Although microcephaly rates increased in Brazil with the introduction of the Zika virus, it is estimated that nearly 90% of cases before the epidemic were not reported, according to findings published in Pediatrics.
“The Zika virus was first noticed in Brazil in May 2015. Shortly thereafter, in October, increasing notification of microcephaly at birth was reported, and a possible link with the Zika virus was announced,” Antônio A. Silva, MD, PhD, from the department of public health at the Federal University of Maranhão, São Luís, Brazil, and colleagues wrote.
“… Prevalence rates of severe microcephaly at birth in Brazil based on the Brazilian Live Birth Information System are underestimated, and the Latin American Collaborative Study of Congenital Malformations estimated that the rate of underreporting is 66%,” the researchers continued. “Thus, baseline estimates of the prevalence of microcephaly before the Zika virus epidemic are confined to a few geographical areas and are not population based.”
To evaluate the baseline prevalence of microcephaly at birth within two Brazilian cities and the risk factors associated with the condition before the Zika outbreak, Silva and colleagues used population-based data collected from cohort studies conducted in Ribeirão Preto RP and São Luís (SL). All children included were born to mothers who were residents and were born in a hospital setting (n = 7,376 in RP, 4,220 in SL).
The researchers determined the gestational age of the child from their mother’s last normal menstrual period or obstetric ultrasonography, when available. The International Fetal and Newborn Growth Consortium for the 21st Century and the Brazilian Ministry of Health’s criteria on microcephaly were used to define the condition at birth. A hierarchized logistical model was also used to assess the risk factors for microcephaly, proportionate and disproportionate microcephaly and severe microcephaly.
Microcephaly (more than two standard deviations [SDs] below the mean for gestational age and sex) was more prevalent in the SL cohort (3.5%) than the RP cohort (2.5%). The SL cohort also had a higher level of severe microcephaly (more than 3 SDs below the mean; 0.7%) than those located within RP (0.5%). The researchers determined that the condition is largely underestimated, with an underreporting rate of around 90%.
Risk factors for the condition consistently included low maternal education level, living in a consensual union or without a companion, maternal smoking during pregnancy, primiparity, vaginal delivery and intrauterine growth restriction.
“In view of the severity of the epidemic of congenital Zika syndrome, it is highly advisable that countries set up head circumference monitoring systems to detect early signs of this syndrome,” Silva and colleagues wrote. “Accurate surveillance of congenital anomalies is a necessity. However, the number of cases not ascertained in Brazil is currently high. If the policy is reporting all head circumferences more than 2 SDs below the mean for gestational age and sex, the current number of reported cases is low compared with our population-based estimates, with underreporting at around 90%.” – by Katherine Bortz
Disclosures: The authors report no relevant financial disclosures.