“This study marks a turning point in the Zika outbreak. It is now clear that the virus causes microcephaly. … We’ve now confirmed what mounting evidence has suggested…”
— Tom Frieden, MD, director of the US Centers for Disease Control and Prevention (April 13, 2016)
The announcement by CDC director Tom Frieden that the Zika virus is definitely the cause of microcephaly does not appear to be based on a NEW STUDY that provides conclusive scientific evidence of a casual relationship, but rather on an EVALUATION of existing data that, for weeks, the CDC has been saying DOES NOT conclusively show a causal relationship.
The evaluation by the CDC’s Sonja A. Rasmussen, MD, Denise J. Jamieson, MD, MPH, Margaret A. Honein, PhD, MPH, and Lyle R. Petersen, MD, MPH CONCLUDES a causal relationship between Zika and microcephaly based on the determination that the available data meets 4 (1 of those only partially) out of 7 “criteria” proposed by pediatrician Thomas H. Shepard, MD in 1994. Shepard is considered to be a “pioneer” in the field of Teratology.
Here’s Wikipedia’s definition of Teratology: “The study of abnormalities of physiological development. It is often thought of as the study of human congenital abnormalities, but it is broader than that, taking into account other non-birth developmental stages, including puberty; and other non-human life forms, including plants.”
The following are the 7 criterion given by Shepard:
1. Proven exposure to the agent at one or more critical times during prenatal development.
2. Consistent findings by 2 or more high-quality epidemiological studies with control of confounding factors, sufficient numbers, exclusion of positive and negative bias factors, prospective studies, if possible, and relative risk of equal to or greater than 6.
3. Careful delineation of clinical cases; a specific defect or syndrome, if present, is very helpful.
4. Rare environmental exposure that is associated with rare defect.
5. Teratogenicity in experimental animals important but not essential.
6. Association should make biological sense.
7. Proof in an experimental system that the agent acts in an unaltered state.
The four CDC researchers concluded that 1, 3, 4 criteria were met and that criteria 2 was partially met. In other words, the hypothesis that Zika infection during pregnancy causes birth defects such as microcephaly has been met.
CDC evidence for Criteria 1: On the basis of case reports, case series, and epidemiological studies of microcephaly that are associated with laboratory-confirmed or presumed Zika virus infection, the timing of Zika virus infections associated with severe microcephaly and intracranial calcifications appears to be in the late first or early second trimester.
CDC evidence for Critieria 2: On the basis of data from Brazil, the temporal and geographic association between Zika virus illness and cases of microcephaly is strong. Two epidemiological studies have been published. In a study in Brazil that used a prospective cohort design, 29% of women with Zika virus infection at any time during pregnancy had abnormalities on prenatal ultrasonography, some of which have not been confirmed postnataly. In a study in French Polynesia, restrospective identification of eight cases of microcephaly and the use of serologic and statistical data and mathematical modeling suggested that 1% of fetuses and infants born to women with Zika virus infection during the first trimester had microcephaly; the risk ratio in this analysis was approximately 50, as compared with the baseline prevalence of microcephaly. No other epidemiological studies have examined this association to date.
CDC evidence for Criteria 3: The phenotype has been well characterized in fetuses and infants with presumed congenital Zika virus infection, including microcephaly and other serious brain anomalies, redundant scalp skin, eye findings, arthrogryposis, and clubfoot. The phenotype in some infants appears to be consistent with the fetal brain disruption sequence, which has been observed after infection with other viral teratogens.
CDC evidence for Criteria 4: Reports of fetuses and infants with microcephaly who are born to women with brief periods of travel to countries with active Zika virus transmission are consistent with Zika virus being a rare exposure. The defect, congenital microcephaly, is rare, with a birth prevalence of approximately 6 cases per 10,000 liveborn infants, according to data from birth-defects surveillance systems in the United States.