Saturday, 28 August 2010

Simplified sewerage: health impacts

The city of Salvador, capital of the Brazilian state of Bahia, has one of the largest simplified sewerage systems in the country (details here). In a recent paper in Environmental Health Perspectives: Impact of a city-wide sanitation programme in northeast Brazil on intestinal parasites infection in young children, by Professor Barreto and colleagues at the Universidade Federal da Bahia in Brazil (including Professor Sandy Cairncross of the London School of Hygiene and Tropical Medicine), it was found that, in children under five:

The prevalence of Ascaris lumbricoides infection was reduced from 24.4% to 12.0%, Trichuris trichuria from 18.0% to 5.0% and Giardia duodenalis from 14.1% to 5.3%. Most of this reduction appeared to be explained by the increased coverage of each neighborhood by the sewerage system constructed during the intervention. The key explanatory variable was thus an ecological measure of exposure and not household-based, suggesting that the parasite transmission prevented by the program was mainly in the public (as opposed to the domestic) domain.

[For ‘public’ and ‘domestic’ domains see the 1996 paper by Cairncross et al. in Tropical Medicine & International Health: The public and domestic domains in the transmission of disease.]

This EHP paper is a sequel to the same group’s 2007 paper in The Lancet: Effect of citywide sanitation programme on reduction in rate of childhood diarrhoea in northeast Brazil: assessment by two cohort studies, which reported that in children less than 3 years of age:

Diarrhoea prevalence fell by 21% (95% CI 18−25%) − from 9.2 (9.0−9.5) days per child-year before the intervention to 7.3 (7.0−7.5) days per child-year afterwards. After adjustment for baseline sewerage coverage and potential confounding variables, we estimated an overall prevalence reduction of 22% (19−26%) … Our results show that urban sanitation is a highly effective health measure that can no longer be ignored [emphasis added].