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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.

Russell TL, Lwetoijera DW, Maliti D, Chipwaza B, Kihonda J, Charlwood JD, Smith TA, Lengeler C, Mwanyangala MA, Nathan R, Knols BG, Takken W, Killeen GF - Malar. J. (2010)

Bottom Line: 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%.

View Article: PubMed Central - HTML - PubMed

Affiliation: Biomedical and Environmental Thematic Group, Ifakara Health Institute, P.O. Box 53, Ifakara, Tanzania. t.russell2@uq.edu.au

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 approximately 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.

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Graphical comparison of the historical and recent estimates of sporozoite prevalence (A-C), biting rate (D-F) and entomological inoculation rate (G-H) for Anopheles gambiae complex and Anopheles funestus and overall. The corresponding bed net usage recorded for the same areas and time periods (J-L) demonstrate the negative association between bed net usage and EIR. Note: Data for 1990-94 was from [29-34,52] and 2001-03 was from [22]. The B and EIR values for 1990-94 were recalculated by [22].
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Figure 3: Graphical comparison of the historical and recent estimates of sporozoite prevalence (A-C), biting rate (D-F) and entomological inoculation rate (G-H) for Anopheles gambiae complex and Anopheles funestus and overall. The corresponding bed net usage recorded for the same areas and time periods (J-L) demonstrate the negative association between bed net usage and EIR. Note: Data for 1990-94 was from [29-34,52] and 2001-03 was from [22]. The B and EIR values for 1990-94 were recalculated by [22].

Mentions: Similar entomological surveys using light traps were conducted in the same villages between 1990 and 1994 (before wide-spread bed net use) and again between 2001 and 2003 (after high coverage of untreated nets had been achieved), allowing a comparison of current malaria transmission intensities with historical rates. In 2008, bed net coverage levels were extremely high with 91.5% of the population sleeping under a net the previous night and 46.8% sleeping under an ITN. Since 1990-94 there has been an 8.4-fold reduction of the sporozoite prevalence of the A. gambiae complex, but surprisingly little change in the sporozoite prevalence of A. funestus (Table 2, Figure 3). The biting rate of the A. gambiae complex has reduced by 2.5-fold and for A. funestus by 13-fold. Between 1990-94 and 2001-03 the EIR was reduced by 4.2-fold [22] and between 2001-03 and 2008 by a further 4.6-fold. Thus, compared with the exposure of non-users in 1990-94 by 2008 there had been an 18-fold (95%) overall community level reduction in transmission intensity for non-users of bed nets. Considering that users of bed nets receive both personal and community level protection, the exposure of bed net users was calculated by adjusting for personal protection from 40% of bites for an untreated net user and 70% of bites for an ITN user [57]. As such, users of untreated nets probably experienced a 30-fold (97%) reduction and users of ITNs experienced a 60-fold (98%) reduction. In 2008, the mean EIR of an average community member calculated as an average weighted according to the recorded bed net use (Table 1) was 33.9 ib/p/y.


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.

Russell TL, Lwetoijera DW, Maliti D, Chipwaza B, Kihonda J, Charlwood JD, Smith TA, Lengeler C, Mwanyangala MA, Nathan R, Knols BG, Takken W, Killeen GF - Malar. J. (2010)

Graphical comparison of the historical and recent estimates of sporozoite prevalence (A-C), biting rate (D-F) and entomological inoculation rate (G-H) for Anopheles gambiae complex and Anopheles funestus and overall. The corresponding bed net usage recorded for the same areas and time periods (J-L) demonstrate the negative association between bed net usage and EIR. Note: Data for 1990-94 was from [29-34,52] and 2001-03 was from [22]. The B and EIR values for 1990-94 were recalculated by [22].
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2902500&req=5

Figure 3: Graphical comparison of the historical and recent estimates of sporozoite prevalence (A-C), biting rate (D-F) and entomological inoculation rate (G-H) for Anopheles gambiae complex and Anopheles funestus and overall. The corresponding bed net usage recorded for the same areas and time periods (J-L) demonstrate the negative association between bed net usage and EIR. Note: Data for 1990-94 was from [29-34,52] and 2001-03 was from [22]. The B and EIR values for 1990-94 were recalculated by [22].
Mentions: Similar entomological surveys using light traps were conducted in the same villages between 1990 and 1994 (before wide-spread bed net use) and again between 2001 and 2003 (after high coverage of untreated nets had been achieved), allowing a comparison of current malaria transmission intensities with historical rates. In 2008, bed net coverage levels were extremely high with 91.5% of the population sleeping under a net the previous night and 46.8% sleeping under an ITN. Since 1990-94 there has been an 8.4-fold reduction of the sporozoite prevalence of the A. gambiae complex, but surprisingly little change in the sporozoite prevalence of A. funestus (Table 2, Figure 3). The biting rate of the A. gambiae complex has reduced by 2.5-fold and for A. funestus by 13-fold. Between 1990-94 and 2001-03 the EIR was reduced by 4.2-fold [22] and between 2001-03 and 2008 by a further 4.6-fold. Thus, compared with the exposure of non-users in 1990-94 by 2008 there had been an 18-fold (95%) overall community level reduction in transmission intensity for non-users of bed nets. Considering that users of bed nets receive both personal and community level protection, the exposure of bed net users was calculated by adjusting for personal protection from 40% of bites for an untreated net user and 70% of bites for an ITN user [57]. As such, users of untreated nets probably experienced a 30-fold (97%) reduction and users of ITNs experienced a 60-fold (98%) reduction. In 2008, the mean EIR of an average community member calculated as an average weighted according to the recorded bed net use (Table 1) was 33.9 ib/p/y.

Bottom Line: 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%.

View Article: PubMed Central - HTML - PubMed

Affiliation: Biomedical and Environmental Thematic Group, Ifakara Health Institute, P.O. Box 53, Ifakara, Tanzania. t.russell2@uq.edu.au

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 approximately 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.

Show MeSH
Related in: MedlinePlus