The arrival of Zika virus in Brazil doubled the rate of birth defects involving the nervous system, including microcephaly, researchers reported Wednesday.
Rates of Guillain-Barré syndrome — a rare, paralyzing side-effect of some infections — nearly tripled, the researchers said. And rates of other inflammatory conditions such as encephalitis doubled in the northeastern part of Brazil that was hardest hit by Zika.
Separately, the World Health Organization tweaked its statement on Zika and the rise in rates of birth defects and Guillain-Barré syndrome (GBS), saying the virus is the "most likely explanation" for both.
For their report, the team at Brazil's Oswaldo Cruz Foundation looked at rates of birth defects, GBS and nervous system inflammatory conditions before and after the arrival of Zika virus.
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"Beginning in mid-2014, we observed an unprecedented and significant rise in the hospitalization rate for congenital malformations of the nervous system, Guillain-Barré syndrome, encephalitis, myelitis, and encephalomyelitis," the organization explained in the report, which was published in the Center for Disease Control and Prevention journal Emerging Infectious Diseases.
"We found increases in the number of hospitalizations for congenital malformations of the nervous system, GBS, and some inflammatory diseases of the central nervous system. These complications began to cause more hospitalizations, with strong fluctuations over the course of the study period, beginning even before the first warning in November 2015 about the possible effects of infection with Zika virus on microcephaly and other malformations."
Zika has swept across much of Latin America and the Caribbean, and there's no doubt that it can cause birth defects that permanently damage the brain and cause miscarriages. The virus invades brain and nervous system tissue, with catastrophic effects in a developing fetus.
Like other infections, it can also cause Guillain-Barré, which causes usually temporary paralysis, and on very rare occasions, other inflammations of the nervous system.
Zika's also arrived in south Florida, infecting at least 56 people locally. Thousands of travelers have carried the virus to the U.S.
It's spread mostly by mosquitos but also through sex.
Christovam Barcellos and colleagues in Brazil looked at data from Brazil's hard-hit northeastern region. Before 2014, about 40 out of every 100,000 babies born there had birth defects affecting their nervous systems, including their brains.
After Zika's arrival, this rate quadrupled to 170 per 100,000 births. By February of this year, the rate of such birth defects across Brazil had doubled, the team reported.
Similarly, rates of Guillain-Barré syndrome soared.
"In the Northeast region, the hospitalization rate for GBS was 0.05 per 100,000 residents until May 2015, when an outbreak occurred, which peaked in July 2015," they wrote.
"From June 2015 through February 2016, the hospitalization rate was 0.11 per 100,000 residents, an increase by a factor of 2.7."
There was a similar increase in rarer nervous system conditions, including encephalitis, myelitis, and encephalomyelitis, the team found. These are inflammations of the brain and spinal cord that can be deadly.
The researchers say it's possible that other factors help Zika cause such devastating effects, including other infections such as Zika's cousins dengue virus and chikungunya virus, which were also circulating in Brazil's northeast.
"The region had one of the worst droughts in its history, during 2010-2013, resulting in contamination of drinking water, food insecurity, and outbreaks of diarrhea across the region, which could have affected the immunity status of the population," they wrote.
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WHO's expert panel on Zika said it's worth at least ruling out this possibility.
"The expert panel recognizes that Zika virus alone may not be sufficient to cause either congenital brain abnormalities or GBS. It is not known whether these effects depend on as yet uncharacterized co-factors being present. Nor is it known whether dengue virus plays a part; dengue virus is carried by the same species of mosquito and has circulated in many countries during the same period," WHO said in the statement posted Wednesday.
But WHO also said Zika at the very least plays an important role in causing GBS and birth defects.
"The most likely explanation of available evidence from outbreaks of Zika virus infection and clusters of microcephaly is that Zika virus infection during pregnancy is a cause of congenital brain abnormalities including microcephaly," it said.
"The most likely explanation of available evidence from outbreaks of Zika virus infection and Guillain-Barré syndrome in whole populations is that Zika virus infection is a trigger of Guillain-Barré syndrome."