Tropical Health Matters

Mufaro Kanyangarara and her PhD thesis adviser, Luke Mullany of the Johns Hopkins Bloomberg School of Public Health Department of International Health, have been looking into the challenges of controlling and eventually eliminating malaria in a multi-country context in southern Africa. We are sharing abstracts from her pioneering work including the following which explores indoor residual spraying in Zimbabwe in a District near the Mozambique border.

In order to reduce the vector population and interrupt disease transmission, IRS with appropriate insecticides is essential. In response to local vector resistance, the Zimbabwe NMCP with support from PMI began a large-scale IRS campaign with organophosphates in four high transmission districts in Manicaland province – Chimanimani, Mutare, Mutasa and Nyanga. Using HMIS data, the present study reports on the effect of switching from pyretheroids to OP on malaria morbidity in one of the four high transmission districts selected. In the subsequent high transmission season following the switch from pyretheroids to organophosphates, there was evidence of a 43% decline in malaria incidence reported by health facilities from wards in Mutasa District treated with organophosphates, after accounting for possible confounding by environmental variables. Previous research shows that switching to organophospates effectively reduced biting rates and vector densities in areas with pyretheroid resistant strains in Ghana, Benin and Tanzania. Although previous research focused on using entomological data to show the reduction in the vector population following application of Actellic, organophosphates, this study adds to the literature by showing a decline in malaria transmission using health facility surveillance data.

In the present study, there were variations in rainfall and temperature over the study period, and these changes were associated with changes in malaria incidence. The study results also indicated malaria transmission in Mutasa District was driven by rainfall, proximity to second order streams, elevation and temperature. These results concur with previous research, which found that elevation, temperature, and rainfall are positively associated with malaria incidence. After adjustment for climatic variables and seasonality, malaria incidence rates a downward trend following the 2014 IRS campaign and thus supporting the plausible conclusion that switching to organophosphates in this setting contributed to the observed public health benefits. No major political, socio-economic, or health-care changes with the potential to reduce malaria morbidity by almost half occurred in Mutasa District during the study period.

Typically data from health facilities only includes data on the number of suspected cases. The HMIS in Zimbabwe is more sophisticated in that it allows reports of confirmed malaria cases. In calculating of incidence rates, the denominator used was the catchment area population size. The reliability of this value has been questioned as this assumes that people will visit the closest health facility/health facility in their catchment area. It is noteworthy to mention that in the present study the main results did not chance after including an offset for catchment area population size. This indicates that in the Zimbabwean context, the reported catchment area population size may be a reliable estimate. The study also underscores the utility of HMIS data in the evaluation of population level interventions. The HMIS has the advantage of providing quality data quickly and easily, with minimal additional investment. Additionally, HMIS reflects the burden of disease on the health system. Results from this study further suggest that passive surveillance data from the HMIS in Zimbabwe was sufficiently sensitive to detect IRS related reduction in malaria morbidity among residents of Mutasa District.

There are several important limitations of this study that should be highlighted. Causal inferences between spraying and improvements in malaria incidence should be made with caution as spraying was not implemented as an intervention in a randomized control trial. However, data from 14 health facilities located in unsprayed wards were included in the analysis to serve as a comparison and help understand any possible changes in malaria morbidity unassociated with the 2014 IRS campaign. Although the univariate model indicated that health facilities in unsprayed wards carried a lower burden of malaria, the multivariable model showed no significant differences between health facilities in sprayed and unsprayed wards prior to the IRS pilot, suggesting that climatic variables included in the model adequately adjusted for differences. However, it should be noted that although the study adjusted for environmental factors, it did not account for other factors like population movement, changes in treatment seeking behaviors, changes in the coverage of ITNs during the study period. The model developed in this analysis assumed that these factors remained constant over the study period. This seems reasonable given that the rural population of Mutasa is relatively stable, with access to health facilities providing malaria diagnosis and treatment. Additionally, although the number of suspected malaria cases was not explicitly model, a descriptive analysis does not indicate changes in diagnostic practice over the study period (data not shown). The HMIS in Zimbabwe has been in place for decades and has previously been used to evaluate the impact of changes in malaria morbidity, construct empirical seasonality maps and describe the spatial and temporal distribution of malaria.

Despite these potential limitations, health surveillance systems provide a feasible and efficient means of collecting longitudinal data on measures of malaria morbidity. The pronounced decline in malaria morbidity observed in this study is evidence supporting the benefit of switching to an insecticide class with a different mode of action in response to pyretheroid resistance. Although the IRS strategy implemented by ZNMCP and PMI was successful, continued entomological monitoring will be necessary. Additionally, with emerging resistance to multiple insecticides, this approach may not be sustainable over time. There is need for the development of novel strategies to manage insecticide resistance.

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