The strongest evidence yet of a link between the Zika virus and Guillain-Barré syndrome was announced Friday afternoon at the American Academy of Neurology conference in Vancouver.

Dr. James Sejvar, a neuroepidemiologist at the U.S. Centers for Disease Control and Prevention, said that currently 12 countries with Zika outbreaks are showing a rise in cases of Guillain-Barré syndrome, a rare disease that can cause temporary paralysis, sometimes leading to death.

A study CDC conducted in Brazil in January showed that almost 90 per cent of cases of Guillain-Barré syndrome reported earlier symptoms associated with the Zika virus, five times the level scientists would have expected to find, Sejvar said.

He said blood and urine samples taken during the study are currently being analyzed in the hope of learning what it is about Zika that triggers Guillain-Barré syndrome.

Earlier this week, the CDC confirmed the Zika virus is a cause of microcephaly and other serious birth defects.

"As a neurologist, I have never seen anything like what we are seeing [right now]," Sejvar said Friday.

Normally, Sejvar said, one or two cases of Guillain-Barré syndrome per 100,000 people worldwide are expected each year.

In Brazil, that rate is currently at 7.5 cases per 100,000, nearly four times higher, Sejvar said, noting that Colombia is also affected with a large number of cases of Guillain-Barré syndrome.

Guillain-Barré syndrome is a disorder in which the body's immune system attacks the nervous system. What often begins as weakness in the legs begins spreading to the arms and upper body of the person affected. The body can become completely paralyzed, and without quick and effective treatment a patient may die.

With successful treatment and rehabilitation, those affected usually recover.

Returning travellers cautioned

The faster Guillain-Barré syndrome is diagnosed and treatment begun, the better the outcome, Sejvar said, noting that physicians treating patients recently returned from Zika-affected areas should be aware of the disorder's symptoms.

In countries with limited access to intensive care units and ventilators, the level of morbidity and mortality connected with Guillain-Barré syndrome is far higher, he said.

Sejvar said it is critical that research into the link continue. The CDC is preparing for a large outbreak of Zika in Puerto Rico this spring and summer.

He said that scientists would be there early in an effort to find out more about the disease and its relationship with Guillain-Barré syndrome, microcephaly and other birth defects.