Background
Malaria remains the most important vector-borne disease globally and continues to severely undermine the socio-economic growth in sub-Saharan Africa where it exacts its greatest toll, particularly in children and pregnant women [
1,
2]. In 2012, there were an estimated 207 million malaria cases and 627,000 deaths globally with almost 84% of the cases and 92% of the deaths reported from sub-Saharan Africa [
3]. The post-2015 global technical strategy for malaria supports countries in reducing the disease burden and accelerating progress towards elimination [
4]. The strategy accentuates the strategic direction, sets milestones and targets to 2030, and provides a framework for countries to adapt to the national strategic plans. While several documents to guide the transition from malaria control to elimination exist [
5,
6], endemic countries have faced a multiplicity of challenges on their path to malaria elimination [
7].
To mitigate vector control constraints, the World Health Organization (WHO) recommends adoption and implementation of integrated vector management (IVM) along the five key strategic elements as a platform for effective vector control [
8] and the global plan for insecticide resistance management (GPIRM) along its five pillars as a means of preventing the development and spread of resistance [
9]. Furthermore, guidelines for vector surveillance to facilitate entomological data collection [
10], guidance documents on strengthening of reporting systems on dichloro-diphenyl-trichloroethane (DDT) in relation to disease vector control [
11] and training modules for vector control and entomology [
12] have been developed. To facilitate efficient and timely implementation, the WHO has developed operational plans for IVM, a framework for the GPIRM [
13] and operational manuals for indoor residual spraying (IRS) [
14], larval source management (LSM) [
15], and long-lasting insecticidal nets (LLINs) [
16].
In 2009, the elimination eight (E8) initiative was established consisting of countries with potential of malaria elimination: Botswana, Namibia, South Africa and Swaziland including their neighbours Angola, Mozambique, Zambia and Zimbabwe [
17]. The government of Namibia launched a malaria elimination campaign in 2010 and reoriented from the objective of reducing disease morbidity and mortality to the goal of achieving elimination by 2020 [
17]. Accordingly, systems are being strengthened to identify and clear all infections, and significantly reduce human–mosquito contact [
18]. Namibia envisions progressing towards elimination through a phased approach with some districts targeting identified foci to interrupt transmission and others consolidating control before entering the pre-elimination phase [
18].
Adaptation of interventions both to the biology and behaviour of local vector species counting the geography and epidemiology of the targeted malaria foci is imperative in the preparatory phase [
10]. As Namibia transitions towards elimination, strengthening vector control as a vital attack weapon of reducing transmission will be critical. Thus, guarding against potential challenges and the need for evidence-based and sustainable vector control prompted the consolidation of strategic frameworks by: adopting the IVM strategy [
18]; initiating implementation of the GPIRM [
19]; intensifying malaria vector surveillance [
20]; strengthening data collection and reporting systems on DDT [
21]; updating the IRS data collection and reporting tool; and, improving geographical reconnaissance through geographical information system (GIS)-based satellite imagery [
21,
22]. This manuscript outlines the processes undertaken in strengthening tactical planning and operational frameworks for vector control to facilitate informed decision-making for expeditious attainment of malaria elimination in Namibia.
Insecticide-treated nets
Distribution of ITNs to women only began in 1993 in northern Namibia. In 2002, targeting of ITNs to only children <5 years of age and pregnant women was commenced. Between 2002 and 2004, about 52,000 ITNs were distributed at no cost [
17]. In 2005 a policy change instituted broader targeting of at-risk groups, including children under 5 years of age and pregnant women with LLINs complimented by mass distribution. LLIN distribution was augmented in 2008 to compensate for lower IRS coverage [
18]. In 2009, the distribution policy further shifted towards universal coverage with one LLIN per two people in moderate transmission areas. In the same year, 53% of households had at least one LLIN and usage among children <5 years and pregnant women was 34.0 and 25.9%, respectively [
34]. However, since 2010, distribution of LLINs targeted the general population to accelerate universal coverage. From 2005 to 2011, over 625,000 LLINs were distributed at health facilities, outreach sites, antenatal clinics, and via mass campaigns to villages [
27]. In 2012, the NVDCP set a new goal to achieve 95% LLIN coverage of the entire population, shifting from just vulnerable populations to all those living in regions with any risk of malaria transmission by 2014. In 2013, a mass distribution of 87,900 LLINs was targeted to villages with the highest malaria caseloads in Zambezi, Kavango,and Omusati [
27].
Consolidating malaria vector surveillance
As Namibia transitions to malaria elimination, vector surveillance will be an integral aspect of the IVM strategy in two ways: (1) to provide evidence for decision-making in IVM and (2) for evaluating a programme’s impact on vector populations. It will also be used for monitoring and evaluation where the surveillance sites are located in or near the implementation settings. These investigations would provide information on malaria vector species composition, their distribution, population density, feeding and resting behaviour, infectivity rate, longevity of vectors, seasonal activities, larval habitats, susceptibility to insecticides, and quality and residual effect of insecticides used for malaria control [
20]. Because of a long sustained IRS programme in Namibia which reduced vector populations to very low levels, some of the indicators mentioned here may be difficult to measure. In this regard, vector surveillance will support case detection and stratification of malaria foci using both epidemiology and entomology data for focal application of IRS in areas where malaria cases are very few to justify blanket spraying, which is important in malaria elimination programmes.
