CDC declares Zika virus teratogenic

Officials from the Centers for Disease Control and Prevention (CDC) have declared that Zika virus is a teratogen that causes microcephaly and other serious brain anomalies. That conclusion was announced in a special report in The New England Journal of Medicine which outlined the investigator’s systematic analysis of epidemiologic evidence on the virus to date.

The goal of the effort by CDC researchers was to determine whether cause and effect could be established between prenatal Zika virus and subsequent birth defects. The data on Zika virus were evaluated against the criteria proposed by Shepard in 1994 as a “yardstick” for establishing human teratogenicity. 

The 4 key criteria of Shepard’s that the Zika virus has met, said the CDC authors, and which justify the new classification, are as follows:

·       Exposure to Zika virus occurs at a critical time in prenatal development, as documented in a report of laboratory-confirmed transmission in specific areas of Brazil and an increase in the number of cases of microcephaly identified in the first trimester;

·       Clinical cases with the teratogenic effect have been clearly delineated, as seen in what the authors described as descriptions of the typical pattern of findings in Zika-exposed fetuses and infants, such as severe microcephaly, intracranial calcifications, and other brain abnormalities; and

·       Verification that the association involves a rare exposure and a rare defect, as supported by evidence indicating that microcephaly is rare in the United States, with an incidence of 6 per 10,000 live births, and that adverse birth outcomes have been seen in offspring of women who spent only a limited period of time in an area where Zika virus was endemic.

With Zika virus confirmed as teratogenic, the CDC authors said, the focus of research can now shift to “understanding the full spectrum of defects caused by congenital Zika virus infection; if Zika virus is similar to other teratogens…, quantifying the relative and absolute risks among infants who are born to women who were infected at different times during pregnancy…and identifying factors that modify the risk of an adverse pregnancy or birth outcome.”