Why we can’t say how likely Zika is to leave you temporarily paralyzed

We are still mostly in the dark.

About six months ago, as the scale and seriousness of the Zika pandemic was starting to sink in, a concerning link emerged between Zika and a serious neurological condition, Guillain-Barré Syndrome (GBS).

Arguably the most serious consequence of Zika exposure for non-pregnant adults, GBS is a serious autoimmune disorder triggered by bacterial or viral exposure (for instance, mononucleosis can cause it), which can cause numbness, muscle weakness and paralysis for up to a few months. Typically reversible, in rare cases GBS can cause permanent damage, and even be fatal.

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Much as with birth defects, understanding how, and how often, a person infected with Zika develops GBS is no easy task, considering testing for Zika is not straightforward. So far, especially from findings on the correlation between Zika and GBS in French Polynesia (pdf), we know that, while GBS typically occurs about three weeks after the primary infection that triggered it, it occurs sooner in Zika—about a week after the infection, according to findings presented at Women Deliver in Copenhagen.

But many things remain unknown, including the likelihood of occurrence. We don’t have much data to understand whether anything makes developing GBS more likely—partially because we don’t have much data about GBS, full stop. While it was possible to quantify the cases in French Polynesia, where the population size is small, it’s not as easy to know the situation in highly populated areas.

“The difficulty of GBS is that while it’s a serious disease it’s not a notifiable condition in any country,” Christopher Dye, the World Health Organization (WHO)’s strategist, told Quartz. This means there are no comprehensive registers kept of cases of GBS, which makes it difficult to understand whether an increase is occurring, and of what size.

“We calculated that there was an excess of reported [GBS] cases,” Dye says, “but my very strong suspicion is that there were many more.” Dye says the WHO has made an informal recommendation to keep records of GBS cases: since it’s a disease difficult to miss, an increase in its occurrence could actually even function as a proxy for Zika epidemics.

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