The World Health Organization says it could take four to six months to say with certainty whether the Zika virus is associated with the birth defect microcephaly.
At a news conference in Geneva on Friday, Dr. Bruce Aylward, WHO's executive director of outbreaks and health emergencies, said causation has not been proved, but they continue to proceed as if the association is causal or "guilty until proven innocent" for both microcephaly and Guillain-Barré syndrome, a rare disorder of muscle weakness and sometimes paralysis.
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Aylward said they are getting more information showing a stronger association between microcephaly, characterized by an abnormally small head and incomplete brain development at times, and infection with the virus. The balance will probably tip once women in Colombia and other countries give birth, meaning mid-summer at the latest.
"A lot can be done in terms of reducing the intensity of Zika transmission and the accumulating evidence is that this has got to be done and done very, very urgently, because there is a very real possibility that this virus could be responsible for some of the horrific consequences we've been talking about in terms of children affected by this disease," he said.
The investigation testing the link between Zika virus and microcephaly was sparked after Brazilian doctors noticed an increase last fall in babies with the birth defect. There are many causes, both infectious and non-infectious. The cases closely followed the country's first outbreak of the tropical virus Zika.
WHO has declared the clusters of microcephaly and other neurological complications associated with Zika "an extraordinary event and public health threat to other parts of the world."
Preliminary results of the two case control studies conducted by the U.S. Centers for Disease Control and Prevention
and Brazilian biomedical research centres in the northeastern states of Bahia and Paraiba should be ready "this spring," said CDC principal deputy director Anne Schuchat.
"Scientists are increasingly confident that Zika is causing microcephaly, but people may have different judgments about how much proof is enough," Schuchat told reporters during a two-day meeting in the Brazilian capital.
In the case-control studies, "cases" or babies born with microcephaly are compared in detail with those born without the condition. Researchers are checking for risk factors such as whether the mothers had symptoms of the Zika virus during their pregnancies, and if so, in which trimester, as well as looking at other potential exposures during pregnancy, such as other infections or environmental toxins.
The virus itself causes a mild illness for about a week, and on its own wouldn't have triggered an emergency.
To reduce transmission, efforts focus on controlling the Aedes aegypti mosquito that the UN health body has described as an "opportunistic and tenacious menace."
Dr. Pedro Alonso, director of WHO's global malaria program, described proven and experimental mosquito control strategies, such as fogging to get rid of adults, use of fish with an appetite for larvae, and ways to eliminate the larvae.
"It is not that we're waiting for a magic bullet of one of these new, fancy, potentially important new technologies, but a lot can be done in terms of controlling this outbreak and massively reducing the disease burden with currently available tools and strategies," Alonso said.
As of today, 36 countries are known to have transmission of the virus, including 28 in the Americas, Aylward said.