Study: First Zika microcephaly wave in Brazil was outlier

mother with newborn

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About 70% of all Zika-related microcephaly cases in Brazil resulted from the first wave of infections in 2015, according to a study yesterday in The Lancet by Brazilian researchers who describes the two major waves of Zika infections and the ensuing microcephaly incidence.

The study is based on data obtained through the Brazilian Ministry of Health's surveillance systems from Jan 1, 2015, to Nov 12, 2016, and shows two distinct waves of Zika infections: An initial wave in late 2015 in the northeastern part of Brazil was linked to 70.4% of microcephaly cases documented in 2015 and 2016, while a second wave in 2016 dispersed across the whole country accounted for the remaining cases.

Very high rate in NE Brazil

The researchers found a total of 1,950 Zika-related cases of microcephaly in 2015 to 2016 in Brazil. At the height of the first wave in 2016, peak monthly occurrence was estimated at 49.9 cases per 10,000 live births. During the second wave, the estimated monthly peaks varied from 3.2 cases to 15 cases per 10,000 live births, or at most less than a third of the rate during the 2016 peak.

This is the first study that looked at the microcephaly cases in terms of regional differences, the authors said. They added that the results, taken with data from other countries battling Zika, provide more evidence that the first wave in Brazil was an outlier.

But there are no clear answers yet as to what caused the increase in microcephaly cases. The authors said, "The marked variation in the frequency of infection-related microcephaly across regions and time is puzzling."

In a commentary on the study, two UK experts expand on the data: "The difference in the association between Zika virus and microcephaly in northeastern Brazil and the rest of the country—which the authors do not present—would be more than 100 times greater."

"Other cohorts assessed in other regions of Brazil and Latin America will provide further robust evidence on whether variations exist in risk, and if so, why," said Laura C. Rodrigues, PhD, and Enny S. Paixao, MSc, of the London School of Hygiene & Tropical Medicine. "Could it be because of severity of Zika infection, viral load, or cofactors (the most popular being previous or co-infection with dengue)?"

Placenta testing for Zika evidence

In other Zika news, a report today in Morbidity and Mortality Weekly Report (MMWR) describes the usefulness of testing placental and fetal tissue for Zika virus after a live birth. The study is based on tissue specimens from 627 completed pregnancies that involved possible maternal Zika virus exposure from the 50 states and Washington, D.C., that were submitted to the Centers for Disease Control and Prevention (CDC) in 2016 for testing.

These specimens included placental tissues from 546 live births and placental and fetal tissues from 81 pregnancy losses. "Among live births, placental tissue RT-PCR [reverse-transcription polymerase chain reaction] provided confirmation of recent maternal Zika virus infection for 47 (10%) women who otherwise did not have a definitive diagnosis," the authors said.

The authors write that testing the placental tissues for Zika virus can offer a definitive diagnosis that separates Zika infection from other flaviviruses, or if the mother's test results are inconclusive.

"Tissue analysis can be valuable when maternal serologic testing either cannot differentiate between Zika virus and other related flaviviruses, or has been conducted >12 weeks after possible maternal exposure, and infant Zika virus testing is not definitive, negative, or not performed. Tissue analysis provides another opportunity to confirm maternal Zika virus infection, which can be important to both families and health care providers," the authors concluded.

They add, however, that a positive placental specimen does not differentiate between maternal or congenital infections, making prenatal testing still the most valuable tool in the management of pregnant women with possible Zika exposure.

See also:

Jun 21 Lancet study

Jun 21 Lancet commentary

Jun 23 MMWR study

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