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

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.

Increasing the potential for malaria elimination by targeting zoophilic vectors

“…gambiae ss to zoophilic An. arabiensis that has occurred following wide scale deployment of LLINs333435.…”

The impact of host species and vector control measures on the fitness of African malaria vectors

“…For example, recent estimates suggest that approximately 75-91% of households in the Kilombero Valley are covered by untreated nets [36]. However, in the past 5 years, these simple interventions are being rapidly replaced by the distribution of more effective insecticide-treated (ITN) and long-lasting insecticidal (LLINs) nets in many African countries.…”

Delayed mortality effects cut the malaria transmission potential of insecticide-resistant mosquitoes

“…LLINs provide physical protection from mosquito bites to people sleeping under them, but the main reason for their success is that the insecticides in them kill mosquitoes within a few hours of contact. The addition of insecticides to nets can almost double the preventive effect of LLINs (11). Only one class of insecticides, the pyrethroids, has World Health Organization (WHO) approval for use on LLINs (12), and their widespread use has led to the rapid emergence and increase of pyrethroid resistance all across Africa (13).…”