The new england journal of medicine Special Report Zika Virus and Birth Defects — Reviewing the Evidence for Causality Sonja A. Rasmussen, M.D., Denise J. Jamieson, M.D., M.P.H., Margaret A. Honein, Ph.D., M.P.H., and Lyle R. Petersen, M.D., M.P.H. Summary Potential Relationship between Zika Virus Infection and Birth The Zika virus has spread rapidly in the Americas Defects since its first identification in Brazil in early 2015. Prenatal Zika virus infection has been linked to Since the identification of the Zika virus in Bra- adverse pregnancy and birth outcomes, most no- zil in early 2015, the virus has spread rapidly tably microcephaly and other serious brain anom- throughout the Americas (www . cdc . gov/ zika/ alies. To determine whether Zika virus infection geo/ active-countries . html). An increase in the during pregnancy causes these adverse out- number of infants with microcephaly in Brazil comes, we evaluated available data using criteria was first noted in September 2015, after the that have been proposed for the assessment of recognition of Zika virus transmission in the potential teratogens. On the basis of this review, country earlier in the year1; this was followed by we conclude that a causal relationship exists be- the recognition of a similar increase in French tween prenatal Zika virus infection and micro- Polynesia after an outbreak there in 2013 and cephaly and other serious brain anomalies. Evi- 2014.2 Despite accumulating evidence that sup- dence that was used to support this causal ports the link between Zika virus infection and relationship included Zika virus infection at times microcephaly, most experts have taken care not during prenatal development that were consis- to state that Zika virus infection is causally re- tent with the defects observed; a specific, rare lated to these adverse outcomes.3 This cautious phenotype involving microcephaly and associated approach toward ascribing Zika virus as a cause brain anomalies in fetuses or infants with pre- of birth defects is not surprising, given that the sumed or confirmed congenital Zika virus infec- last time an infectious pathogen (rubella virus) tion; and data that strongly support biologic caused an epidemic of congenital defects was plausibility, including the identification of Zika more than 50 years ago, no flavivirus has ever virus in the brain tissue of affected fetuses and been shown definitively to cause birth defects in infants. Given the recognition of this causal rela- humans,4 and no reports of adverse pregnancy tionship, we need to intensify our efforts toward or birth outcomes were noted during previous the prevention of adverse outcomes caused by outbreaks of Zika virus disease in the Pacific congenital Zika virus infection. However, many Islands.5,6 questions that are critical to our prevention ef- On the basis of the available evidence, the forts remain, including the spectrum of defects public health response to the outbreak of Zika caused by prenatal Zika virus infection, the de- virus disease has moved forward, with the dis- gree of relative and absolute risks of adverse out- tribution of health messages about the impor- comes among fetuses whose mothers were in- tance of mosquito-bite prevention, recommenda- fected at different times during pregnancy, and tions by public health authorities in some of the factors that might affect a woman’s risk of ad- most severely affected countries to delay preg- verse pregnancy or birth outcomes. Addressing nancy, and advisories that pregnant women avoid these questions will improve our ability to reduce travel to areas with active Zika virus transmission.7 the burden of the effects of Zika virus infection However, communications regarding Zika virus during pregnancy. have been challenging: a recent survey showed n engl j med nejm.org 1 The New England Journal of Medicine Downloaded from nejm.org at Stephen B. Thacker CDC Library on May 6, 2016. For personal use only. No other uses without permission. Copyright © 2016 Massachusetts Medical Society. All rights reserved. The new england journal of medicine low levels of knowledge and concern about Zika about causation in teratology-related litigation30 virus in the United States.8 The recognition of and to assess other potential teratogens.10 We used Zika virus as a cause of microcephaly and other Shepard’s criteria9 as a framework to evaluate serious brain anomalies would allow for more whether the currently available evidence supports direct communication, which might lead to im- the hypothesis that prenatal Zika virus infection proved understanding of and adherence to public is a cause of microcephaly and other brain anom- health recommendations. Therefore, a review of alies (Table 1). the evidence linking Zika virus infection and ad- According to these criteria, causality is estab- verse pregnancy and birth outcomes is needed. lished when either criteria 1, 3, and 4 (rare ex- As is typically the case in epidemiology and posure–rare defect approach) or criteria 1, 2, and medicine, no “smoking gun” (a single definitive 