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ARTICLE

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

MAY 04, 2016 | NICOLA M. PARRY, BVSC, MRCVS, MSC, DIPACVP
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.
 


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