One year into the Zika outbreak: how an obscure disease became a global health emergency

The virus reaches Brazil

The virus was almost certainly brought to Brazil by a traveller from French Polynesia, as molecular studies showed that viruses from the two countries were virtually identical, and clearly from the Asian, not the African, lineage. Researchers looking at flight patterns initially suspected that the 2014 World Cup soccer competition, held in Brazil from June to July 2014, was the seminal event, but no teams from countries with a Zika outbreak attended that event. Researchers now believe the virus was introduced during the August 2014 World Sprint Championship canoe race, held in Rio de Janeiro, which attracted participants from four Pacific Island nations, including French Polynesia, with active Zika transmission.

Retrospective investigations show that the virus was already causing illness in the northeastern part of Brazil near the end of 2014. Cases were retrospectively identified in Rio de Janeiro as early as January 2015. Research published in late April 2016 indicates that Zika was causing cases in Haiti as early as December 2014, though the outbreak was not detected and reported until January 2016. Comparisons of viruses suggest that the virus from French Polynesia first stopped on Easter Island before entering the Americas.

Once established in Brazil, Zika spread explosively within the country and then throughout Latin America and the Caribbean. Within a year, the virus had been detected in nearly every country or territory infested with Aedes aegypti, the principal mosquito species that transmits Zika, dengue, and chikungunya. Two factors favoured explosive spread: the lack of population immunity and the behaviour of the mosquito.

Ae. aegypti is the ultimate “citified” mosquito, having adapted to thrive in tropical areas that have undergone rapid, sprawling urban growth. The mosquitos flourish in the litter, open ditches, clogged drains, containers for water storage, old tyre dumps, and crowded flimsy dwellings typically seen in urban and periurban areas where population growth has outstripped the capacity to construct essential infrastructure, like piped water and sanitation.

Ae. aegypti mosquitoes are exquisitely adapted to city life. They like humans best, live with people in their homes, bite aggressively during the day, don’t mind stagnant water, and prefer to breed in artificial containers. Even something as small as a bottle cap or a discarded plastic wrapper will do.

Fondly known to some entomologists as the “gothic cockroaches” of the mosquito world, these mosquitoes are attracted to black things, like suitcases piled on closets or used tyres dumped in vacant lots. Inside homes, they hide in shadowy places, inside closets, under beds, and beneath sinks. They can breed in toilet tanks, flower vases, and water dishes for pets. The difficulty of their control makes the diseases they spread, including Zika, a much larger menace.

The virus, in its first run through the Americas, profited greatly from the demise of the massive mosquito control programmes of the 1940s and 1950s that virtually eliminated yellow fever from the western hemisphere. As so often happens in public health, when a disease subsides the control programme dies. In addition, options for control shrank dramatically as more and more mainstay insecticides were rendered useless as mosquitoes developed resistance.

The outbreak in Brazil also carried a high risk of exported cases, with around 10 million travellers departing each year for international destinations. Although several hundred imported cases have been reported worldwide, none is known to have sparked an outbreak. This is likely more a matter of luck than evidence of little potential for spread, as most cases were exported to countries during a season of no or low mosquito activity.