The findings demonstrated an overall decreasing trend in malaria morbidity in Mutale during most seasons of the study period, most remarkebly after a peak during the 2007/08 season. The decreasing trend is in line with patterns that have been reported elsewhere in Limpopo Province and in other malarious provinces in South Africa [9, 16]. The decrease has been attributed to the synergistic effect of the scaling up of IRS in the country and the introduction of artemisinin-based combination therapy (ACT) for the treatment of uncomplicated malaria in 2004 . Implementation of widespread use of ACT in the treatment of malaria has been shown to directly decrease malaria transmission, in addition to improving the malaria cure rates [18–20].
Furthermore, the implementation of the tri-national Lubombo Malaria Protocol as part of the Lubombo Spatial Development Initiative (LSDI) aimed at enhancing economic development in the Lubombo region (a mountainous region shared by South Africa, Swaziland and Mozambique) probably contributed to the decline of malaria in Mutale [21, 22]. Although Mutale does not directly fall within the Lubombo region, the municipality significantly benefited from the reduction of malaria in neighbouring countries such as Mozambique as a result of the LSDI malaria control efforts. Findings from this study seem to support this view since only a small proportion of the imported malaria cases were from Mozambique despite Mutale being situated along her border, and the majority of the imported cases were from Zimbabwe which was not part of the LSDI. The LSDI initiative has also been complimented for reducing the burden of malaria in areas neighbouring the Lubombo region in southern Mozambique and in Swaziland .
Interestingly Mutale municipality experienced a surge in the incidence of malaria during the 2007–2008 season. Despite the high IRS coverage achieved during this season (exceeding 100%), Limpopo Province recorded an outbreak of malaria from March through April 2008. The outbreak was related to excessive rainfall recorded in the province from January to March 2008. Excessive rainfall has been shown to increase malaria transmission in various ways. Firstly, the increased rainfall provides more breeding pools for mosquitoes leading to an increased population of the malaria vector (Anopheles arabiensis) [23, 24]. Secondly, the high temperatures and humidity often associated with high rainfall, compel people to spend the nights in cooler open areas outside the houses (or with open windows) thus further exposing them to the malaria vectors. Thirdly, prolonged heavy rainfall interrupts the IRS programme by disrupting the movement of the teams that will be conducting IRS, as some communities become inaccessible due to bad roads and possibly flooding. Consequently, the reported high IRS coverage (of 104%) possibly resulted from mop-up spraying of households in response to the outbreak, which normally includes re-spraying of some of the previously sprayed households.
One of the targets of the malaria control programme in South Africa is to maintain malaria case fatality rates (CFR) below 0.5% . The present study revealed that malaria case fatality rates in Mutale were high in the older population (above 25 years of age). In a study conducted in Limpopo Province in 2008, Gerritsen and colleagues reported higher malaria CFRs in the older population, and they attributed this to the possibility of more severe malaria in the elderly and also the possibly of poor health-seeking behaviour in this age group . However the study findings are in contrast to figures reported in most developing countries with high malaria endemicity, where the majority of malaria deaths occur in children below the age of five years . According to statistics released by the Global Malaria Action Plan in 2009, at least 85% of malaria deaths that occurred globally in 2008 were in children below the age of five years .
Mutale maintained fairly high IRS coverage throughout the study period. However further enquiry on how the province derives IRS targets revealed that the actual population of household structures in Mutale municipality is not documented. When the malaria control programme is planning the IRS season the programme managers estimate the targeted structures based on the number of household structures that were sprayed in the previous season. This method of estimating the number of structures targeted for spraying does not take into account the actual population at risk of malaria. IRS for malaria control is a method for community protection, and to be effective, implementation has to be targeted carefully, treating only where and when necessary [14, 17]. According to WHO guidelines , planning for an IRS season should include accurate estimation of the total population and determination of the total number of rooms that are used for sleeping or relaxing at night in the target area. This enables the programme managers to calculate the proportion of the population and the proportion of rooms they managed to protect through the IRS efforts at the community level, which are very useful process indicators for monitoring the performance of the IRS programme. The malaria control programme should work closely with the local community leaders in order to obtain accurate figures for the population figures at community level.
The decreasing malaria incidence in Mutale suggests that the malaria control efforts implemented during the period under study have been fairly effective. If the declining trend is sustained, it is feasible that Limpopo Province will soon be entering the pre-elimination phase of malaria. When a country/province/district is transitioning into the so-called “pre-elimination” phase of malaria, the WHO recommends a re-orientation of the malaria programme, which should be characterized by the adoption of more rigorous surveillance strategies in order to accurately measure and document the malaria incidence rates [4, 6]. Accordingly, the malaria control programme in Limpopo should strengthen the surveillance, reporting and the capturing of data in the provincial malaria information system. This is especially important for areas like Mutale Municipality where there is high potential for imported malaria through the porous borders with Zimbabwe and Mozambique, countries that remain highly malaria endemic. The surveillance strategy should therefore ensure that all diagnosed malaria cases are investigated to determine the source of the malaria, in order to accurately monitor local transmission and document transmission due to importation. The malaria control programme should also ensure regular update of the denominator data required to compute rates of malaria morbidity and mortality as well as other programmatic indicators.
Several limitations should be taken into account when interpreting the findings from this study. Firstly, 24% of the malaria cases had no documentation of the location where malaria was presumably contracted. Some of these could have been people who came for treatment from outside Mutale, and thus could have resulted in an overestimation of the true incidence of malaria in Mutale. Secondly, though the population of Mutale is administratively divided into villages, village-specific population figures could not be obtained, and thus the study could not calculate the malaria incidences at the sub-municipality level to determine the problematic areas in terms of the high risk of malaria transmission. Similarly the number of household structures per village could not be obtained; therefore the study could not determine the IRS coverage down to the village level. The findings would have been more useful if the analysis had managed to map the malaria incidence rates and IRS coverage in Mutale at sub-municipality level. Thirdly, the data used in this study is routinely collected through the provincial malaria surveillance system, which relies mostly on passive reporting. It is therefore possible that some malaria cases (or deaths) could have been missed by the system (because of non-reporting or misdiagnoses), thereby underestimating the true incidence of malaria (or the malaria CFRs). However there were no changes in the reporting requirements for malaria during the period under study, it is therefore unlikely that any cases missed by the system could have affected the observed trends.