More research needed into Zika-microcephaly link: Scientists

The release of new figures apparently finding fewer cases of microcephaly in Brazil than first feared is adding force to calls for more research into the link between the rare birth defect and the spreading Zika virus.

Health experts have been looking at 4,180 suspected cases of microcephaly reported since October in Brazil, where authorities said the birth defect could be linked to the virus and announced that 220,000 military personnel were being deployed to help eradicate the Aedes aegypti mosquito that transmits Zika.

But yesterday, Health Ministry officials said they had done a more intense analysis of more than 700 of those cases, confirming 270 cases and ruling out 462 others.

What this means is hard to say, according to some experts. It does not answer whether the tropical Zika virus is causing the babies to have unusually small heads. Nor does it really tell us how big the problem is.

"I don't think we should lower our alarm over the Zika outbreak," said Paul Roepe, co-director of Georgetown University's Center for Infectious Disease.

Brazilian officials still say they believe there's a sharp increase in cases of microcephaly and strongly suspect the Zika virus is to blame.

The concern is strong enough that the US Centers for Disease Control and Prevention this month warned pregnant women to reconsider visits to areas where Zika is present, and officials in El Salvador, Colombia and Brazil have suggested women stop getting pregnant until the crisis has passed.

But the World Health Organisation and others have stressed that any link between Zika and the defect remains circumstantial and is not yet proven scientifically. And the new figures were a reminder of just how little is known about the disease and its effects.

The arrival of the mosquito-borne illness in Brazil initially caused little alarm as the virus' symptoms are generally much milder than those of dengue.

Then late last year, after noting what they said was a spike in the birth defect, Brazilian authorities for the first time asked doctors to report cases of patients in their care.

So there are no solid numbers to compare with the new tally. In 2014, only about 150 cases were reported in Brazil in a year a surprisingly small amount for a large country with nearly 3 million births a year. The United States, with about 4 million births a year, has an estimated 2,500 cases of microcephaly a year, said Margaret Honein, a CDC epidemiologist.