US and Brazilian researchers are recruiting Brazilian mothers and babies for one of the biggest government-led studies investigating if the Zika virus is linked to microcephaly, a rare birth defect.
Brazil has confirmed more than 500 cases of microcephaly, defined by an unusually small head, since the start of the outbreak. Over 3900 additional suspected cases are being investigated.
Brazil normally only sees about 150 cases a year. The spike in microcephaly prompted the World Health Organisation on February 1 to declare Zika an international health emergency.
It will pair researchers at the US Centers for Disease Control and Prevention with Brazilian experts to examine babies already born with microcephaly and their mothers to determine whether Zika or some other infection caused their malady.
"What we're trying to do is to better define the association between children that have been diagnosed with microcephaly and whether or not they might have evidence of congenital Zika virus infection," said Dr Erin Staples, a CDC medical epidemiologist leading the study in Paraiba.
In an interview, Staples offered a first glimpse at the long-anticipated study. Researchers hope to enrol 100 mothers and their babies with microcephaly. These will be matched with 300 to 400 pairs of healthy mothers and their babies. Initial results are expected this spring, a senior CDC official said.
Current diagnostic tests looking for Zika antibodies are limited because they closely resemble dengue, a related virus common in Brazil. The team hopes that taking samples from both mothers and their babies will give a more precise picture of whether Zika was involved. They will also look for other exposures that might explain why a baby developed microcephaly.
The study's design should help determine the relative risk of microcephaly in babies whose mothers were infected with Zika.
Staples said the work could provide stronger confirmation of a link, but that it would take years of scientific investigation to prove whether Zika actually causes microcephaly.