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.
There are many hypotheses about what might be going on. Marinho says that her team's data, submitted for publication, hint that socio-economic factors might be involved. For example, the majority of women who have had babies with microcephaly have been young, single, black, poor and tend to live in small cities or on the outskirts of big ones, she says. Another idea is that co-infections of Zika and other viruses, such as dengue and chikungunya, might be interacting to cause the high intensity of birth defects in the area. A third possibility was put forward in a paper published last month1, in which researchers from Brazilian labs noted a correlation between low vaccination rates for yellow fever and the microcephaly clusters.Brazil has called in spatial epidemiology experts Oliver Brady from 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 help sort out the issues. Not mentioned among the possibilities is the debunked theory put out early on that this was all due to experimental research on larvicides being conducted in the region by Sumitomo, which has a relationship to Monsanto.