Malaria parasites: elimination is not eradication

Abstract

For those of us who participate in the fight against malaria, the present time stands in stark contrast to what we have always known. In comparison with the eve of this century, less than a dozen year ago, there is presently some unanimously accepted good news. The global burden of malarial disease has been reduced in a large variety of settings, regardless of transmission intensities and endemic patterns. Malaria prevalence, morbidity and mortality are now low enough in several areas of tropical Africa to challenge some well-established principles of malariology. There are three major reasons for this welcome improvement: (i) the unprecedented increase in funding devoted to malaria control; (ii) generalization of the shift from chloroquine to artemisinin-based combination therapy for the treatment of clinical malaria attacks; and (iii) deployment of a much better prevention strategy, largely owing to insecticide-treated bed-nets and, in some places, to residual wall spraying with insecticides. Two other approaches also proved to be very effective where they were deployed: intermittent preventive treatment, probably equivalent to targeted chemoprophylaxis, focused at the two most vulnerable populations—pregnant women and children; and improvement of case detection with rapid diagnostic test kits that are easily usable at the patient’s bedside by both local health workers and nurses. Although prompt access to effective drugs has been shown to prevent most malaria deaths, even in a context of intense malaria transmission, and the implementation of insecticide-treated nets has dramatically reduced the burden of malaria, there is no doubt that the combination of different interventions has had a synergistic effect, resulting in a much higher impact than the separate use of individual control measures. Forty countries worldwide have recently reduced malaria deaths and cases by half. Some countries, such as Morocco, have recently eliminated malaria completely, and others have made impressive progress, such as South Africa and Swaziland, where cases have decreased by approximately 90%. A recent report on malaria research underlines that investment has more than quadrupled in the past 16 years, from US$121 million in 1993 to US$612 million in 2009 [1]. Between 2004 and 2009, 28% of this funding was used for vaccine development, 38% for new drugs or combinations of drugs, 23% for basic research, but only 4% for vector control products (mainly new insecticides) and 1% for new diagnostic testing systems. This contrasting picture largely reflects the donor funding preferences. In this special issue of Clinical Microbiology and Infection, devoted to malaria elimination, Brian Greenwood and Geoffrey Targett provide definitions of malaria control, elimination and eradication, before they focus on malaria vaccines and explain why second-generation vaccines are needed even though the first malaria vaccine has not yet been licensed [2]. Meredith McMorrow [3] emphasizes the importance of malaria diagnostic tests in the context of elimination, and discusses how these tests are useful and what improvements are needed in the future. Roly Gosling, Lucy Okell, Jacklin Mosha and Daniel Chandramohan express their views on active case detection and malaria treatment for clinical cases, as well as asymptomatic parasite carriers. Unsurprisingly, modelling approaches advocate maximum efficacy when drug administration programmes are implemented at the same time as antivector activities [4]. Kaliyaperumal Karunamoorthi [5] focuses on vector control measures. Since the pioneering studies that discovered the efficacy of bed-nets when they were treated by dipping them in pyrethroid insecticides [6,7], longlasting factory-coated or impregnated bed-nets have been developed that do not need to be repeatedly impregnated. Indoor and/or outdoor residual spraying with insecticides constitutes the second main tool for vector control. Many other vector control tools exist, but are used on a much smaller scale or remain at the stage of proof-of-concept [8]. The remarkable successes achieved in malaria elimination must not be allowed to disguise their delicate nature [9]. Elimination, by definition, implies that both capacity and commitment are needed to sustain this status indefinitely [10].