Country-specific vector surveillance guidelines have been developed and include: entomological field and laboratory techniques, WHO contact and susceptibility bioassays, mosquito rearing in the insectary, organization of entomological teams, geographical information systems (GIS) and supervision of entomological teams and operations [
20]. Namibia is implementing an elimination strategy as such, stratification (mapping) of active, passive, new foci becomes imperative. Malaria is generally heterogeneous in distribution except in areas along the Kavango river, where disease distribution remains generally homogenous. In this regard spot checks to support surveillance at sentinel sites with active vector surveys conducted together with case detection teams are recommend and investigations will be carried out regularly at fixed locations to: (1) reduce natural variation, costs and labour intensity; (2) to increase the usefulness of timely collected data from surveillance in decision-making, and (3) to maximize the use of available resources. Thus sentinel sites are being strengthened and training of national staff in basic entomological monitoring, field and laboratory techniques including insectary management has been embarked upon in Namibia [
20]. While only one insectary exists in Namibia, it has been characterized by a long state of inactivity.
Discussion and evaluation
Further to the evidence-based deployment of effective and proven malaria control interventions by the NVDCP, tremendous change in the epidemiology of the disease has been experienced across Namibia [
30]. The country is at an exciting turning point with its elimination potential. However, there is an inherent risk of complacency and lack of motivation that needs to be guarded against, as historically evidenced in other countries who have reached elimination [
17]. If efforts are not maintained, a resurgence of malaria could easily occur, threatening the progress and gains made to date. A sustained political and financial commitment will be necessary for Namibia to remain malaria free once elimination is achieved. While case management and surveillance are key, strengthened vector control will be critical during the pre-elimination phase in Namibia [
18].
The IVM strategy defined by the WHO as “a rational decision-making process to optimize the use of resources for vector control” is a combination of proven malaria vector control methods based on the knowledge of local vector biology and ecology of the disease transmission [
8]. This can be a singular intervention, but is usually a combination of multiple interventions in a synergistic approach. Integrated vector control methods include IRS, distribution of LLINs, larviciding and personal protection methods both synthetic and traditional. IRS and LLIN distribution are the hallmark vector control interventions in Namibia [
18]. However, these should be implemented based on annual review of malaria vector bionomics, susceptibility studies and disease transmission. In this regard, the NVBCP is considering adoption of contemporary innovative vector control strategies including eave tubes and Entomopathogenic bacteria traps, durable wall linings including personal protection to combat outdoor transmission through Entomopathogenic fungus-impregnated targets, attractive toxic sugar baits and spatial repellents.
The fact that insecticide resistance selection pressure is driven primarily by gene flow, agricultural and public health use of insecticides as well as cross resistance has got implications for malaria vector control in Namibia. While no insecticide resistance in malaria vectors has been reported so far in Namibia, resistance has been detected in all the neighbouring countries except Botswana. In Angola, knockdown resistance (
kdr) west—the L1014F mutation—has been found in
An. gambiae M-form individuals [
37]. In Zambia
kdr west (1014F) and east (1014S) alleles to pyrethroids and DDT, and over expression of cytochrome P450s in pyrethroid resistance and glutathione S-transferases (GSTs) in DDT resistance has been reported in
An. gambiae s.s, including over expression of GSTs in pyrethroid resistance and carbamate resistance in
An. funestus s.s. [
38,
39]. In Zimbabwe,
An. arabiensis resistance to both DDT and permethrin has been reported in Gokwe District with high activity levels of P450s, GSTs and general esterase activity detected [
40]. In South Africa, high resistance to pyrethroids in
An. arabiensis has been demonstrated [
41], including P450-based metabolic resistance to DDT in
An. gambiae s.l. [
42] and P450 monooxygenase and/or GST-mediated pyrethroid resistance in
An. funestus [
43]. This situation highlights the extent of the resistance problem in southern Africa and necessitates frequent and extensive resistance monitoring in well chosen sentinel sites across Namibia and an urgent need for pre-emptive resistance management strategies preferably annual rotations of insecticides.
In an elimination setting, quality surveillance data is crucial in informing and guiding the targeting of interventions. Though Namibia is transitioning towards malaria elimination, there are still very limited data available on the spatiotemporal bionomics and insecticide resistance status of malaria vector species to guide targeted and effective control, including monitoring of potential vectors and the role they could play in disease transmission. To ascribe the impact to vector control activities, such information on vector attributes must be known and data needs to be collected in a timely manner to serves as an evidence base for decision-making in IVM. However, some of these activities may require special expertise and equipment. For the purpose of evaluating local effects of IVM, new sentinel sites should be selected in the intervention and control areas [
20]. While vector surveillance systems are often concentrated on one disease, implementation of an IVM strategy will enable vector surveillance to cover the vectors of other vector-borne diseases, thus improving the efficiency of resource use in Namibia. The NVDCP should establish data quality self-assessment (DQS) system to ensure accuracy, timeliness and completeness [
17]. The DQS should consist of tools that are designed for programme staff at the national, regional and district levels. It helps to evaluate different aspects of the programme monitoring system at region and district levels in order to determine the quality of reported data and address the identified gaps.
