(CNN) A new study lends support to one of the biggest fears about Zika virus: It might increase the risk of the rare neurological syndrome Guillain-Barre.
Years before Zika virus first appeared in Latin America, it spread from Africa and Asia to Pacific islands. From 2013 to 2014, French Polynesia had 32,000 suspected cases of Zika virus. During this same period, there was also an uptick in cases of Guillain-Barre.
Researchers in French Polynesia and Europe looked at those 42 cases of Guillain-Barre in the Pacific Island nation between October 2013 and April 2014. They asked whether there was any evidence these patients had been exposed to Zika virus.
The researchers found that 98% of the Guillain-Barre patients had antibodies to Zika, suggesting they had been infected with the virus, compared with only 56% of the individuals in a control group that did not have Guillain-Barre. In addition, 88% of patients had experienced typical symptoms of Zika, including fever, rash and joint pain, usually about a week before developing Guillain-Barre.
"It is a well-done paper and clearly shows a link between Zika and Guillain-Barre in Polynesia," said Dr. Peter Jay Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine. Hotez was not involved in the study, which was published on Monday the Lancet, a medical journal.
There has also been an increase in cases of the neurological syndrome, along with a birth defect called microcephaly, in other areas with Zika outbreaks. According to a report by the World Health Organization released on Friday, eight countries or territories have seen increases in Guillain-Barre since 2015. In some of those areas, including Puerto Rico and Venezuela, there's evidence of Zika infection among Guillain-Barre patients
"Given the time course of clusters of Guillain-Barre' after peak Zika virus infection, I think most epidemiologists would say it's certainly related," Dr. Tom Frieden, director of the Centers for Disease Control and Prevention, said.
To date, there have been 107 cases in the United States among U.S. travelers returning from Zika-infected areas. None of these resulted in Guillain-Barre, but that is not surprising considering how rare it appears to be for infected individuals to develop this syndrome, Hotez said.
But that could change, at least in parts of the United States.
"It's going to be a big concern as we go into the warmer spring and summer months, when we start seeing a large uptick in the Aedes aegypti (that carries Zika) on the Gulf Coast," Hotez said. "We could see an uptick in Zika ... and if we start seeing a large number of Zika cases, the prediction would be that we would start to see Guillain-Barre and microcephaly."
In February, the World Health Organization declared Zika virus a "public health emergency of international concern" because of its suspected links to Guillain-Barre and microcephaly.
Although Guillain-Barre is very rare, affecting about one in 100,000 people in the United States, the study suggests that Zika could make it less rare. The researchers estimate, based on the rates of Zika and Guillain-Barre in French Polynesia, that one in 4,000 people infected with Zika could develop the neurological syndrome.
"In the vast majority of people, Zika is still causing mild illness to no symptoms," Hotez said. "It's an important question that remains unanswered: What is it about the subset of people that get Guillain-Barre?"
Guillain-Barre is an autoimmune disorder in which the body's immune system attacks the nervous system. The disease typically affects axons, the parts of peripheral nerves that transmit nerve signals, or the myelin sheath covering the axons.
The result is that patients, over the course of several weeks, experience tingling in their legs, then weakness in their legs and arms, and in some cases even lose the ability to use muscles. In general, it can take patients weeks to years to recover, and 30% of them still have weakness three years later.
In the current study, 74% of patients had muscle weakness, 64% had weakness specifically in facial muscles and 29% needed assistance to breathe. Breathing difficulties can be a fatal complication of the syndrome, although no patients in the study died.
Among the patients in the study, 57% were able to walk on their own within three months. There are treatments that can reduce the symptoms, help recovery and treat complications.
Although it is not clear what causes Guillain-Barre, a number of viruses have been linked to the syndrome before Zika, including flu, Epstein-Barr, and other mosquito-borne viruses including dengue. Research suggests that infections with the bacteria campylobacter could lead to Guillain-Barre in one to three in 4,000 cases.
The thinking is that when the immune system responds to viral and bacterial infections, it makes antibodies that recognize them, but the antibodies also happen to recognize components of the nervous system, Hotez said.
Even though dengue and chikungunya have been implicated in Guillain-Barre, the evidence is probably stronger that two other mosquito-born viruses, West Nile and Japanese encephalitis, increase the risk of the syndrome, said Jae Jung, director of the Institute of Emerging Pathogens and Immune Diseases at the Keck School of Medicine at the University of Southern California.
Although Jung called the study, "one of the best showing a link between Zika virus and Guillain-Barre," he is not very concerned about an increase in the syndrome in the United States.
"There's a very high chance that we will see an increase in Zika in southern parts of the United States, (but) our medical system is much, much better than in Brazil or Polynesia ... and our environment is a lot cleaner," which could reduce the risk of mosquito bites, Jung said.
Although there are no treatments for Zika, like other mosquito-borne viruses, Jung hopes that they will become available in coming years and help control spread of the Zika.