zika virus.

Mosquitoes, including Aedes aegypti, spread Zika virus.

James Gathany/CDC

Fast-spreading virus may cause severe birth defects

An emerging virus that is causing an unprecedented epidemic in Brazil and is quickly spreading through Latin America may be responsible for a spike in severe birth defects. The Zika virus, a little-known pathogen that until 2007 hadn't been seen outside of Africa and Asia, spread earlier this year to South America and has infected more than 84,000 people in Brazil. Zika usually causes relatively mild symptoms, including fever and rashes. Many infected people do not get sick at all.

But the Brazilian government is now warning that the virus may be responsible for a dramatic increase in cases of microcephaly, a severe birth defect in which the brain fails to develop properly and the head is much smaller than normal. Children with microcephaly frequently have developmental delays, learning disabilities, impaired motor function, and seizures. 

The connection is not yet proven, but if Zika virus is causing birth defects, it would be a serious public health issue. Transmitted by mosquitoes, the pathogen is rapidly expanding its range. In October and November, cases were identified in Colombia, Suriname, Guatemala, El Salvador, and Mexico. There is no vaccine or treatment for the little-studied virus, and its mosquito hosts are common as far north as the southern United States.

Microcephaly can be caused by genetic factors, infections, or injuries. In recent years, there have been between 150 and 200 cases in Brazil per year. As of 30 November, more than 1200 cases had been reported in 10 states, all of which have also reported Zika virus infections, says Ana Maria Bispo de Filippis, head of the flavivirus laboratory at the Oswaldo Cruz Institute in Rio de Janeiro, Brazil.

Doctors in the state of Pernambuco first noticed the unusual increase. That state has seen at least 487 microcephaly cases so far this year, compared with an average of 10 cases annually between 2010 and 2014. New cases are still appearing, Bispo says. “We are overwhelmed,” she says. “Dozens of suspected cases are appearing every day in our laboratory.”

Several lines of evidence point toward Zika as a culprit. The virus has been found in the amniotic fluid of two fetuses diagnosed with microcephaly via ultrasound. It has also been found in tissues of a baby with microcephaly that died shortly after birth. It seems that in some cases the virus can cross the placenta and infect the fetus directly, says Patricia Garcez, a neurodevelopment expert at the Federal University of Rio de Janeiro in Brazil. It’s possible that the virus then attacks brain cells, she says. If that happens during the key phases of brain development in the first 3 to 4 months of pregnancy, the overall size of the brain would be dramatically reduced, leading to microcephaly.

Other regions that have experienced recent Zika outbreaks—mostly Pacific islands—have not reported an increase in microcephaly. But on 24 November, authorities in French Polynesia reported that there were 17 cases of unusual central nervous system birth defects following a Zika outbreak there in 2013 and 2014. None of the mothers reported being sick with Zika virus, but all four who were tested had antibodies to a flavivirus, the family to which Zika belongs. (Other flaviviruses include the dengue and West Nile viruses.) Tests of other mothers are underway, the authorities said.

It isn’t clear why Zika virus would be causing more frequent or severe birth defects in Brazil than it apparently has elsewhere. There’s a chance the virus has somehow become more pathogenic, Bispo says. Or perhaps the closely related dengue virus, which is endemic in Brazil, has made people more vulnerable to Zika, says Patricia Sequeira, a virologist in Bispo's lab. People who have antibodies to dengue might mount a less intense immune response to Zika, which might make it more likely that the virus crosses the placenta and infects a fetus.

One way to nail down the connection, Bispo says, would be to show that the virus can cause similar brain anomalies in developing animals. But Zika is so obscure that it isn’t clear whether the virus will affect lab animals like mice in the same way it does humans. Garcez and her colleagues at the Instituto D'Or in Rio de Janeiro in Brazil are starting experiments in which they will infect so-called cerebral organoids—tiny models of the developing human brain—with Zika virus and see whether their development is affected. (The organoid model has already been used to help explain how a genetic mutation causes microcephaly.) 

For now, the only way to fight Zika is to target the mosquitoes that carry it. Brazilian health authorities are warning women who are pregnant or trying to conceive to do what they can to avoid mosquitoes. That means wearing protective clothing, keeping doors and windows screened, and using insect repellant.

*Update, 7 December, 11:30 a.m.: This item has been updated to indicate that although Zika has been shown to infect mice, it is not clear whether it would have similar effects on pregnant animals as it may in humans.