Microcephaly Risk Highest With Zika Virus Infection During First or Early Second Trimester of Pregnancy

According to a recent study published by the Centers for Disease Control and Prevention (CDC), data from Bahia State, Brazil, show that Zika virus infection during the first trimester of pregnancy, or early in the second trimester, is associated with the observed increase in infants born with microcephaly.

In their study, Jennita Reefhuis, PhD, from the CDC, Atlanta, Georgia, and colleagues used published data to highlight the expected periods of exposure and weeks of delivery for pregnant women potentially infected with Zika virus during outbreaks in the city of Salvador, Bahia State, Brazil.

Zika virus is an emerging mosquito-borne virus that is transmitted to people predominantly through Aedes mosquitoes. Zika virus disease was first identified in 1947 in Uganda. For decades, the virus predominantly infected monkeys and rarely infected humans. However, in 2007, the virus spread further, causing an outbreak of disease on Yap island in the Federated States of Micronesia. From there, it spread to French Polynesia in 2013, and Easter Island in 2014. Then in 2015, Zika virus disease outbreaks were confirmed in Brazil and Colombia.

Most people who are infected with Zika virus experience no symptoms. However, if symptoms do occur, they commonly include low grade fever, skin rash, conjunctivitis, joint pain, and headache. However, Zika virus infection of women during pregnancy has been linked to birth defects, in particular microcephaly.

Indeed, in February 2016, the World Health Organization declared a Public Health Emergency of International Concern, because of the significant increase in cases of microcephaly and other neurological disorders.

“The marked increase in infants born with microcephaly in Brazil after a 2015 outbreak of Zika virus disease suggests an association between maternal Zika virus infection and congenital microcephaly,” the authors write.

With this in mind, they conducted a study to project the effects of Zika virus infections, and also to identify the gestational period of highest risk. They produced a graphical illustration that combined data on cases of Zika virus disease and microcephaly in the city of Salvador, with weekly birth cohort information.

Their analysis showed that Zika virus transmission was highest from March to June 2015, during which time a cohort of pregnant women could have been infected at different times during their pregnancies. Across all birth cohorts, Zika virus activity was also shown to be highest from March 22 to May 31, 2015.

For pregnancies that began in December 2014 or January 2015, Zika virus infection most likely occurred late in the first trimester of pregnancy, or during the second trimester, and term births would have occurred during September and October 2015.

For pregnancies that began at any time from late February to May 2015, Zika virus infection most likely occurred during the first trimester, and term births would have occurred from November 2015 to February 2016.

According to the authors, the spike in the number of microcephaly cases in Bahia State began with October births, and increased substantially between November 2015 and January 2016. In Salvador, these births were associated with Zika virus infection that most likely occurred in the first trimester of pregnancy, or early during the second trimester.

The authors also developed a modifiable spreadsheet tool that public health officials and researchers in areas with local Zika virus transmission can use to plan for births in women who were infected with Zika virus at different stages of pregnancy.

“Conducting surveillance, [not only] for microcephaly, but also other pregnancy outcomes, such as pregnancy loss and other birth defects, will enable continued evaluation of any effects” that Zika virus disease might have on pregnancy, the authors conclude.

Zika virus has now also been reported in the US, in travelers returning from areas where the disease is spreading. Consequently, the CDC recommends testing of potentially exposed individuals with signs or symptoms consistent with Zika virus disease.

In another recent CDC report, Sharoda Dasgupta, PhD, and colleagues shared data from Zika virus testing that was performed from January 3 to March 5, 2016, for 4,534 individuals from the US states and District of Columbia (DC)—3,335 (73.6%) of whom were pregnant women.

“Among persons from US states and DC receiving testing for Zika virus, few persons had confirmed Zika virus infection. Approximately 99% of asymptomatic pregnant women who received testing did not have Zika virus infection,” Dr Dasgupta and colleagues write.

Among 1,541 individuals who reported one or more Zika-virus associated symptoms, 182 (11.8%) had confirmed Zika virus infection.

However, among 2,425 asymptomatic pregnant women, only seven (0.3%) had confirmed Zika virus infection.

However, because of the risk of serious pregnancy and neonatal outcomes that have been linked to Zika virus infection in pregnant women, the CDC recommends that health care providers should continue to also test pregnant women with possible exposure to the virus, even if they do not have symptoms.

Dr. Parry graduated from the University of Liverpool, England in 1997 and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals, and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee.