Sunday, February 13, 2011

Polio outbreak in Congo-Brazzaville demystified

Since the polio outbreak in Congo-Brazzaville began in October, the atypical epidemic had leaders of the polio eradication program of WHO “scratching [their] heads.” The outbreak has paralyzed 476 and killed at least 179, according to WHO December estimates, and also has been unusual in that most victims of the disease were males between the ages of 15 and 25. In addition to the strange demographics, the outbreak has established an exceptionally high mortality rate, killing 42% of those ill.

The outbreak is a considerable blow to the eradication campaign given recent success in the Congo. National immunization programs and rigorous epidemiological investigation led to the national eradication in 2000, and until 2010 the Congo remained polio-free despite outbreaks in neighboring Angola and the D.R.C. The WT 1 strain now circulating in Congo was confirmed as a single import from Angola, where the polio epidemic is “garden variety,” featuring the characteristic under-5 demographic. If the virus type was identical in both countries, what made disease so different? The possibilities include common underlying health problems, environmental cofactors, or more likely, demographic differences, convincing because we know that polio is a more serious disease for older individuals.

A letter to the editor of Science has suggested that we should not be surprised by this outbreak and that this type of “virgin soil” outbreak in young adult populations was in fact predictable. The writer cited M. Burnet for description of similar outbreaks that mainly challenged young adults (e.g. Eskimos in the Hudson’s Bay arctic suffered a 40% paralysis rate) and also for the specific prediction of “catastrophic epidemics” that would ensue if young adults who did not receive exposure as children would come into contact with the virus. The writer also suggests that we should learn from this outbreak and vaccinate young adults in regions with high-risk of infection, such as Chad.



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