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Optimizing impact assessment of entomological intervention for malaria control in an operational setting in Zambia

Chanda, Emmanuel (2011) Optimizing impact assessment of entomological intervention for malaria control in an operational setting in Zambia. Doctoral thesis, University of Liverpool.

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Abstract

The study aimed at optimally assessing the impact of indoor residual spraying (IRS) and insecticide treated nets (ITNs) on vector species abundance, their infectivity and resistance status, and Plasmodium falciparum prevalence, malaria deaths and case fatality rates in the human population. Malaria prevalence surveys were conducted and routine surveillance data was retrospectively analyzed. The average P. falciparum prevalence in children between the ages of 1 and 14 years was below 10% across the study period. The intervention effect was more pronounced in IRS areas than in ITNs localities but with an incremental protective effect of their combined use. Age-specific comparison showed better intervention effect on children below 5 years than older children 5 to 14 years old. While the average number of deaths and case fatality rates in children under the age of five plunged precipitately, the reductions were more significant in IRS districts than in ITNs districts. Results indicate the need for supplementing parasite prevalence survey data with routine surveillance data in low transmission intensity areas and demonstrate the significance of evidence-based age-specific deployment of interventions. To monitor vector species abundance and infectivity, mosquitoes were collected daily using exit window traps. The three major vectors; An. gambiae s.s, An. arabiensis and An. funestus s.s, and three potential vectors of malaria, An. nili, An. rivulorum and An. funestus-like species were identified. Overall, the biggest impact of IRS and ITNs was on An. gambiae s.s, and An. funestus abundance. No An. gambiae s.s was collected in IRS localities, thus validating the fact that An. gambiae s.s and An. funestus are characteristically more amenable to control by IRS and ITNs than An. arabiensis. The transmission potential for all malaria vectors, as expressed by the calculated transmission index, was zero as none of the trapped mosquitoes tested positive for P. falciparum sporozoites. The identification of An. nili, An. rivulorum and An. funestus-like necessitate further research to determine their role in malaria transmission in the country. The low numbers of mosquitoes collected also indicate a compromise in the efficiency of exit window traps in low transmission settings, suggesting the need for their replacement with a more robust collection tool like the CDC light trap. While the persistence of An. arabiensis suggests the presence of resistance segregating in this population or, that this outdoor species is not in contact with IRS or ITNs, it could as well imply that it’s the one species perpetuating malaria transmission in these meso-to hypo- endemic areas. To determine the impact of interventions on insecticide resistance status of malaria vectors, susceptibility assays using the WHO standard protocol were conducted in 17 localities. High levels of resistance were detected in both An, gambiae s.l and An, funestus s.l to pyrethroids and DDT but with 100% susceptibility to malathion and bendiocarb. The level of resistance was significantly higher in IRS areas than in ITN areas. These findings indicate that resistance has been selected for following extensive vector control. Resistance to both DDT and deltamethrin in IRS localities and ITN areas with intense cotton growing was detected suggesting selection due to either historical use of DDT, gene flow or cross-resistance. All An. gambiae s.s were molecular s-forms and only the west (leu-phe) kdr was detected. Complete susceptibility to the organophosphates and carbamates provides a possibility to switch to these alternative insecticide classes for IRS. The detected increases in the malaria prevalence in localities with high insecticide resistance levels indicate vector control failure. These findings point to the need for information on underlying biochemical and molecular resistance mechanisms to make possible the design of an effective resistance management strategy, and for the assessment of the impact of resistance on interventions. The results indicate that the impact of malaria control can be optimally assessed by using a combination of epidemiological (routine surveillance and prevalence data) and entomological indicators, in the context of a malaria decision support system, to enhance policy formulation for objective implementation of malaria control interventions and rational use of available resources.

Item Type:Thesis (Doctoral)
Subjects:R Medicine > RA Public aspects of medicine > RA0421 Public health. Hygiene. Preventive Medicine
Departments, Research Centres and Related Units:Academic Faculties, Institutes and Research Centres > Liverpool School of Tropical Medicine
Status:Unpublished
ID Code:3413
Deposited On:04 Jan 2012 10:11
Last Modified:04 Jan 2012 10:11

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