A new study published Monday in the Lancet offers the first evidence that Zika Virus, which has swept across Latin America and has been linked to severe congenital defects, might cause a severe neurological disorder. The study was based on analysis of blood samples from 42 patients diagnosed with Guillain-Barré syndrome (GBS) in French Polynesia.

“This is the first study to look at a large number of patients who developed Guillain-Barré syndrome following Zika virus infection and provide evidence that Zika virus can cause GBS,” the study’s lead author Arnaud Fontanet from the Institut Pasteur in Paris said in a statement released Monday. “Most of the patients with GBS reported they had experienced symptoms of Zika virus infection on average 6 days before any neurological symptoms, and all carried Zika virus antibodies.”

The current outbreak of the virus occurred in Brazil in May 2014. Since then, an estimated 1.5 million people are believed to have been infected in the country, and a total of 46 nations have reported some level of Zika infection. Although the Aedes mosquito-borne virus normally causes mild symptoms, such as fever, rash, joint pain and conjunctivitis, or no symptoms at all, health officials across the globe are concerned because the latest outbreak has been linked to a drastic increase in the rate of microcephaly — a birth defect in which babies are born with small heads and undeveloped brains.

Prior to the latest outbreak, French Polynesia experienced a massive Zika outbreak between October 2013 and April 2014. An estimated 32,000 patients consulted a doctor about a suspected Zika virus infection, and 42 patients were diagnosed with GBS between November 2013 and February 2014.

GBS is a disorder that affects the immune and nervous systems, and can trigger temporary paralysis. Symptoms develop rapidly and include weakness in the legs and arms, muscle weakness and pain. It is usually triggered by an infection and can sometimes develop following infections of herpes, influenza or dengue virus. In about 20 to 30 percent of cases, severe GBS can lead to respiratory failure, and about 5 percent of patients die.

In the latest study, 37 people, or 88 percent of the patients with GBS reported symptoms of Zika virus infection approximately six days before the onset of neurological symptoms. While none tested positive for a Zika virus infection once in hospital, blood tests showed that 98 percent were carrying Zika virus antibodies — indicating that they had been infected with the virus in the past.

By contrast, only half of a control group of 98 people had the antibodies.

The researchers estimated that for every 100,000 people infected with Zika, 24 would develop the syndrome.

“Although it is unknown whether attack rates of Zika virus epidemics will be as high in affected regions in Latin America than in the Pacific Islands, high numbers of cases of Guillain-Barré syndrome might be expected in the coming months as the result of this association,” Fontanet said in the statement. “The results of our study support that Zika virus should be added to the list of infectious pathogens susceptible to cause Guillain-Barré syndrome.”