A recent study is spurring newfound fears about the Zika virus, with evidence showing third-trimester babies can still suffer brain damage from the virus, even if they don't suffer from microcephaly.
"Congenital Zika virus syndrome in Brazil: a case series of the first 1501 livebirths with complete investigation," was published on June 29 in The Lancet, a British medical journal.
Worryingly, the study concluded that "screening criteria must be revised in order to detect all affected newborn babies," because the usual indicators of Zika damage in babies -- including microcephaly diagnoses, abnormal head measurements, and mothers who suffered from rashes during pregnancy -- no longer cover all Zika diagnoses.
"One in five definite or probable cases presented head circumferences in the normal range, and for one-third of definite and probable cases there was no history of a rash during pregnancy," researchers found.
Microcephaly is a birth defect in which a baby's head is smaller than healthy babies of the same gender and age, according to the Centers for Disease Control. In addition to the physical deformities, newborns afflicted with the defect also suffer from cognitive defects and have smaller brains. Many of those children suffer from slower development as they age, as well.
Zika can be transmitted through mosquito bites and is passed from mother to child during pregnancy. But it can also be transmitted through sexual intercourse, blood transfusions, and laboratory exposure, the CDC says.
As scientists began studying Zika, microcephaly was considered a reliable indicator of whether a mother had been infected with the virus. Rashes during pregnancy were also a telltale symptom that spurred pregnant women to undergo additional tests.
“We really want to understand which time period in pregnancy poses the highest risk, and if there are periods of pregnancy that pose no risks,” Peggy Honein, of the CDC's Zika Response Team, told The Atlantic. “That would be really helpful. It is a critical question.”
With the new study -- and a similar study published in the New England Journal of Medicine in March -- it's become clear that babies can be impacted by Zika even if they don't exhibit the common symptoms.
“In view of the huge interest in the epidemic, we believe that under-reporting of microcephaly cases is rare," the researchers wrote, "but newborn babies affected late in pregnancy might fail to be reported as their heads will be in the normal range.”
The researchers singled out one group of more than 600 babies diagnosed with "probable or definite" cases of Zika virus syndrome, and found that 1 in 5 of them had normal-sized heads, The Atlantic reported.
“There is no question,” the researchers wrote, that “focusing on microcephaly alone will underestimate the true magnitude of this major epidemic.”