How Does The Zika Virus Cause Birth Defects?

In February of 2016, the World Health Organisation (WHO) declared the Zika virus an international concern for public health. This came after reports that cases of fetal microcephaly had increased within South America, believed to be due to the 2015 outbreak in Brazil.

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Similar to dengue and yellow fever, Zika is an arbovirus, meaning it spreads through mosquito bites. Transmission may also occur through sexual intercourse and blood transfusion, as well as through mother-child during pregnancy.

The symptoms of infection are mild, with the CDC reporting that sometimes no symptoms are seen at all. Common symptoms include headache, fever, and conjunctivitis. With no specific treatment needed, other than the usual bed rest and fluids, people may not even know they have caught the Zika virus. This could prove problematic, especially for pregnant women when considering the devastating defects observed in newborns after infection during pregnancy.

The mechanisms of how the virus causes the devastating birth deformities have recently been elucidated. Studies published in May of this year describe experimental data showing how infection causes growth restriction, microcephaly and other abnormalities in developing embryos.

One study has been done using rodents to show how the Brazilian strain of the Zika virus crosses the placenta during pregnancy to come in contact with the embryo. Consequently, certain cells called cortical progenitor cells (which eventually give rise to the brain) are infected. The virus then works to induce cell death in these cells which ultimately leads to poor brain growth and the reduced head size which is seen in microcephaly babies. A second study looking at the Asian strain showed similar findings.

A third study was able to demonstrate how the virus attacks the placenta during pregnancy in mice. A healthy placenta acts as a barrier between mother and baby, preventing transmission of infection and disease whilst allowing essential oxygen and nutrients to pass along. The viral infection compromises the placenta’s function by attacking its cells and blood vessels. This inevitably reduces the nourishment available to the baby, restricting growth.

These studies are crucial - with deeper understanding of how the virus operates, a development of a preventative vaccine or maybe even a therapeutic treatment may be possible.

Right now the Zika virus is definitely a cause for concern. With many in poorer areas not using effective contraception, coupled with the mild almost asymptomatic manifestations of the infection, the number of cases of birth defects may increase. Despite this there is now hope as the mechanisms behind the disease are being unlocked, meaning vaccines and treatment may be just around the corner...

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