Scientists on Tuesday said they had confirmed that the Zika virus sweeping Latin America and blamed for severe birth defects can also trigger a dangerous neurological disorder.
In a study published in the medical journal The Lancet, a team probed Zika's suspected role in a 2013-2014 outbreak in French Polynesia of Guillain-Barre syndrome (GBS) - a rare condition in which the body's immune system attacks a part of the nervous system that controls muscle strength.
Research into patients who fell ill with GBS, supported by blood tests, proved that the mosquito-borne virus was the culprit, they said.
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"This is the first evidence for Zika virus causing Guillain-Barre syndrome," the study said.
The syndrome - which can also be caused by bacterial infections as well as the dengue and chikungunya viruses - provokes muscle weakness in the legs and arms.
In rich nations, GBS is lethal in about five percent of cases, and another five percent suffer lasting disabilities. More than a quarter of patients require intensive care.
With 1.5 million cases of Zika infection already recorded in Brazil, and tens of thousands in neighbouring countries, researchers warn that an outbreak of Guillain-Barre could strain healthcare facilities, especially outside of big cities.
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"In areas that will be hit by the Zika epidemic, we need to think about reinforcing intensive care capacity," said Arnaud Fontanet, a co-author of the study and a professor at the Emerging Diseases Epidemiology Unit of the Institut Pasteur in Paris.
"We know that a certain number of those patients are going to develop GBS, and 30 percent of them are going to need intensive care, especially for assisted breathing," he told AFP.
By itself, Zika is no more threatening than a bad cold or a mild case of the flu. Sometimes there are no symptoms at all.
But the rapidly expanding virus - present in nearly four dozen countries, according to the World Health Organization (WHO) - is suspected to be the cause a sudden increase in cases of neonatal microcephaly, a severe deformation of the brain and skull among newborns.
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Brazil reported last week 583 confirmed cases of babies with the irreversible birth defect since October 2015, four times the previous annual average.
Zika is spread among humans by the Aedes aegypti mosquito, which is found in 130 nations. But recent evidence suggests that it can also be sexually transmitted by men carrying the virus.
In the study, two dozen researchers identified 42 cases of Guillain-Barre in French Polynesia in the aftermath of a Zika epidemic that infected some 200,000 people.
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For Fontanet, there was no doubt that the virus caused the upsurge in GBS cases.
"The links are as strong as they would be for saying that tobacco causes lung cancer," he told AFP.
Three kinds of evidence supported this conclusion, he said.
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The first was a 20-fold increase in the number of GBS cases during the Zika epidemic.
The second was that 90 percent the patients struck with the debilitating syndrome had been infected the week before by the mosquito-borne virus.
Both epidemiological findings were supported by blood analysis.
"We found traces of the recent presence of Zika in 100 percent of the GBS patients," including antibodies built up to fight the virus, said Fontanet.
The researchers were also able to exclude previous infection with the dengue virus - also common in French Polynesia - as a cause.
They did acknowledge, however, the biological mechanism by which Zika triggers the muscle-depleting syndrome has yet to be identified. Some experts who did not participate in the research agreed that it was a breakthrough.
"This study provides the most compelling evidence to date of a causative link between Zika virus infection and the serious neurological condition Guillain-Barre syndrome," said Jeremy Farrar, director of the Wellcome Trust in Britain.
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"The scale of the crisis unfolding in Latin America has taken us all by surprise, and we should be prepared for further unforeseen complications... in the coming weeks and months."
Others, though, cautioned that the findings were not conclusive, and may not apply directly to other affected regions.
"A significant amount of work has still to be undertaken before the same conclusions can be extended to the Zika outbreak in South America," said Peter Barlow, spokesman for the British Society for Immunology.
On February 1, the WHO declared a public health emergency due to rising cases of microcephaly and Guillain-Barre syndrome, even though the link to Zika remained circumstantial.