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An unusually deadly outbreak of poliomyelitis (polio) in Congo has scientists scratching their heads.

Close to 500 people were left paralysed and almost 200 succumbed to polio since the outbreak began in October. The viral disease killed around 42 per cent of people infected, unlike typical outbreaks that kill 5–10 per cent. And although the disease usually attacks children under five years of age, most victims in this outbreak were men aged between 15 and 25 years.

Polio was wiped out in Congo in 2000. "[The country] was not considered at high risk. That is why we were all surprised," said Mark Pallansch, who is leading efforts to analyse the virus at the US Centers for Disease Control and Prevention.

Polio was not initially suspected to be the culprit of this unusual outbreak, and this hindered data collection. Results from genetic analysis of a small number of biological samples collected from patients so far indicate that the outbreak is caused by a wild poliovirus type 1 imported from Angola.

But in Angola, type-1 poliovirus is infecting young children as expected, not adult men. Scientists speculate that an infection with another, as-yet unidentified virus might be adding to the problem in Congo. But more research is needed to test this theory.

Others speculate that political instability that affected the country in the mid-1990s might have disrupted vaccination efforts and as a result, the virus is now infecting young men who missed out on childhood immunisation against the disease.

A third possibility is that Congo is undergoing a much larger outbreak, with many mild cases going undetected.

"Everyone's got an opinion. But there are few data," said Bruce Aylward of the WHO in Geneva, who runs the organisation's global programme to eradicate polio. "We are scratching our heads."

Polio expert Neal Nathanson, at the University of Pennsylvania, agreed: "There are too many things that don't fit or are unexpected".

Meanwhile, the epidemic has started spreading to Angola and the Democratic Republic of Congo. Large vaccination campaigns are underway to prevent further spread, and experts say that if they are successful, in three-to-four months the epidemic will be stopped.

But the incident leaves a sense of uncertainty about the possibility of similar outbreaks occurring in other polio-free parts of Africa. "We don't know where the susceptibles are," said Aylward, "where the next Congo could be."

Link to full article in Science

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