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Limited Impact of indoor residual spraying (IRS) for malaria control in Tanzania 2014 - 2020
A retrospective analysis
Bisanzio, D., Kabula, B. I., Mkali, H. R., Lalji, S. M., Al Mafazy, A.-W., Joseph , J., International, RTI., Ally, M., Gitanya, P., Abbas, F. B., Haji, K. A., Khatib, B., Kitojo, C., Serbantez, N., Reaves, E., McKay, M., Eckert, E. L., Ngondi, J. J. M., & Reithinger, R. (2021). Limited Impact of indoor residual spraying (IRS) for malaria control in Tanzania 2014 - 2020: A retrospective analysis. American Journal of Tropical Medicine and Hygiene, 105(5 Supplement), 102.
Indoor residual spraying (IRS) has been a key malaria intervention in Tanzania since 2006. This retrospective analysis aims to evaluate the impact of IRS on malaria incidence in Tanzania’s high burden Lake Zone (district level) and Zanzibar (shehia [sub-district] level). Routine data from 2014 to 2020 were collected (District Health Information System 2 for mainland Tanzania and Malaria Case Notification system for Zanzibar) and separately analyzed. Districts and shehias were grouped into ‘No IRS’ and multiple years of IRS in the same locations (‘1-2’; ‘3-4’; ‘>4’ years). The annual epidemiological trend was obtained by calculating the percentage difference between the incidence in the month pre-IRS (baseline month) and the incidence during the 9 months post-IRS (i.e., corresponding to the organophosphate insecticide’s residual efficacy used for the corresponding IRS year). Spatial regression models, with random effect (to adjust for other factors), were developed to evaluate IRS’s effect on incidence between and within years, and accounting for multiple IRS rounds. During the study period, 23 districts and 205 shehias received at least one round of IRS. For the entire period, model results showed a significant protective effect of IRS on malaria incidence in mainland Tanzania (-63.9%: 95% CI: -183.3%; 17.6%) and Zanzibar (-24.9%; 95% CI: -52.6%, -3.2%). However, on a yearly basis, the protective effect of IRS was temporally and spatially heterogeneous: in Mainland Tanzania, the highest number of sprayed districts where IRS showed a protective effect was in 2016 and 2017 (7/9, of which 4 were statistically significant); in Zanzibar, 19% of sprayed shehias in 2018 showed a significant protective effect versus 4% in 2019. In Zanzibar, incidence (OR 1.15; 95% CI: 1.10, 1.19) and cases in the prior year (OR 0.98; 95% CI: 0.96, 0.99) were positively and negatively associated with malaria incidence trends, respectively. Our analytical approach can be used to improve IRS monitoring as well as targeting, ensuring that the impact of IRS on malaria is maximized.
