As the world learned of the emerging Zika virus outbreak in Brazil in 2015 and of the astutely identified association with microcephaly, questions quickly arose regarding the dramatic increase in reported microcephaly and the level of risk to pregnant women. There were 2 overarching questions at that time: how large was the increase in the incidence of microcephaly in Brazil, and how much of it could be attributed to Zika virus? Appropriate concern regarding this new association abounded, along with questions regarding whether increased awareness may have led to increased reporting. Initial assessments were further compounded by comparison with a pre-Zika baseline that relied on a passive surveillance system with likely significant underreporting.1,2 Further complicating the situation was the lack of an internationally accepted, standardized approach to the measurement and definition of microcephaly, making comparisons of rates across regions and time periods difficult. Although several advances in the response to Zika virus required the use of cutting-edge technologies (eg, diagnostic advances, vaccine development), these urgent, first, and overarching questions that arose during the Zika virus epidemic in the Americas necessitated the use of traditional epidemiologic study and surveillance methods. In the study published in this issue of Pediatrics by Silva et al,3 “Prevalence and Risk Factors for Microcephaly at Birth in Brazil in 2010,” the authors begin to answer these questions and provide insight into the elusive pre-Zika microcephaly baseline.
In the study, Silva et al3 retrospectively evaluated microcephaly prevalence in 2010 by using population-based surveillance methods (the gold standard of surveillance) before the introduction of Zika virus into Brazil.4,5 Building on progress made in 2016 when the global community came to a consensus on a definition of microcephaly (by using INTERGROWTH-21st birth growth charts), Silva et al3 used these standards to establish baseline microcephaly rates in 2 cities in different regions and then estimated prevalence throughout Brazil in 2010.6
Previous estimates of pre-Zika microcephaly prevalence rates vary widely on the basis of definition used, region, population studied, and methodology, with estimates in the literature ranging from 0.6 to 9 per 10 000 Brazilian live births; United States estimates range from 2 to 12 per 10 000 live births.7–11 By comparison, the 2010 microcephaly prevalence described in the study by Silva et al3 is estimated at 290 per 10 000 live births (defined as more than 2 SDs below the mean) and 62 per 10 000 live births for severe microcephaly (defined as more than 3 SDs below the mean). These dramatic differences are likely affected by varying study populations, microcephaly definitions, and the vastly different reporting mechanisms and study methodologies used (passive versus active case ascertainment and case-based versus population-based surveillance). Nevertheless, this significantly higher than expected baseline is important, and the authors were led to suggest the data reveal a “silent endemic of microcephaly.”
Likewise, although it is tempting to compare these results to the reported post-Zika occurrence of microcephaly from more recent publications, one must use caution given the different reporting mechanisms and study methods used. This underscores the importance of this study (and that of further studies) in establishing accurate methods of measuring and reporting congenital birth defects that can be used consistently to compare results across populations and time periods.
Although it is important to note that there are other birth defects associated with congenital Zika virus beyond microcephaly not explored in this study, this study also serves as a reminder that Zika virus is but 1 cause of microcephaly. Silva et al3 also identified risk factors associated with having a microcephalic infant, including maternal schooling level, marital status, smoking during pregnancy, primiparity, vaginal delivery, and intrauterine growth restriction. Because these risk factors are not unique to Brazil, an exploration of pre-Zika background rates of microcephaly in subpopulations and in regions that have since experienced a Zika outbreak would be useful to determine if the high rates seen in Brazil were seen elsewhere. In addition, conducting future studies to determine etiologies of the pre-Zika and ongoing cases of microcephaly would also be important (eg, cytomegalovirus, toxoplasmosis, factors associated with poverty, etc).
Although Silva et al3 add considerably to our understanding of the pre-Zika microcephaly baseline, many unanswered questions remain. A better understanding of microcephaly and the role of Zika virus requires continued population-based, active surveillance and epidemiologic study (as produced here) in the post-Zika era, which can then be followed prospectively to assess trends and understand risk factors. As the world moves on from its intense interest in microcephaly and the Zika virus outbreak, pediatric providers and the public health community must continue to further our understanding of the risks of Zika-associated microcephaly and other birth defects, as well as the causes and preventive interventions of this now-revealed “silent endemic of microcephaly.”
Thank you to Deb Fox and Lou Smith for their reviews of the manuscript.
- Accepted November 17, 2017.
- Address correspondence to Elizabeth Dufort, MD, Division of Epidemiology, New York State Department of Health, Corning Tower Room 503, Empire State Plaza, Albany, NY 12237. E-mail:
Opinions expressed in these commentaries are those of the authors and not necessarily those of the American Academy of Pediatrics or its Committees.
FINANCIAL DISCLOSURE: The authors have indicated they have no financial relationships relevant to this article to disclose.
FUNDING: No external funding.
POTENTIAL CONFLICT OF INTEREST: The authors have indicated they have no potential conflicts of interest to disclose.
COMPANION PAPER: A companion to this article can be found online at www.pediatrics.org/cgi/doi/10.1542/peds.2017-0589.
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- Copyright © 2018 by the American Academy of Pediatrics