A retrospective study of a Zika outbreak in Tahiti in 2013-14 has found evidence of a link between the virus and a rare form of paralysis - the latest sign that the virus may be more threatening than once believed.

As concerns over Zika mounted in recent months, officials in Tahiti went back and analyzed blood samples from 42 adults who were diagnosed with Guillain-Barré syndrome around the time the virus was raging in the French Polynesian island. All but one showed signs of Zika antibodies, indicating they had been previously infected.

Writing in the journal Lancet, scientists warned that at-risk countries in the Americas where the virus is spreading rapidly "need to prepare for adequate intensive care beds capacity to manage patients with Guillain-Barré syndrome."

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The condition occurs when the immune system goes awry and attacks the nerves. Typically, this happens two to eight weeks after an infection and begins with a tingling sensation. It can progress to paralysis, although that is usually temporary. However, one study found that a quarter of patients need respiratory assistance and that the death rate is as high as one in 20.

In the cases documented in Tahiti, the syndrome progressed quickly. The patients reported symptoms consistent with the Zika virus a median of six days before they showed neurological symptoms. Most appeared to have a variant of the syndrome known as acute motor axonal neuropathy, or AMAN, which is characterized by paralysis and loss of reflexes. The median age of the patients was 42 years. Nearly all of the 31 men and 11 women were born in French Polynesia.

The researchers speculated that past infection with the dengue virus might have made the group more vulnerable to the syndrome, but they did not find evidence of this.

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The study did contain some positive news. The patients' outcomes were "generally favorable," the researchers wrote, and they had a "faster recovery than usually observed in typical Guillain-Barré syndrome."

Those outcomes are in sharp contrast to what health officials are reporting on the ground in Latin America.

Turbo, Colombia, has reported a recent outbreak of Guillain-Barré that totaled five cases, all of them severe. According to Washington Post correspondent Nick Miroff, who visited the city, "Three patients have died. One is fighting for his life in an intensive care unit. The fifth, a 10-year-old girl, hasn't been able to move her legs in a week."

In total, Colombia has reported 97 cases of Guillain-Barré.

In the United States, the Centers for Disease Control and Prevention has confirmed three cases, one in Puerto Rico, of Guillain-Barré related to Zika. Last weekend, Honduras reported that a 29-year-old pregnant woman with Zika symptoms had been diagnosed with Guillain-Barré, according to the Associated Press.