CDC Identifies Risk of Developing Zika Complications Based on Trimester of Infection

The Centers for Disease Control and Prevention (CDC) just published a Morbidity and Mortality Weekly Report (MMWR) highlighting data on the largest number of completed pregnancies with lab-confirmed Zika virus infection. In addition, during a telebriefing, CDC Acting Director, Anne Schuchat, MD, shared the CDC’s stance on the end of the Zika epidemic in Puerto Rico.

This MMWR report is the first of its kind from US territories, including the Federated States of Micronesia, American Samoa, the Republic of Marshall Islands, the US Virgin Islands, and the Commonwealth of Puerto Rico.

The report reflects data collected between January 1, 2016 and April 25, 2017, and includes data on 2,549 completed pregnancies, 1,508 of which had lab-confirmed Zika virus infection. Of the completed pregnancies, 122 infants (5%) exhibited Zika-related birth defects, with 5% of afflicted infants born to symptomatic mothers and 4% born to asymptomatic mothers. The CDC notes that this number does not constitute a clinical significance in terms of risk of infants born to symptomatic mothers developing complications versus those born to asymptomatic mothers.

Zika lab testing results were reported for only 59% of liveborn infants included in the registry. Of those, 52% received neuroimaging and 79% had hearing tests. The CDC stresses the importance of performing head ultrasounds on infants born to mothers with confirmed Zika infection, as these tests can show abnormalities that aren’t apparent after birth.

The report also highlights that the greatest incidence of Zika-associated birth defects was apparent among infants born to mothers infected during the first trimester: the results showed that 8% of infants born to Zika-infected mothers in US territories developed complications compared with 15% of those born in US states. According to a press release, “these estimates are based on relatively small numbers and are not statistically different.” Infection during the second trimester resulted in the second highest incidence of complications (5%), and infection during the third trimester was found to have the lowest incidence of complications (4%). On average, the CDC estimates that 1% to 13% of Zika infections that occur during the first trimester of pregnancy lead to microcephaly. However, this MMWR report is the first to identify “preliminary estimates of risk for each trimester.”

Another important finding that Peggy Honein, PhD, co-lead on the Pregnancy and Birth Defects Task Force for CDC Zika Response, shared during the telebriefing, is that these findings are consistent with a previous CDC report, published in early April 2017, which noted that, of completed pregnancies with a possible Zika virus infection, 5% may result in Zika-related complications.

At the moment, the CDC acknowledges that congenital Zika virus infection can cause brain abnormalities and vision problems. Some infants may even have trouble swallowing and sitting up due to constricted joints, while others may cry continuously and appear to be inconsolable. Although much is known about Zika-associated complications there is still much to be discovered, especially in infants who are seemingly healthy upon birth. This makes follow-up care even more crucial, since it can also inform management strategies.

During the telebriefing, Dr. Schuchat and Dr. Honein commented on surveillance efforts in US territories, noting that previously, US territories did not follow a standard case definition. However, now, all US states and territories are reporting using the CDC’s standard case definition, which can be found online.

During the telebriefing, the CDC acknowledged the gap in Zika case reporting that was occurring in Puerto Rico last year. Those on the call were assured that health authorities in Puerto Rico are now doing a “very good job” in terms of evaluating and reporting Zika infection cases, specifically among pregnant women.

In addition, Puerto Rico recently declared the Zika virus epidemic over on the island. Health officials made this announcement after assessing the situation and finding that there has been a sharp drop in the number of reported cases of Zika virus infection. The CDC is “very pleased the incidence is low;” however, they continue to advise pregnant women against travel to areas with Zika transmission, including Puerto Rico, especially in the coming months. When asked if the CDC acknowledges the epidemic to be over in Puerto Rico, CDC officials stated that they believe there are “low levels of transmission in some areas.”

According to Dr. Schuchat, this report indicates that Zika is a “very serious virus when acquired during pregnancy,” regardless of when the mother was infected during pregnancy. CDC officials also acknowledge that Zika has taken an economic toll on both families with infants who have Zika-related complications, and on tourism to areas where the virus is endemic. However, the CDC is working with the Centers for Medicare and Medicaid Services to reduce the economic burden of care for these infants.

Based on past experience with Dengue and Chikungunya outbreaks, the CDC reports that the Zika virus is an ongoing threat in Puerto Rico and other areas. The CDC recommends that individuals living in areas with active Zika virus transmission take the necessary precautions to avoid mosquito bites.