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 Russell,Christian Lengeler,Deodatus Maliti,Thomas A Smith,Rose Nathan,Willem Takken,J Derek Charlwood,Dickson W Lwetoijera,Mathew A Mwanyangala,Japhet Kihonda,Beatrice Chipwaza,Bart Gj Knols,Gerry F Killeen

doi:10.1186/1475-2875-9-187

Abstract

BackgroundThe 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.MethodsThe 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.ResultsCompared 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%.ConclusionInsecticide 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.

Highlights

The communities of Namawala and Idete villages in southern Tanzania experienced extremely high malaria transmission in the 1990s
The current study investigated whether the treatment of bed nets with longer-lasting insecticide produced any further impact on the intensity of malaria transmission beyond the 4.2-fold reduction observed from the use of untreated nets alone [22]
Members of the A. gambiae complex dominated the biting burden while A. funestus contributed to only 3% of the total Anopheles bites occurring in these two villages (Table 1)

Summary

Introduction

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. While it is intuitively clear that ITNs provide protection to individual users, what is less obvious is the impact of widespread ITN use at the community level. ITNs are able to reduce the density, feeding frequency and survival of mosquitoes [8,9,10,11] and wide-scale use can mediate protection of all community members, including the vulnerable portion without a bed net [12,13,14,15]. Field studies have demonstrated that the beneficial impacts on malaria transmission outweigh any such inequitable biting burden for the unprotected [8,14,19,20]

Methods

Results

Discussion

Conclusion