Zika and Other Potential Causes of Microcephaly in Brazil: Status March 9, 2016

We review the current information about causes of microcephaly in Brazil in 2015-6, including speculative ones, in order to provide the best possible clarity about the information that is available as of the publication date. We review the leading candidate, Zika virus infections, the second candidate, the pesticide pyriproxifen, as well as DPT immunizations, and GM mosquitoes. Each of these candidates has been considered because of an increase at very roughly the time of the increase in microcephaly cases. The strongest evidence is in favor of Zika, through the observation of Zika virus in neural tissue, though a key piece of evidence is missing in the expected rise of cases in other locations, specifically Colombia. Evaluation of the potential role of pyriproxifen is difficult due to the limited number and nature of available studies, which should be revisited as they include some evidence for neurodevelopmental toxicity. The possibility of DPT immunizations of pregnant women as a factor is largely ruled out by an increase in immunization in countries in which microcephaly cases are not being reported. There is no direct evidence for GM mosquitoes as a cause. If there is a dramatic increase in cases of microcephaly in Columbia in the next three months, the case for Zika will be dramatically strengthened, and the case for pyriproxyfen and GM mosquitoes will be essentially ruled out. On the other hand if the cases do not materialize, Zika will essentially be ruled out and pyriproxyfen would become the strongest case with GM mosquitoes a speculative alternative along with other environmental toxins.