What caused the Zika outbreak?

18 May 2017


More than 70 years has passed since the Zika virus was first identified in Africa; now a new study has revealed why it's suddenly changed tactics and has spread rapidly across the Americas.

Zika is transmitted by Aedes aegypti mosquitoes. It takes its name from the Ugandan Forest where it was first isolated from monkeys in the 1940s. The first human cases were confirmed about a decade later, and the virus has since been shown to be present across Africa and parts of Asia, although the numbers of human cases have historically been low.

In 2012, though, something changed. The virus spread to Polynesia and produce a significant outbreak, and Brazil followed hot on its heels. The first signs that something was happening was a dramatic increase in reports to Brazilian health authorities of cases of newborns with abnormally small heads, a condition called microcephaly.

This has now been confirmed to be a consequence of Zika infection during pregnancy. Other adult disorders, including a condition called Guillain-Barre syndrome that leads to nerve damage and weakness, have also been linked to prior exposure to Zika.

Now Chinese scientists at Tsinghua University in Beijing, writing in Nature, have identified a change in the genetic blueprint of Zika virus that appears to explain the recently accelerated trajectory.

Researcher Yang Liu and his colleagues compared an Asian isolate of Zika virus collected in 2010 with a sample of the virus circulating in 2013. The 2013 strain, they found, produced very high blood levels of a viral protein called NS1.

This dramatically increases the likelihood that a mosquito feeding on an infected individual will pick up the virus.

When the Chinese team compared the genetic signatures of the two viral strains to account for why the 2013 isolate should produce so much more of this "infectivity factor", they found that the 2013 virus had slightly altered the gene that encodes NS1.

The alteration replaces one of the amino acids from which NS1 is built, resulting in much greater secretion of the protein into the blood.

This, the team argue, powerfully enhances the transmissibility of the agent into mosquitoes, increasing the pool of infected insect vectors and therefore increasing the chance of transmission between humans.

The carriage of this new "weaponised" strain of Zika to South America, possibly during a football fixture, led to it finding large numbers of humans with no pre-existing immunity and a thriving population of the ideal insect vector to transmit it. The rest, as they say, is history...


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