First solid evidence
Neurologists from Johns Hopkins University working with researchers at six hospitals in Colombia said they detected Zika virus in the body fluids of a group of people suffering symptoms of Guillain-Barre syndrome.
Guillain-Barre is a disorder that causes potentially life-threatening muscle weakness when the immune system attacks the nerves, according to background information with the study.
"This is the first solid evidence that the virus is present in patients with Guillain-Barre," said senior researcher Dr Carlos Pardo, an associate professor of neurology at Johns Hopkins in Baltimore. "In a large population from multiple centres affected by Zika outbreaks, we were able to detect and culture the virus that was affecting those patients."
The study was published in the New England Journal of Medicine.
At least 14 patients cited in the study were diagnosed with both Zika infection and Guillain-Barre syndrome using the highest-level tests available for both illnesses, said Dr Jennifer Frontera, a neurologist with the Cleveland Clinic. She is co-author of an accompanying editorial.
Zika is the first mosquito-borne virus known to cause terrible birth defects, most of them brain-related. In the average person, however, the virus generally causes either mild symptoms or none at all.
A tentative link
Because of the potential for birth defects, public health workers have placed special emphasis on protecting pregnant women from the virus.
However, there has also been an increase in cases of Guillain-Barre syndrome within South American countries hit hardest by Zika, prompting public health officials to draw a tentative link between the two.
Guillain-Barre is rare, normally afflicting about one person in 100,000, according to the US National Institutes of Health. But when Zika struck French Polynesia several years ago, about one in 4,000 people infected with the virus also developed Guillain-Barre, Pardo said.
"There was an overwhelming increase in the number of cases of Guillain-Barre" in Colombia early in 2016, right about the time of that country's Zika outbreaks, Pardo said. "It went from one or two cases a year to 10 to 15 cases a week. That was very striking."
People with Guillain-Barre experience muscle weakness that can range from slight weakness and tingling in the legs all the way up to full paralysis requiring a ventilator, Frontera said.
Most advanced genetic test
"The vast majority of people will improve from Guillain-Barre over time, but it can be very serious," she said.
To bring more light to this potential connection, Pardo and his colleagues evaluated 68 patients with Guillain-Barre syndrome at Colombian hospitals. They performed tests on 42 of the patients to detect the presence of Zika virus.
Using the most advanced genetic test available for the virus, researchers determined that 17 patients tested positive for Zika in their urine.
Another 18 patients also displayed potential evidence of Zika infection, the researchers noted. However, Frontera said this evidence also could point to dengue or yellow fever infections, since these viruses are from the same family as Zika.
The research team went on to confirm Guillain-Barre syndrome in 14 of the patients with genetically validated Zika infection. The researchers used nerve conduction exams that are critical to accurately diagnose the neurological disorder, Frontera said.
Guillain-Barre has been linked to other viral and bacterial infections, including campylobacter, influenza, yellow fever and Epstein-Barr virus, Frontera said.
Direct attack on nerve cells
The researchers said that almost one-half of study participants complained of neurologic symptoms within four days of the onset of Zika symptoms. That's an unusually fast response compared with others who develop Guillain-Barre symptoms following infections from other viruses, the study authors said.
Zika and these other infectious agents could be creating a condition called "molecular mimicry" to provoke an attack on the nervous system by the immune system, Pardo and Frontera said.
Zika proteins may share some similarities to nerve cells in the body, or to the fatty white substance called myelin that sheaths nerve cells, Frontera said.
So, antibodies created to attack Zika might become confused and wind up attacking the nervous system, she said.
It's also possible that Zika is somehow attacking nerve cells directly, Frontera added.
Doctors typically treat Guillain-Barre by exchanging the patient's plasma or infusing immunoglobulin, an antibody, into their bloodstream. Both tactics aim to short-circuit the faulty immune system response, Pardo and Frontera said.