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Cases of Zika-linked birth defects dropped in Brazil

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Staff Writer | April 3, 2017
Brazil experienced a smaller-than-expected increase in cases of microcephaly in 2016, despite the continued spread of the mosquito-borne Zika virus.
Virus in Brazil   Zika is now endemic in the Americas
Researchers predicted 1,133 cases of microcephaly would occur between May and December 2016, but only 83 cases were reported by local health officials, said senior researcher Christopher Dye.

He is director of strategy, policy and information for the World Health Organization in Geneva, Switzerland.

Zika causes microcephaly, a birth defect where babies are born with abnormally small skulls and underdeveloped brains.

Brazil served as the epicenter of the 2015 Zika outbreak in South America, and it was the country that endured the highest rates of microcephaly and other Zika-related neurological birth defects that year.

Zika reemerged in Brazil early in 2016, and so researchers expected more cases of microcephaly to crop up as the months passed.

"We expected to see microcephaly cases increasing from May onwards," Dye said. Instead, few cases of microcephaly occurred.

The researchers think the supposed Zika resurgence early last year might have been due to doctors mistakenly diagnosing it when a patient's illness actually was caused by another tropical virus.

"Most likely, we think, is that cases reported as Zika were actually due to another virus, Chikungunya, which causes fever and rash -- similar symptoms to Zika -- but not microcephaly as a result of infections in pregnancy," Dye said.

This is possible, the study authors said, because the 2015 Zika outbreak likely created "herd immunity" against the virus among Brazilians. Most people in that country have endured infection and now are immune against the virus, providing little opportunity for mosquitoes to spread Zika person-to-person through their bite, the researchers suggested.

"It is likely that Zika is now endemic in the Americas and that, once the susceptible population builds up again -- mainly through the births of children who have not previously been exposed -- then we will see new outbreaks, unless the mosquito population is reduced substantially or we have a vaccine," Dye said. "The outbreaks we saw in 2015 may not happen again for a few years -- perhaps a decade."

Another possibility is that Zika may interact with some other factors -- possibly other tropical disease viruses -- during pregnancy to cause microcephaly in developing fetuses, the researchers added.