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PMI Activity TZ-1,2: IRS and LLIN: Integration of Methods and Insecticide Mode of Actions for Control of African Malaria Vector Mosquitoes

Okumu, F. O. and Moore, S. J. (2012) PMI Activity TZ-1,2: IRS and LLIN: Integration of Methods and Insecticide Mode of Actions for Control of African Malaria Vector Mosquitoes. UNSPECIFIED. (Unpublished)

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Long lasting Insecticidal nets (LLINs) and indoor residual spraying (IRS) are the preferred techniques for malaria vector control in Africa, where their application has a proven contribution to the recent significant reductions in the burden of the disease. Even though both methods are commonly used together in the same households, evidence of improved malaria control due to the use of combinations as opposed to use of either method alone has been minimal and inconclusive.To measure the mode of action of three classes of insecticides used for IRS at the WHO recommended dose: the organochlorine DDT 70 wettable powder (AVIMA, South Africa) at 2g/m2; the pyrethroid lambda-cyhalothrin capsule suspension ICON CS, (Syngenta, Switzerland), at 0.03g/m2; and the organophosphate pirimiphos-methyl (PM) emulsified concentrate, also known as actellic (Syngenta, Switzerland), at 2g/m2 used alone or in combination with three leading LLIN brands: PermaNet 2.0® nets (Vastergaard, Switzerland), Olyset® nets (manufactured by A-Z, Tanzania), and Icon Life® nets (Bestnet Europe ltd, Denmark). All LLINS were used intact and were not subjected to repeated washing to reflect their optimum performance. The control was untreated polyester net. Data were collected from experimental huts developed during the project to measure both behavioral and toxic modes of actions of insecticides in Southern Tanzania. The primary malaria vector is Anopheles arabiensis with >90% susceptibility to insecticides of all classes at diagnostic doses in WHO susceptibility assays. Two rounds of data collection were performed: 1) 4 months during the dry season 2) six months during the wet season. Data generated from the experimental hut studies were analysed with Poisson-lognormal generalized linear mixed effects models (GLMM). Data was also simulated using deterministic mathematical model to measure potential impacts of each IRS, LLIN and combination thereof on malaria at a community level. Bite prevention (feeding inhibition): During both rounds, all the IRS treatments, LLINs and the controls (which consisted of intact untreated mosquito nets), provided greater than 99% protection from potentially infectious bites by the malaria vector, An. arabiensis, for the entire duration of the study. Most of the mosquitoes were caught inside the exit traps as opposed to inside the experimental huts, regardless of whether the huts were had LLINs, IRS or non-insecticidal nets. More than 95% of An. arabiensis, Culex pipiens quinquefasciatus and Mansonia africana / uniformis mosquitoes were caught inside the exit traps while exiting the huts. Toxicity: All IRS treatments, all the LLINs and the majority of LLIN/IRS combinations significantly increased proportions of dead An. arabiensis mosquitoes, relative to the control huts. The most toxic IRS relative to the controls was PM (RR = 2.21 (1.82 – 2.68), P < 0.001), followed by ICON CS (RR = 1.55 (1.27 – 1.89), P < 0.001) and then DDT (RR = 1.44 (1.18 – 1.77), P < 0.001). The most toxic LLIN relative to the controls was PermaNet 2.0® nets (RR = 1.65 (1.58 – 1.74), P < 0.001), followed by Icon Life® nets (RR = 1.55 (1.42 – 1.69), P < 0.001) and then Olyset® nets (RR = 1.33 (1.12 – 1.47), P < 0.001). Combinations of IRS and LLINs relative to LLINs alone: In most cases, there was no significant increase in An. arabiensis mortality in huts combining LLINs plus IRS, relative to huts having LLINs only, except in cases where the specific IRS treatment was PM. Addition of PM significantly increased proportional mortality of An. arabiensis when combined with Olyset® nets (RR = 1.38 (1.14 – 1.65), P = 0.001), PermaNet 2.0® nets (RR = 1.42 (1.18 – 1.71), P <0.001) and Icon Life® (RR = 1.24 (1.03 – 1.49), P = 0.023). Combinations of LLINs and DDT or lambda cyhalothrin resulted in marginal increases in An. arabiensis mortality relative to huts with LLINs alone although none of these combinations resulted in a statistically significant increase. Combinations of IRS and LLINs relative to IRS alone: There was a trend of significant increases in An. arabiensis mortality in huts having IRS plus LLINs, relative to huts having just the IRS alone, except for the