Liz Szabo

USA TODAY

For the first time, researchers have shown that the Zika virus can cause Guillain-Barre syndrome, a rare condition in which the immune system attacks nerve cells and causes paralysis.

Five Latin American countries with Zika epidemics — Brazil, El Salvador, Venezuela, Colombia and Suriname — have reported increases in Guillain-Barre syndrome. Until now, however, doctors didn't have scientific evidence showing the virus actually triggers the condition.

The new evidence comes from a study published Monday in The Lancet, in which researchers analyzed 42 cases of Guillain-Barre syndrome that were diagnosed during a Zika outbreak in French Polynesia from 2013 to 2014. Scientists estimate about 66% of the territory's residents were infected.

In the study, doctors compared patients with Guillain-Barre to those hospitalized for other reasons. All 42 of the Guillain-Barre patients had antibodies designed to neutralize Zika's effects on the body, compared with about half of control patients.

About 88% of the Guillain-Barre patients had experienced Zika-like symptoms — such as fever, rash and joint pain — about six days before developing paralysis, according to the study.

That's a significant detail, according to an accompanying commentary from researchers David Smith at the University of Western Australia and John Mackenzie of Curtin University, also in Australia.

Only about 20% of patients with Zika infections develop symptoms, according to the Centers for Disease Control and Prevention. The study's results suggest that people who are infected with Zika but don't develop symptoms might be at lower risk from complications, Smith and Mackenzie wrote.

Guillain-Barre typically begins with weakness in the legs, then moves up the body. One in four patients end up on a ventilator because the condition paralyzes the muscles they need to breathe. Although some patients recover fully, 20% are left with a significant disability and about 5% die, Smith and Mackenzie wrote.

About 38% of Guillain-Barre patients in French Polynesia needed intensive care, usually because patients needed breathing machines, according to the study.

Yet patients in this outbreak recovered more quickly than usual; about half were able to walk without help three months later, according to the study.

Guillain-Barre syndrome typically develops after people recover from a virus, such as influenza, or a bacteria, such as Campylobacter, one of the most common causes of food poisoning. Other infections that can cause Guillain-Barre syndrome include dengue virus and West Nile virus, which are both transmitted by the same mosquito that spreads Zika.

Although most patients in the study had been infected at some point in the past with dengue, researchers found no evidence that a history of dengue increased the risk of Guillain-Barre.

Zika was first isolated in a monkey in Uganda in 1947 and caused its first known human case in 1954 in Nigeria, according to background information in The Lancet study. For the next 50 years, Zika caused only sporadic cases.

The first epidemic in people was reported in 2007 on Yap island in the Pacific nation of Micronesia, where 73% of residents were infected, said Arnaud Fontanet, an author of The Lancet study and head of the emerging diseases epidemiology unit at the Institut Pasteur in Paris.

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Fontanet said it's not possible to say what percentage of people in Latin America and the Caribbean will be infected with Zika or affected by Guillain-Barre. He notes that Zika may spread differently on a large continent than it did on these islands.

More than 30 countries and territories in the Americas are now battling Zika outbreaks, including Puerto Rico and the U.S. Virgin Islands.

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Guillain-Barre is normally rare, with about 1.1 cases per 100,000 people in Europe and North America. According to the new study, that rate climbed to 24 cases per 100,000 during the French Polynesia outbreak. The condition is more common as people age and slightly more common in men than women.

Scientists are also investigating an increase in birth defects in Brazil, which has had a surge of infants born with microcephaly, a condition in which babies are born with abnormally small skulls. Although scientists can't yet definitely say the Zika virus causes microcephaly, officials at the World Health Organization say the virus should be "presumed guilty until proven innocent."