Seventy-year journey of Zika virus


Zika virus structure – Image: Purdue University/Kuhn and Rossmann Research Groups

Zika is an arbovirus infection which occurs through the bite of several different species of Aedes mosquitoes, including the Aedes aegypti which is active during the day; it can also be transmitted sexually. The virus was first isolated from a rhesus monkey in Uganda in the Zika forest, near the western shore of Lake Victoria, in 1947, and hence the name. The mosquito-borne virus spread in many African and Asian countries but caused no harm.

But in 2007, more than a hundred cases were reported at Yap, a tiny island in the south-western Pacific. Six years later, Zika spread to French Polynesia, where nearly 30,000 people required medical attention. Among them, more than 70 people had severe neurological symptoms and 40 contracted the Guillain-Barré syndrome, in which the immune system attacks the nervous system, sometimes resulting in paralysis.

How did it come about?

In north-eastern Brazil, towards the end of 2015 and within months after the outbreak, the Zika virus was seen to have a possible link to birth defects in babies. While nearly 80% of the people infected with Zika did not have symptoms, the infection became dangerous if it occurred during the early stages of pregnancy. In some cases, if the virus attacked the brain tissue of the foetus, it led to microcephaly, a condition that results in babies being born with tiny heads, causing severe neurological disorders.

By the end of January 2016, 4,200 suspected cases of microcephaly were reported, and the virus spread to several other Latin American countries and to the Caribbean, with El Salvador, Jamaica and Colombia advising women to delay pregnancy. Last year, on February 1, the World Health Organisation (WHO) declared Zika a public health emergency of international concern. The WHO issued an advisory to pregnant women to avoid travelling to countries with an outbreak of the Zika virus.

Why does it matter?

Besides being spread by mosquitoes, the sexual route of transmission was confirmed in France by last February-end; in a matter of days the WHO said sexual transmission of the virus is “relatively common.” And by mid-April, in a turning point in the Zika virus outbreak, the U.S.-based Centers for Disease Control and Prevention confirmed that the virus caused severe foetal brain defects. Though it was widely suspected, scientists finally pinned down the Aedes aegypti mosquito, common in many tropical countries including India, as the vector responsible for transmitting the virus.

The virus can be spread through blood transfusion. It has also turned up in urine, tears and saliva, but it is not confirmed that it can spread through them. Since the virus can stay in semen longer than in blood, the WHO recommended that couples abstain from sex for at least six months after a man has been diagnosed with Zika. Besides spreading to a few States in the U.S, the virus reached closer to India when it was reported in Singapore last August: 330 cases in two months.

The virus soon spread to Malaysia, the Philippines, Thailand and Myanmar. In mid-November 2016, the WHO declared that the Zika emergency was over. By the end of the year, Brazil had confirmed about 2,230 microcephaly cases. The U.S. reported nearly 4,600 Zika cases by December.

Even as Brazil announced the end of its public health emergency on May 12 this year, the virus appeared in India. Between November 2016 and February 2017, India reported three locally transmitted cases (two women and a man) of Zika in Gujarat; but the news came to light only on May 26, 2017 when the WHO published it on its website.

What next?

A year after the WHO declared Zika a public health emergency, there has been some positive news on the vaccine front. A single dose of Zika vaccine made from the Zika virus showed promise in mice and monkeys. More than 40 Zika vaccine candidates are in the pipeline and five are entering human clinical trials (Phase I).

Back home, the Hyderabad-based Bharat Biotech’s Zika vaccine was found to confer 100% protection on mice. The company has just begun clinical trials in humans (Phase I) in two centres in India.

Published in The Hindu June 3, 2017