3 (epidemiologic approach) are fulfilled. The first piece of evidence that confirms Zika virus as a criterion states that a proven exposure to an agent cause of congenital defects) should have been an- must occur at a critical time during prenatal de- ticipated. Instead, the determination of a causal velopment. The severe microcephaly and other relationship would be expected to emerge from brain anomalies that have been observed in many various lines of evidence, each of which suggests, infants are consistent with an infection occurring but does not on its own prove, that prenatal Zika in the first or early second trimester of pregnan- virus infection can cause adverse outcomes. Two cy. Several case reports and studies have shown approaches have been used to identify potential that women who had fetuses or infants with teratogens (exposures to a mother during preg- congenital brain anomalies that were believed, nancy that have a harmful effect on her embryo on the basis of the mother’s symptoms or labo- or fetus)9: first, the identification of a combination ratory confirmation, to be due to Zika virus in- of a rare exposure and a rare defect (sometimes fection were infected in the first or early second referred to as the astute clinician approach),10 and trimester of pregnancy, as determined either second, the use of epidemiologic data to confirm according to the timing of the symptoms or ac- an association. Many teratogens were first iden- cording to the timing of travel to an area where tified by means of the rare exposure–rare defect Zika virus is endemic.14-20 An analysis of the tim- approach, including rubella virus, which was iden- ing of laboratory-confirmed Zika virus transmis- tified after an ophthalmologist noted a character- sion in certain states in Brazil and of the in- istic form of cataracts in infants whose mothers crease in the cases of microcephaly identified had rubella during pregnancy,11 and heavy alcohol the first trimester as the critical time period for use, which was identified as a teratogen after the infection.1 Zika virus infections that occur later recognition of a characteristic pattern of malfor- in pregnancy have been associated with poor in- mations that became known as the fetal alcohol trauterine growth, fetal death, or in some preg- syndrome.12 In contrast, some teratogens have nancies, defects on prenatal imaging that have been identified on the basis of epidemiologic not yet been confirmed postnatally because the studies (e.g., valproic acid was identified as a pregnancies are ongoing.14 We conclude that teratogen after a case–control study showed an Shepard’s first criterion has been met. odds ratio of 20 for the association of spina bifida Shepard’s second criterion requires that two with use of this drug during the first trimester of epidemiologic studies of high quality support the pregnancy).13 association. Although ecologic data do not neces- sarily qualify as an epidemiologic study, data from Shepard’s Criteria Brazil regarding the temporal and geographic association between Zika virus infection and the In 1994, Thomas Shepard, a pioneer in the field later appearance of infants with congenital mi- of teratology, proposed a set of seven criteria for crocephaly are compelling.1,31,32 Two epidemio- “proof” of human teratogenicity (Table 1) that logic studies also provide support.2,14 In a study incorporated both approaches.9 These criteria conducted during the outbreak in Brazil, 88 preg- were an amalgamation of criteria developed by nant women who had had an onset of rash in the other teratologists and guided by methods that previous 5 days were tested for Zika virus RNA. were used to identify previous teratogens. These Among the 72 women who had positive tests, criteria have been used to guide discussions 42 underwent prenatal ultrasonography, and fe- 2 n engl j med nejm.org The New England Journal of Medicine Downloaded from nejm.org at Stephen B. Thacker CDC Library on May 6, 2016. For personal use only. No other uses without permission. Copyright © 2016 Massachusetts Medical Society. All rights reserved. Special Report Table 1. Shepard’s Criteria for Proof of Teratogenicity in Humans as Applied to the Relationship between Zika Virus Infection and Microcephaly and Other Brain Anomalies.* Criterion No. Criterion Evidence Criterion Met? 1 Proven exposure to the agent at one or On the basis of case reports, case series, and epidemiologic studies of Yes more critical times during prenatal microcephaly that are associated with laboratory-confirmed or pre- development sumed Zika virus infection, the timing of Zika virus infection associ- ated with severe microcephaly and intracranial calcifications appears to be in the late first or early second trimester.14-20 2 Consistent findings by ≥2 high-quality On the basis of data from Brazil, the temporal and geographic associa- Partially epidemiologic studies, with con- tion between Zika virus illness and cases of microcephaly is strong.1 trol of confounding factors, suffi- Two epidemiologic studies have been published.