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Impact of promoting longer-lasting insecticide treatment of bed nets upon malaria transmission in a rural Tanzanian setting with pre-existing high coverage of untreated nets

Tanya L Russell123*, Dickson W Lwetoijera14, Deodatus Maliti13, Beatrice Chipwaza1, Japhet Kihonda1, J Derek Charlwood5, Thomas A Smith6, Christian Lengeler6, Mathew A Mwanyangala1, Rose Nathan1, Bart GJ Knols7, Willem Takken8 and Gerry F Killeen1236

Author Affiliations

1 Biomedical and Environmental Thematic Group, Ifakara Health Institute, P.O. Box 53, Ifakara, Tanzania

2 Department of Biological and Biomedical Sciences, Durham University, South Road, Durham, DH1 3LE, UK

3 Vector Group, Liverpool School of Tropical Medicine, Pembroke Place, Liverpool, L3 5QA, UK

4 Department of Zoology and Marine Biology, University of Dar es Salaam, P.O. Box 35064, Dar es Salaam, Tanzania

5 DBL Centre for Health Research & Development, 57 Thorvaldensvej, Fredriksberg -C, DK 1870, Denmark

6 Department of Public Health and Epidemiology, Swiss Tropical Institute, Socinstrasse 57, Basel, CH 4002, Switzerland

7 Division of Infectious Diseases, Tropical Medicine & AIDS Academic Medical Center, F4-217, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands

8 Laboratory of Entomology, Wageningen University and Research Centre, P.O. Box 8031, 6700 EH, Wageningen, The Netherlands

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Malaria Journal 2010, 9:187  doi:10.1186/1475-2875-9-187

Published: 28 June 2010

Abstract

Background

The communities of Namawala and Idete villages in southern Tanzania experienced extremely high malaria transmission in the 1990s. By 2001-03, following high usage rates (75% of all age groups) of untreated bed nets, a 4.2-fold reduction in malaria transmission intensity was achieved. Since 2006, a national-scale programme has promoted the use of longer-lasting insecticide treatment kits (consisting of an insecticide plus binder) co-packaged with all bed nets manufactured in the country.

Methods

The entomological inoculation rate (EIR) was estimated through monthly surveys in 72 houses randomly selected in each of the two villages. Mosquitoes were caught using CDC light traps placed beside occupied bed nets between January and December 2008 (n = 1,648 trap nights). Sub-samples of mosquitoes were taken from each trap to determine parity status, sporozoite infection and Anopheles gambiae complex sibling species identity.

Results

Compared with a historical mean EIR of ~1400 infectious bites/person/year (ib/p/y) in 1990-94; the 2008 estimate of 81 ib/p/y represents an 18-fold reduction for an unprotected person without a net. The combined impact of longer-lasting insecticide treatments as well as high bed net coverage was associated with a 4.6-fold reduction in EIR, on top of the impact from the use of untreated nets alone. The scale-up of bed nets and subsequent insecticidal treatment has reduced the density of the anthropophagic, endophagic primary vector species, Anopheles gambiae sensu stricto, by 79%. In contrast, the reduction in density of the zoophagic, exophagic sibling species Anopheles arabiensis was only 38%.

Conclusion

Insecticide treatment of nets reduced the intensity of malaria transmission in addition to that achieved by the untreated nets alone. Impacts were most pronounced against the highly anthropophagic, endophagic primary vector, leading to a shift in the sibling species composition of the A. gambiae complex.