Puzzling distribution of cases suggests Zika is not the only factor in reported microcephaly surge.

Government researchers in Brazil are set to explore the country’s peculiar distribution of Zika-linked microcephaly — babies born with abnormally small heads.

Zika virus has spread throughout Brazil, but extremely high rates of microcephaly have been reported only in the country’s northeast. Although evidence suggests that Zika can cause microcephaly, the clustering pattern hints that other environmental, socio-economic or biological factors could be at play.

“We suspect that something more than Zika virus is causing the high intensity and severity of cases,” says Fatima Marinho, director of information and health analysis at Brazil’s ministry of health. If that turns out to be true, it could change researchers’ assessment of the risk that Zika poses to pregnant women and their children.

The ministry has asked Oliver Brady, an epidemiologist at the London School of Hygiene & Tropical Medicine, and Simon Hay, director of geospatial science at the Institute for Health Metrics and Evaluation in Seattle, Washington, to collaborate with researchers in Brazil. “The aim is to understand why we are only observing elevated rates in the northeast,” says Brady, who flew into Brasilia this month to begin work.