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

The first outbreaks

After six decades of apparent slumber, the virus broke out of oblivion in 2007 when it caused its first outbreak on Yap Island in the Federated States of Micronesia. The outbreak was surprising, but ultimately reassuring. Although an estimated 5,000 people were infected, representing more than 70% of the island’s tiny population, no one was hospitalized and no one died. The outbreak was brief, lasting just three months. At that time, researchers speculated that a new virus strain, with greater fitness and epidemic potential, had likely emerged, as happened with the related dengue virus when it hopped among the Pacific Islands during the 1970s.

The next surprise was more ominous. Having demonstrated its ability to spark an outbreak, Zika did so again in French Polynesia from 2013-2014, causing an estimated 30,000 infections. During and after the outbreak, the virus spread to an additional seven island nations, though the outbreaks were much smaller. Again, no deaths were reported, but the disease had established a strong foothold in the Pacific and no longer looked so harmless.

As the outbreak in French Polynesia evolved, doctors began seeing usually rare neurological complications, including 42 cases of Guillain-Barré syndrome, a severe neurological disorder that required long hospital stays for several patients, with 12 needing respiratory assistance. For the 16 patients admitted to intensive care, the median duration of hospital stay was 51 days. Three months after hospital discharge, only 24 patients (57%) were able to walk without assistance. If Zika was indeed linked to the syndrome, the burden on health services would be considerable.

The number of cases of the syndrome was striking, representing a 20-fold increase over previous years. But the concomitant presence of dengue, which had been previously linked to the syndrome, precluded any firm conclusions about a causative role for Zika. Nonetheless, the possible association with neurological complications changed the image of Zika from a benign disease to one with more sinister potential.

Some virologists argued that a more virulent strain of the virus emerged as the outbreak swept through French Polynesia. Others were more sceptical: since Guillain-Barré syndrome was so rare, an outbreak with a large number of cases was needed to detect it. In their view, it was the numbers, and not an altered virus, that explained the presumed association of Zika with neurological complications.