Zika virus disease is mainly spread by mosquitoes

"Scientists have amassed the strongest evidence yet that the Zika virus can cause the serious neurological condition Guillain-Barré syndrome," The Guardian reports.

A large outbreak of Zika in the French Polynesian islands saw a spike in the numbers of people with Guillain-Barré syndrome – a nerve disorder that causes temporary muscle paralysis.

Researchers analysed blood tests of people from that outbreak, which showed 41 of the 42 people diagnosed with Guillain-Barré syndrome at the time had antibodies against Zika virus.

There have also been reports of increased numbers of people with Guillain-Barré syndrome being admitted to hospitals in Central and South America, which has seen an outbreak of Zika virus.

This new study provides further evidence to suggest a link between Zika virus and Guillain-Barré Syndrome.

However, figures from French Polynesia suggest just 0.24 cases for every 1,000 Zika infections. This means the individual chances of getting Guillain-Barré syndrome after being infected by Zika are pretty low.

Guillain-Barré syndrome can paralyse the respiratory muscles, which may require treatment in intensive care. Most people make a full recovery, although it may take a year or more to fully recover.

Despite some alarming headlines, nobody died of the syndrome in the French Polynesian outbreak.

Advice for pregnant women If you're pregnant, or planning to become pregnant, and are about to visit somewhere known to be affected by the Zika virus, you should discuss your travel plans with your doctor. Public Health England provides regular updates about the current spread of the disease. Read more advice about the Zika virus.

Where did the story come from?

The study was carried out by researchers from a number of institutions, including L'Institut Louis Malardé, Centre Hospitalier de Polynésie Française, Bureau de Veille Sanitaire, and Service de santé des armées in French Polynesia, L'Institut Pasteur and Pitié-Salpêtrière Hospital in France, and the University of Glasgow.

It was funded by the French government, the European Union and the Wellcome Trust.

The study was published in the peer-reviewed journal The Lancet on an open access basis, so it is free to read online.

This research has been widely covered, and most of the UK media reported the study accurately. The Guardian and The Independent focused on the danger of intensive care services in areas with current Zika outbreaks being overwhelmed.

What kind of research was this?

This was a case control study, comparing blood tests taken from people diagnosed with Guillain-Barré syndrome with those from people with acute Zika infection without Guillain-Barré syndrome, or other illnesses.

Researchers wanted to see whether signs of Zika infection were more common in the group that had Guillain-Barré syndrome.

They also wanted to see whether signs of infection with dengue virus were more common in people with Guillain-Barré syndrome than in people with Zika infection, but no signs of nerve damage.

A case-control study cannot prove infection with the Zika virus causes Guillain-Barré syndrome. However, it does provide evidence to support the theory.

What did the research involve?

Researchers took blood samples from all the people admitted to hospital with Guillain-Barré syndrome in French Polynesia from October 2013 to February 2014.

They tested them for a range of antibodies to Zika virus and dengue virus, and for signs of active viral infection.

They then tested blood samples from two comparison groups to look for links between Zika virus, Guillain-Barré syndrome and dengue virus.

Group one were people of the same age and from the same island being treated for an illness that did not cause a fever, which means it was unlikely to be Zika virus.

Group two were people approximately the same age as those with Guillain-Barré syndrome being treated for acute Zika infection, without any signs of Guillain-Barré syndrome.

Everyone in the study with Guillain-Barré syndrome had been diagnosed using the same internationally accepted criteria at one central hospital.

People with the syndrome also had other tests, which looked at how their nerves were affected by the illness and which immune responses had been involved.

What were the basic results?

Most of the people with Guillain-Barré syndrome said they had symptoms similar to those caused by the Zika virus – such as rash, aches and fever – about six days before their symptoms of muscle weakness started.

Out of 42 patients diagnosed, 41 (98%) had antibodies to Zika virus in their blood, showing they had recently been infected with Zika. Of the 98 people in the first comparison group, only 35 (36%) had Zika antibodies.

While all the patients being treated for Zika had signs of Zika viral infection in their blood, none of the Guillain-Barré patients did, suggesting the syndrome happens once the body has fought off the virus.

Although most people with Guillain-Barré syndrome also had signs of previous infection with dengue virus (95%), so did most of the people in the comparison groups (89% and 83%).

How did the researchers interpret the results?

The researchers said: "The results of our study support that Zika virus should be added to the list of infectious pathogens susceptible to cause Guillain-Barré syndrome."

However, they admit they don't know exactly how Zika virus could trigger the immune reaction that leads to Guillain-Barré syndrome. They dismissed the possibility that infection with the dengue virus played a part.

They warned that countries in Central and South America currently experiencing an epidemic of Zika infection should expect "high numbers of cases of Guillain-Barré syndrome … in the coming months".

They added that, "Because Zika virus is spreading rapidly across the Americas, at-risk countries need to be prepared to have adequate intensive care beds capacity to manage patients with Guillain-Barré syndrome."

Conclusion

There were already suspicions that the current outbreaks of Zika virus were triggering an unusually high number of Guillain-Barré syndrome cases in affected areas.

While a case-control study cannot provide absolute proof, this study does give the first direct evidence that most people who got Guillain-Barré syndrome during an outbreak of Zika were infected with Zika virus.

Zika infection was also much more common in this group than in similar people being treated for other illnesses.

The researchers are right to warn that Central and South American countries could see many more cases of Guillain-Barré syndrome than usual, whether or not it is directly caused by the Zika virus. It's important for these countries to be prepared.

For individuals wondering whether or not to travel to countries affected by Zika virus, this news should not cause alarm. Figures from the French Polynesian outbreak show about 1 in 4,000 people infected with Zika virus went on to develop Guillain-Barré syndrome.