PARIS • Scientists say they have 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 GBS patients, supported by blood tests, proved that the mosquito-borne virus was the culprit, they said.

"This is the first evidence for Zika virus causing Guillain-Barre syndrome," the study published yesterday 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 5 per cent of cases, and another 5 per cent 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.

"In areas that will be hit by the Zika epidemic, we need to think about reinforcing intensive care capacity," said Dr Arnaud Fontanet, a co-author of the study and a professor at the Institut Pasteur in Paris.

By itself, Zika is no more threatening than a bad cold or mild flu. Sometimes, there are no symptoms at all.

But the rapidly expanding virus - present in nearly four dozen countries, according to the World Health Organisation (WHO) - is suspected to be the cause of a sudden increase in cases of neonatal microcephaly, a severe deformation of the brain and skull among newborn babies.

Brazil reported last week 583 confirmed cases of babies with the irreversible birth defect since October last year, four times the previous annual average.

Zika is spread among humans by the Aedes aegypti mosquito, which is found in 130 nations. 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.

For Dr 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 said.

Three kinds of evidence supported this conclusion, he said.

The first was a 20-fold increase in the number of GBS cases during the Zika epidemic. The second was that 90 per cent of the patients struck had been infected the week before by the mosquito-borne virus.

"We found traces of the recent presence of Zika in 100 per cent of the GBS patients", including antibodies built up to fight the virus, said Dr 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, that the biological mechanism by which Zika triggers the muscle-depleting syndrome has yet to be identified.

AGENCE FRANCE-PRESSE