This malaria survey in Mutasa District showed a decline in suspected number of malaria cases from a peak in 2008 following increased coverage with malaria control interventions. What was striking was the greater decline in RDT-positive cases among children younger than five years compared to older children and adults. As scale up of diagnostics, effective treatment regimens and vector control was implemented, the asymmetric decrease in malaria among different age groups poses a challenge to control and elimination efforts.
Although the malaria vectors in Mutasa District are not well characterized, malaria transmission increased despite coverage with IRS and ITNs [12, 13]. Coverage with vector control interventions as reported by the National Malaria Control Programme, particularly for ITNs, was higher in Mutasa District than that reported for many endemic countries , although district level data do not reflect national or provincial coverage levels. The modest impact of these control methods, particularly in older children and adults, may indicate vector species that are biologically or behaviourally resistant to the interventions. Presumably, indoor-based interventions worked better in protecting young children but not persons of older ages who are more likely to be bitten during farming and other occupational activities.
The malarial burden in Africa remains heterogeneous among areas undergoing apparently similar intervention efforts . In Ethiopia, there was a >60% reduction in malaria incidence following the introduction of IRS , but resurgent malaria despite extensive availability of ITNs was reported from Kenya . Cryptic outdoor-feeding vectors have been reported recently from western Kenya as a potential cause of sustained transmission in areas with high coverage with IRS or ITNs . Extensive use of ITNs has been reported to alter the composition of the vector complex, with relative reductions in An. gambiae s.s. and Anopheles funestus compared with Anopheles arabiensis[16, 18]. Changes in the composition of the vector complex in Tanzania paralleled a dramatic decline in malaria transmission . In areas where An. gambiae has acquired resistance to pyrethroids, ITNs are no more effective than untreated nets in reducing biting . Clearly, understanding the vector biology in Mutasa District is critical to designing effective and sustainable control strategies.
Overdiagnosis of malaria based on clinical suspicion is a known problem in Africa  and elsewhere, and confirmatory diagnosis is recommended. RDTs have been shown to be a cost-effective measure to ensure malaria treatment is directed to those who are infected [22, 23]. In Mutasa District, the use of RDT increased to approximately 80% of suspected cases and up to 75% of RDT were positive. The trend of increasing RDT positivity despite malaria control measures could reflect improved health staff selection of patients likely to have malaria. However, the improvement would have been specific for individuals older than five years. The proportion of all clinic cases that are RDT positive is known to decline when the burden of malaria is reduced, as other conditions become the primary cause for febrile presentations .
Modest reductions in the prevalence of malaria were observed following the introduction of ACT in Tanzania  and other areas in sub-Saharan Africa. These reductions in malaria prevalence appear to have occurred, in some areas, prior to local scale up of the vector control and the introduction of ACT. Low malaria prevalence hinders the ability to measure the impact of malaria control interventions. For example, an attempt to show the efficacy of gametocidal treatment in reducing malaria transmission in a low transmission region of Tanzania was not possible because of the reduction in malaria transmission in control villages .