A 16-member team of epidemiologists from the Centers for Disease Control and Prevention began training dozens of Brazilian counterparts in Joao Pessoa, Brazil, in preparation to begin work on Tuesday. The researchers will gather data on hundreds of Brazilian women and their children.
"Having the data at this point in time are very critically important for understanding the impact Zika might be having in the future and as it spreads in the region," says J. Erin Staples, a CDC medical officer leading the CDC team in Brazil.
This story is part of NPR's ongoing coverage of Zika virus.
Scientists believe there has been a significant increase in microcephaly, a condition that causes babies to be born with abnormally small heads and brain, since Zika started spreading widely in Brazil. That strongly suggests that the virus is to blame. Zika has been found in the brains of a small number of babies with microcephaly, but that still does not prove the virus is to blame.
"Taking that and saying the 4,000 to 5,000 cases of microcephaly reported in Brazil are due to Zika is a very big jump," Staples says.
To try to better understand whether the association is real, the CDC is launching what is known as a "case-control" study, which involves comparing cases of people with diseases with similar people who did not have disease to try to identify the cause.
On Monday, the CDC formed eight teams of American and Brazilian investigators. On Tuesday, the teams fanned out in the region searching for about 100 mothers who gave birth to babies with microcephaly since the Zika outbreak began.
The teams will collect blood samples from the mothers and children to test for signs of Zika infection, examine the babies and gather detailed information from the mothers. Among the information the researchers plan to collect is whether the women experienced any symptoms of Zika infection during their pregnancies and, if so, when.
The investigators will also ask the women a series of questions aimed at identifying any other factors that may have played a role in the birth defects, such as whether the women had any other infections, including toxoplasmosis or cytomegalovirus. In addition, they'll try to determine whether the women were exposed to anything in the environment, such as mercury or pesticides, that could be to blame.
The investigators planned to do the same thing with several hundred other women who are similar in terms of their ages, locations and socioeconomic status who gave birth to healthy babies around the same time. The researchers will then compare the two groups.
The hope is that this study will either provide more definitive evidence establishing the link between Zika and microcephaly or identify other factors that may be to blame, either in combination with Zika or instead of Zika.
"What we're really trying to do is to better understand what's going on in terms of the size, the scope and the causes of microcephaly," Staples told NPR's Shots blog last week. "We want to better understand the role the Zika virus has in the outbreak of microcephaly."
Staples estimated it would take three or four weeks to collect all the data, but was unsure how long the analysis would take.
But "hopefully from all this work we will glean some information that will be able to help us prevent other children from being born microcephaly," Staples says.
The virus is spreading rapidly throughout Latin America and the Caribbean. Dozens of travelers have arrived in the United States after being infected elsewhere, but so far the virus is only known to have spread once in the continental United States, and that was through sexual contact.
The mosquito that spreads the virus is found in parts of the United States, but health authorities have said they are optimistic any transmission in this country would be quickly controlled.
— via NPR