We summarize current evidence on the prevalence of Zika and microcephaly in Brazil and Colombia and conclude that the expectation of a large number of microcephaly cases outside of Brazil has not been realized. The ratio of microcephaly to Zika cases is inconsistent between Colombia and Brazil and between Brazilian states, where the majority of cases are confined to the northeast region. At the rate of microcephaly in Colombia, if all pregnancies in the Brazilian state of Pernambuco were infected by Zika, we estimate there would only be 100 cases of microcephaly in a year, whereas the number of confirmed cases is 386. Other causes and co-factors of microcephaly must be considered, including the pesticide pyriproxyfen which has been added to drinking water in some regions of Brazil since the fall of 2014 and is cross-reactive with retinoic acid which is known to cause microcephaly. Even without confirmation, the continued increase of microcephaly cases in Brazil by 100 per month warrants urgent policy action to stop the use of pyriproxyfen.
Yaneer Bar-Yam, Raphael Parens, Alfredo J. Morales, Is Zika the cause of Microcephaly? Status Report November 4, 2016, NECSI