Namibia polio outbreak puzzling

Unlike most outbreaks, this one is targeting adults rather than young children.

Outbreaks among adults are “as rare as hen’s teeth,” says Bruce Aylward, who directs the global campaign to eradicate polio from World Health Organisation (WHO) headquarters in Geneva, Switzerland, because adults usually have acquired immunity either through vaccination or exposure to infected children. But when an adult in infected, the disease tends to be much more severe.

At least 47 suspected cases were under investigation in Namibia, and wild poliovirus type one had been confirmed in four. Seven people had died, and several others were critically ill.

Shipments of vaccine arrived in Namibia for the first of at least three emergency campaigns, and officials were bracing for yet more cases before the virus can be contained.

Although the latest outbreak is yet another blow to the global campaign to eradicate polio, WHO officials say they are optimistic that the Namibian government can quash it. But the outbreak remains puzzling. A team of epidemiologists has rushed in to investigate, while others have been scouring the genetic sequence of the virus for clues. Genetic evidence suggests that the virus originated in India, one of four countries where polio is endemic, and that it recently jumped across the border from Angola into Namibia. Angola had interrupted transmission of wild poliovirus in 2001 but was reinfected with an Indian virus last year; only a handful of new cases have been reported there.

When it reached Namibia, the virus found fertile soil. Why is not clear, but the prevailing assumption is that the afflicted adults were not immunized as children. Namibia didn’t offer routine vaccinations until the early 1990s, but once the country began polio immunization in earnest, the last case of indigenous poliovirus was reported in 1996. One puzzle, says Aylward, is how the affected adults apparently escaped exposure to the circulating virus before then. In other adult outbreaks, for instance in Albania in 1996 or Cape Verde in 2000, the populations were either culturally or geographically isolated, he notes.

The first case identified in Namibia was a 39-year-old farmer from Aranos, about 150 kilometers from the capital of Windhoek. He went to Windhoek for gall bladder surgery and became ill, with sudden paralysis, about 2 weeks later on 8 May. He is now on a respirator. Within days, other cases were reported around Windhoek, mostly among adults between age 20 and 40. Health authorities initially thought they might be dealing with Guillain-Barre syndrome, an autoimmune disorder that may be triggered by infections. But on 2 June, a WHO-accredited laboratory in South Africa confirmed wild poliovirus.

So far, epidemiologists don’t know where the farmer picked up the virus or even whether he is the first in the chain of transmission, says David L. Heymann, the WHO director general’s representative for polio eradication. Investigators are looking for any connection among the suspected cases. “We may never know,” says Heymann, who notes that many of the victims are unable to talk because they are on respirators. Investigators are also trying to determine whether a cluster of Guillain-Barre cases reported in April may have been polio as well, he adds.

A shipment of monovalent oral polio vaccine arrived in Namibia last week. In the first campaign, which began on Wednesday, and a second scheduled for late July, the government hopes to reach every person in the country, adults and children alike. The age group to be targeted in subsequent campaigns has yet to be determined. “I am very confident [Namibia] will be able to [control the outbreak],” says Heymann. He lauds Namibia’s rapid response and says the campaign will also benefit from the country’s sparse population and the onset of winter.

A bigger challenge may be knocking out polio next door in Angola, says Heymann. The last reported case of polio in Angola was in November 2005, but last month, wild poliovirus from Angola also resurfaced in the Democratic Republic of Congo (DRC), which abuts Angola to the north. “They are clearly not doing enough to stop transmission,” says Heymann. New sequencing data from Angola, the DRC, and Namibia suggest that the virus was introduced into one of these countries a year ago and has been circulating in all three.

Although the new outbreaks are troubling, WHO officials say the most serious threat to polio eradication remains the densely populated states of northern Nigeria, where there has been intense opposition to vaccination and an epidemic is raging out of control. Already, 438 cases have been reported this year, nearly triple the number at the same time last year.

To Ellie Ehrenfeld, a polio biologist at the U.S. National Institutes of Health, the Namibian outbreak provides yet more evidence that “the world cannot be left unimmunized.” Even if the eradication campaign succeeds, she warns, “the potential for a hideous, hideous outcome is really there.” ‘ Science. (See more pictures on Page C4.)

June 2006
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