malariacontrol.net: Science update part III: till June 2013

Dear malariacontrol.net user, In our third installment on the science update, we look at how your cpu cycles helped Olivier Briët and his colleagues explore the pressing issue of how insecticide resistance might affect the cost effectiveness of an intervention, as reported in Effects of pyrethroid resistance on the cost effectiveness of a mass distribution of long-lasting insecticidal nets: a modelling study. The effectiveness of insecticide-treated nets in preventing malaria is threatened by developing resistance against pyrethroids. Little is known about how strongly pyrethroid resistance affects the effectiveness of vector control programmes. In this analysis, data from experimental hut studies on the effects of long-lasting, insecticidal nets (LLINs) on nine anopheline mosquito populations, with varying levels of mortality in World Health Organization susceptibility tests, were used to parameterize models. Both simple static models predicting population-level insecticidal effectiveness and protection against blood feeding, and complex dynamic epidemiological models, where LLINs decayed over time, were used. The epidemiological models, implemented in OpenMalaria, were employed to study the impact of a single mass distribution of LLINs on malaria, both in terms of episodes prevented during the effective lifetime of the batch of LLINs, and in terms of net health benefits expressed in disability-adjusted life years (DALYs) averted during that period, depending on net type (standard pyrethroid-only LLIN or pyrethroid-piperonyl butoxide combination LLIN), resistance status, coverage and pre-intervention transmission level. The basis model features are displayed in a graphic of the useful lifetime of a single ITN distribution. As the nets age, the insecticide in the net wears out and the number of holes in the nets increases. These factors combine to limit the useful lifetime a single net distribution. Note that the slight bump in the baseline malaria level after the net distribution is no longer in effect is real: the cases averted and decreased exposure during the viable net distribution decreases immunity. With no other intervention, the episodes per person over time returns to the baseline level. With the most resistant mosquito population, the LLIN mass distribution averted up to about 40% fewer episodes and DALYs during the effective lifetime of the batch than with fully susceptible populations. However, cost effectiveness of LLINs was more sensitive to the pre-intervention transmission level and coverage than to mosquito susceptibility status. For four out of the six Anopheles gambiae sensu lato populations where direct comparisons between standard LLINs and combination LLINs were possible, combination nets were more cost effective, despite being more expensive. With one resistant population, both net types were equally effective, and with one of the two susceptible populations, standard LLINs were more cost effective. Despite being less effective when compared to areas with susceptible mosquito populations, standard and combination LLINs are likely to still be cost effective against malaria even in areas with strong pyrethroid resistance. So, well done you! for contributing to this work.

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