While the deployment of an IVM based vector control strategy will facilitate heightened advocacy and resource mobilization [
44], the current malaria vector control efforts are characterized by apparent gaps in entomological evidence: key informational gaps that need to be addressed in the short term include; frequent and extensive susceptibility testing, increasing the number of sentinel sites and mapping of malaria vector species. Medium and longer-term knowledge gaps include; vector biting and resting behaviour, host choice or infectivity rates [
35]. Short-term action will be to support monitoring of vector density, infectivity, bionomics and quantifying insecticide resistance to guide informed decision-making and to determine the impact of resistance. Long-term solutions will involve establishing a rational IRM plan to reduce the selection pressure thus preventing or delaying resistance development, and aiming to reduce dependency on chemical insecticides for vector control [
35].
In Namibia, efficient and effective data collection and reporting procedures for evaluating the continued need of DDT for disease vector control have been successfully established [
21]. Potential stakeholders alliance for effective reporting of insecticides used for diseases vector control, particularly DDT, include the Ministry of Environment and the Ministry of Agriculture, water and forestry [
21]. The NVDCP should consistently convene annual stakeholders alliance meetings for effective reporting of public health insecticides principally DDT. Strengthened coordination of the stakeholder alliance to facilitate timely and quality compilation and reporting on DDT based on the standardized format to the VBDCP and to the focal person of the Secretariat of the Stockholm Convention should be prioritized. This will enable the conference of parties of the Stockholm Convention, in consultation with the WHO to evaluate the continued need for DDT [
11].
With the malaria incidence declining to low levels in Namibia, and the country setting 2020 as the target for malaria elimination, many of the residual malaria cases occur in the border regions with Angola. Presently, transmission is sustained in Kunene, Omusati, and Ohangwena regions constituting the Trans-Kunene Malaria Initiative [
45,
46]. Human population movement has been identified as one of the major obstacles to malaria elimination [
23]. It is possible that malaria transmission is associated with cross border movements of people between Angola and Namibia. Moreover, the northern parts are wetter and the potential of sustaining the presence of additional Afro-Tropical malaria vectors is high. Given the variable behaviour of
An. arabiensis, in areas where the mainstay for vector control has been IRS with DDT since the 1960’s, reassessment of the vector species composition and its resting and biting behaviour are important [
47]. It is imperative to further explore the presence of
An. funestus along the border of Kavango and Angola and that of Caprivi and Zambia. The possibility of re-introduction from Angola and Zambia cannot be completely ruled out thus calling for closer coordination of malaria activities through the Trans-Zambezi Malaria Initiative. However, strengthened collaboration and involvement of high level decision makers IN cross-border meetings between the countries will be key and would stimulate and ascertain implementation of recommendations from such meetings.
The mandate of informing, educating and mobilizing the communities on the malaria elimination goals in Namibia rests with the NVDCP. This requires unremitting and well-coordinated information, education and behaviour change communication to promote knowledge and awareness on malaria, create a sense of ownership and guiding all levels of society to achieve malaria elimination. Equally, strategic and effective advocacy to mobilize domestic (public and private) support will be critical for the sustainability of the malaria elimination campaign. To achieve this, the NVDCP should; (1) coordinate the development of all malaria IEC and advocacy material and ensure the harmonization of its content in collaboration with partners, and (2) advocate for increased and continued political, financial, and technical support and mobilize all stakeholders and partners in its efforts to eliminate malaria [
17].
The requisite technical and infrastructure capacity for entomological capacity in Namibia remains minimal or nonexistent at all levels of service delivery i.e. national, regional and local levels. This nominal local science capacity evidenced by the very few publications by in-country scientists remains a major challenge for achieving elimination. Cognizant of the fact that building this capacity will take a long time, deliberate efforts to establish strategies to strengthen this aspect of the NVBDP will be critical for expeditious elimination of malaria. As such, networks for empirical and operations research including malaria vector species, bionomics and insecticide resistance monitoring and management should be established and operationalized. This could be achieved through well coordinated collaboration with research and academia institutions i.e. national universities, as well as by creating an enabling environment for external institutions to develop interest in entomological research in the country.
Namibia has implemented a very successful malaria vector control programme using IRS, LLINs and larval control in the context of IVM. The use of other personal protective interventions is being encouraged by the NVDCP and updated malaria vector control policies and guidelines are in place. The success of the malaria control in Namibia has been a function of total government political commitment and deployment of effective control strategies that is decentralized to district level with a strong partnership support and collaboration including the communities. However, the programme has been grappling with a number of challenges in the implementation of interventions including data collection and reporting, technical capacity due to shortage of staff, operational budgets and other requisite resources. As the country re-orients towards malaria elimination, it is imperative that vector surveillance, data collection and reporting, technical and operational constraints are circumvented within the programme. To expedite the transition towards elimination an emergency mode should be adopted by the control programme.