Zika brain damage is going undetected in babies because tests only screen for one defect

One in five Zika infections in babies may go undiagnosed because tests solely focus on screening for one birth defect caused by the virus, a new study revealed.

Zika screenings in newborns look exclusively for microcephaly, a defect in which babies have underdeveloped brains and abnormally small heads. Yet, not all babies affected by Zika have microcephaly.

But babies infected by the virus late in pregnancy may have brain damage and normal-sized heads, and so their cases are not being accurately diagnosed.

That's why the screenings must look for symptoms and signs of brain abnormalities, regardless of head size, scientists from Universidade Federal de Pelotas in Brazil wrote in the journal The Lancet.

The study was the largest ever of its kind, looking at data from the 2015 Zika outbreak in Brazil that was collected by the country's Ministry of Health. The team analyzed 1,501 live-born suspected Zika cases.

The scientists found that three out of five suspected Zika cases ended up being normal newborns who just had small heads.

"However, one in five definite or probable Zika cases had head circumference values in the normal range. Therefore, the current focus on microcephaly screening alone is too narrow," lead study author Professor Cesar G. Victora said in a statement.

The study also found that mothers who developed Zika-related rashes late in pregnancy were more likely to have children with brain abnormalities, despite normal head sizes.

Cranium development usually happens by week 30, and so that's why babies affected by Zika late in pregnancy can have normal head size.

"Although we believe that the underreporting of microcephaly cases is rare during the epidemic, newborns infected with the virus late in pregnancy may go unreported," Victora said. "Moreover, for a third of these definite or probable cases there was no history of rash during pregnancy."