Zika virus can cause severe neurological disorder, scientists say

Scientists say they’ve confirmed that the Zika virus can cause Guillain-Barré syndrome, a rare but severe neurological disorder that kills 5 percent of people who develop it. Authorities in countries with a Zika outbreak should make sure they have enough intensive care beds to deal with an increase of patients with Guillain-Barré.

In a 2013 outbreak, 32,000 people were assessed for a suspected Zika infection in French Polynesia. That’s where today’s study subjects came from. Almost all 42 patients with Guillain-Barré had signs of a recent Zika infection in their blood. In addition, 37 of those patients said that they had symptoms of Zika six days before they experienced symptoms of Guillain-Barré. The results suggest that Zika should be added to the list of infectious pathogens that can cause Guillain-Barré, the researchers write in The Lancet.

"Until now, everything was anecdotal," says Lee Norman, an intelligence officer in disaster medicine planning in the United States Army National Guard, who didn’t work on the study. This is "the first time that I've had confidence that there’s a definitive link between Zika and Guillain-Barré syndrome."

So far, most of the attention that the Zika virus has garnered has come from a possible link to birth defects. As a result, the CDC has advised pregnant women and their partners to stay away from Zika infected areas. But the association between Guillain-Barré and Zika virus could lead to a shift in scientists’ thinking on managing outbreaks, Norman says; authorities will have to cast a wider net. "It’s not like you go to Brazil and you don’t have to worry about Zika because ‘I’m a man or I’m a woman who’s 55 years old and I don’t have to worry about getting pregnant,’" he says. "We can’t narrowly focus our attention like a razor beam; Zika virus is an equal-opportunity virus" — and scientists will have to treat it as such.

There’s no vaccine or treatment for the Zika virus, which is usually transmitted through mosquito bites. But for most people, it’s relatively harmless. Those who are infected rarely die from the illness, and only about one in five people actually experience symptoms, which can include conjunctivitis, mild fever, and joint pain.

So scientists haven't paid much attention to Zika since its discovery in 1947. But in early 2015, Brazil experienced an outbreak of the virus that coincided with a steep increase in the number of babies born with abnormally small heads. Authorities also noticed an increase in the number of people diagnosed with Guillain-Barré, a syndrome that affects about one in 100,000 people worldwide each year and can cause muscle weakness, respiratory failure, and even death.

Now, scientists are trying to figure out if Zika really does cause birth defects and Guillain-Barré. There’s some evidence Zika exposure during pregnancy can lead to birth defects; today’s study comes close to confirming the virus’s link to Guillain-Barre?.

In the study, researchers sampled blood from patients who were diagnosed with Guillain-Barre? syndrome during an outbreak of Zika virus in French Polynesia, from November 2013 through February 2014. They took samples of blood from 42 patients with Guillain-Barré. The number of patients itself is unusually high; there were only 21 cases of Guillain-Barré between 2009 and 2012. The scientists also analyzed the blood of patients who didn’t have Guillain-Barré or a fever, but who went to the hospital during the outbreak, as well as the blood of patients who had Zika, but no neurological symptoms.

Although none of the patients with Guillain-Barré tested positive for Zika while in the hospital, 39 of them had antibodies in their blood that appear a few days after a Zika virus infection and that can last up to a year. In addition, 37 of these patients reported symptoms of Zika about six days before they developed Guillain-Barré. By comparison, only slightly over half of the 98 patients who didn’t have a fever or Guillain-Barré showed signs of an immune system reaction to Zika.

Several infections are known to cause Guillain-Barré, including dengue. But dengue probably isn't the cause this time, because the proportion of patients with signs of a past dengue infection in their blood was similar in all three groups, the researchers say. The scientists estimate that for every 100,000 people infected with Zika, 24 will develop the syndrome.

The researchers could be overestimating the likelihood of developing Guillain-Barré after a Zika infection, says David Smith, a microbiologist at University of Western Australia who wrote a comment on today’s study.

"The study shows very strong evidence that Zika virus can trigger Guillain-Barré," but because there’s some overlap between antibodies that react to dengue and antibodies that react to Zika, it’s possible that some cases of Guillain-Barré were actually caused by dengue. That means that scientists will have to keep conducting studies like this one to come up with a good estimate of the risks that Zika poses.

Melinda Moore, a public health physician at the Rand Corporation and a former deputy director at the CDC’s Office of Global Health Affairs, agrees but says that the studies need to be conducted elsewhere.

Researchers will need to find out if the cases of Guillain-Barré in French Polynesia are specific to that location, or if Zika in other areas can cause the syndrome as well, she says.

Others are even more cautious. Animal studies are "needed to really prove the link," says Andrew Pekosz, an immunologist at Johns Hopkins University. And more research that shows an increase in Guillain-Barré in areas with Zika wouldn’t hurt. "If Guillain-Barré consistently goes up when Zika infections are occurring, then goes down when the infections are not occurring, we would have additional data supporting a link between the two," Pekosz says. Though the risk of Guillain-Barré is small, the consequences can be tremendous. Most people who develop the syndrome recover from it, but about 30 percent of patients show signs of residual weakness three years later.

So far, the CDC has only cautioned women who are pregnant or who want to be pregnant against traveling to Zika-affected areas. Studies like this that broaden Zika’s reach can potentially change travel recommendations. That may mean trouble for the Brazil Olympics, scheduled to begin in August; experts are already calling for its cancellation.