combinations of 1) Olyset® with ICON CS, 2) DDT with Olyset® or 3) DDT with Icon Life® nets. In the huts that had been sprayed with PM, there was a significant increase in An. arabiensis mortality whenever Icon Life® nets (RR = 1.39 (1.18 – 1.63), P < 0.001), Olyset® nets (RR = 1.32 (1.13 – 1.55), P = 0.001) or PermaNet 2.0® nets (RR = 1.26 (1.08 – 1.48), P = 0.004) were added, relative to the huts where PM IRS was used alone. Similarly, in the huts that had been sprayed with ICON CS, there was a significant increase in An. arabiensis mortality in combination with Icon Life® nets (RR = 1.43 (1.19 – 1.73), P < 0.001) or PermaNet 2.0® nets (RR = 1.70 (1.35 – 2.13), P < 0.001), but not Olyset® nets (RR = 1.16 (0.92 – 1.45), P = 0.210), relative to the IRS alone. In huts sprayed with DDT, none of the LLINs significantly improved proportional mortality of the An. Arabiensis mosquitoes, except PermaNet 2.0® nets (RR = 1.18 (1.06 – 1.32), P = 0.003). Residual efficacy bioassays of IRS: All IRS formulations were highly effective during the first month after spraying and rapidly decayed losing most activity within 1-3 months. In month 1, all An. arabiensis exposed to palm ceilings sprayed with either PM or ICON CS died, and 85% were killed by DDT (despite full susceptibility most likely because it flaked away). On mud walls sprayed with the same chemicals, 100%, 90.0% and 97.5% mortality was observed, respectively, during the first month. Activity of the IRS declined significantly so that by the third month, PM on palm and mud killed 42.5% and 55.0% of exposed An. arabiensis, respectively. ICON CS killed only 46.3% on palm and 52.5% on mud walls. By month 6, PM had nearly entirely decayed, killing only 7.5% of An. arabiensis exposed to sprayed palm ceilings and 27.5% of those exposed to sprayed mud walls; ICON CS killed 30.0% on ceilings and 27.5% on walls. DDT had a longer residual action, killing 42.5% of An. arabiensis exposed to sprayed ceilings, and 36.3% of those exposed to sprayed walls after 6 months. Residual efficacy bioassays of LLINs: While all the LLINs generally performed better (i.e. killed more mosquitoes) on wire frame assays than on the cone assays, their activity rapidly deteriorated by the second month of use relative to new nets. Only PermaNet® nets retained mosquitocidal efficacy of >80% by the sixth month of net use (killing 92.7% on wire ball tests and 84% on cone assays). All the LLINs however retained very high knock-down rates (> 90% in wire ball tests and >80% in cone tests) on the exposed mosquitoes, except Olyset® nets whose knock-down activity reduced to 72.7% on wire ball tests and 62% on cone tests by the sixth month. Both the field studies and the model simulations showed that any synergies or redundancies resulting from LLIN/IRS combinations are primarily a function of modes of action of active ingredients used in the two interventions. None of the IRS or LLINs tested was deterrent so they do not protect by keeping mosquitoes from houses in this setting. Very few mosquitoes were able to obtain a blood meal due to the use of intact LLINs and untreated control nets. Therefore, where households are correctly using and maintaining LLINs there is no added value in the additional application of IRS unless the IRS chemical is highly toxic and non-irritant, as is PM. This compound consistently increased mosquito mortality in combination with any LLIN even though mosquitoes did not rest indoors as they were unable to obtain a blood meal. The average duration of effect of insecticides in this setting was 3 months, far lower than that stated by the manufacturers, so IRS should be carefully timed. Where IRS is the pre-existing intervention, providing households with additional LLINs confers additional protection. Therefore, IRS households should always be supplemented with nets, preferably LLINs, which not only protect house occupants against mosquito bites, but also kill additional mosquitoes. Finally, where resources are limited, priority should be given to providing everybody with LLINs and ensuring that these nets are consistently and appropriately used, rather than trying to implement both LLINs and IRS in the same community at the same time.

Item Type: Other
Keywords: Malaria Vector, Mosquitoes, Long Lasting Insecticidal Nets, Indoor Residual Spraying, Tanzania
Subjects: Malaria > Surveillance, monitoring, evaluation
Malaria > Vector control
Divisions: Ifakara Health Institute > Impact Evaluation
Depositing User: Mr Joseph Madata
Date Deposited: 17 Jul 2013 08:36
Last Modified: 17 Jul 2013 08:36

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