Introduction
Despite the notable progress made in certain domains, malaria continues to pose a significant challenge in the majority of tropical regions, leading to countless instances of illness and hundreds of thousands of fatalities on an annual basis.1 The etiology of malaria can be attributed to the infection caused by a unicellular parasite known as Plasmodium. Four distinct species of Plasmodium serve as the causative agents of malaria in human beings, namely Plasmodium falciparum, P. vivax, P. ovale, and P. malaria.2 The most severe cases of malaria, which often result in mortality, as well as instances of drug-resistant infections, predominantly arise from the infection caused by Plasmodium falciparum.3 This particular species of Plasmodium is renowned for its highly virulent nature.1 The transmission of malaria transpires through the bite of a female Anopheles mosquito, and the prevalence of malaria is endemic in 104 countries situated within tropical and subtropical regions.4 It is worth noting that over half of the global population resides in areas where malaria transmission is prevalent.5 Consequently, this disease imposes a considerable burden on public health within communities located in various parts of Africa, Asia, and Central and South America.4 P. vivax and P. ovale are characterized by their ability to undergo a dormant stage within the liver, which can subsequently lead to relapses months or even years after the initial infection has been cleared. As a result, the treatment of these species necessitates the administration of an additional agent capable of eradicating this latent stage.6 The symptoms associated with malaria exhibit a high degree of variability, encompassing manifestations such as chills, profuse sweating, intense headaches, lethargy, myalgia, and coughing. Moreover, gastrointestinal symptoms can be particularly severe, including nausea, vomiting, diarrhea, and abdominal pain. Furthermore, physical signs such as paleness, rapid heart rate, enlargement of the liver and spleen, jaundice, and an increased respiratory rate may also be present in individuals affected by malaria.7 In instances of cerebral malaria, altered mental status is frequently observed. It is crucial to note that the employment of single-molecule approaches in the treatment of malaria has been shown to significantly contribute to the development of drug resistance, hence rendering such approaches inadvisable in clinical practice.8 For instance, the current guidelines do not recommend the utilization of single-molecule therapies for the treatment of uncomplicated malaria caused by any species of Plasmodium. Instead, these infections can be effectively treated within three days through the administration of artemisinin-based combination therapy (ACT), which entails the amalgamation of a rapidly acting artemisinin derivative with a longer-lasting antimalarial medication. Examples of ACTs include artemether + lumefantrine, artesunate + amodiaquine, artesunate + mefloquine, artesunate + SP, and dihydroartemisinin + piperaquine. It is worth reiterating that single-molecule approaches to malaria treatment have been shown to significantly contribute to the emergence of drug resistance, and are therefore not recommended. As an illustration, the current guidelines do not recommend the administration of single-molecule therapies for the treatment of uncomplicated malaria caused by any species of Plasmodium. Instead, these infections can be effectively treated within three days through the administration of artemisinin-based combination therapy (ACT), which entails the amalgamation of a rapidly acting artemisinin derivative with a longer-lasting antimalarial medication. Chloroquine (CQ) remains the standard drug for the treatment of P. vivax, P. malaria, and P. ovale in the majority of countries. Nevertheless, it is worth mentioning that P. vivax has demonstrated resistance to CQ in certain regions of sub-Saharan Africa and Southeast Asia. Consequently, the World Health Organization (WHO) recommends the use of ACTs for the treatment of P. vivax in areas that have been affected by this species. The avoidance of CQ usage is prevalent in numerous African countries due to the frequent occurrence of side effects. On the other hand, artemisinin derivatives are generally well-tolerated.4
The life cycle of the malaria parasite
The life cycle of the malaria parasite in humans is a complex and intricate process, as depicted in the accompanying Figure. The transmission of the motile infectious form, known as Plasmodium sporozoite, occurs when an insect bites the human skin, leading to the transportation of thread-like sporozoites through the circulatory system to the liver.8 Over a span of 7 to 12 days, these sporozoites undergo a series of growth stages, eventually developing into schizonts and giving rise to up to 30,000 merozoites, which subsequently rupture hepatocytes.9 However, it is worth noting that certain species of the malaria parasite, such as vivax and ovale, have sporozoites that can transform into hypnozoites, a dormant form that can persist in the liver for months or even years, leading to relapses in infected individuals. Interestingly, cases of recurrent falciparum malaria have been reported in patients several years after leaving an endemic area, suggesting the existence of a dormant stage for this particular species.10 This observation highlights the occasional presence of a latent phase in the life cycle of falciparum malaria.
The asexual cycle of the parasite commences with the invasion of red blood cells (RBCs) by merozoites, which derive their nourishment from the consumption of hemoglobin. Within the confines of the host RBC, the parasite undergoes a series of developmental stages, progressing from the early ring stage to the late trophozoite stage, and subsequently undergoing mitotic divisions to form schizonts, each containing anywhere between 6 to 32 merozoites, depending on the specific species of the parasite.11 Upon the rupture of the erythrocytic schizont, the released merozoites proceed to invade other RBCs, thereby perpetuating the life cycle of the malaria parasite. The rupture of the schizonts coincides with the occurrence of cyclical fevers, which typically manifest shortly before or during the lysis of RBCs, as the schizonts rupture and release new infectious merozoites. In tertian malaria, this cycle occurs every 48 hours, while in quartan malaria infection, it takes place every 72 hours. 12
During the repeated cycles of the asexual stage, some merozoites undergo differentiation into male and female sexual forms known as erythrocytic gametocytes, each possessing a single nucleus, and await the arrival of a blood-seeking female Anopheles mosquito. Upon ingestion of the gametocytes by the mosquito, gametogenesis is induced. The microgametes, which are flagellated forms resulting from Ex flagellation, penetrate or fertilize the macrogametes, leading to the generation of zygotes. These zygotes subsequently transform into ookinetes and eventually develop into round oocysts. Within the oocyst, the nucleus undergoes repetitive division, resulting in the production of a significant number of sporozoites and the enlargement of the oocyst. Once the sporozoites are fully formed, the oocyst bursts, freeing the sporozoites into the mosquito's body cavity, known as the hemocoel. These sporozoites then migrate to the salivary glands of the mosquito, thereby completing the life cycle of the malaria parasite. The entrance of the sporozoites from the mosquito's salivary glands into a new human host serves to perpetuate the cycle of malaria infection.13
Antimalarial drugs
Antimalarial drugs are primarily designed to eradicate the erythrocytic stages of malaria parasites that are accountable for causing illness in humans. The treatment regimens for the two most prevalent malaria parasites, P. falciparum, and P. vivax, differ substantially. It is now recommended to use artemisinin-based combination therapy (ACT) for the treatment of uncomplicated falciparum malaria in almost all regions. In the majority of areas, the first-line regimen for radical cure of vivax malaria remains to be the combination of chloroquine and primaquine. ACT comprises a potent artemisinin component, which swiftly eliminates the majority of parasites, along with a longer-acting partner drug that eradicates the remaining parasites and restricts the selection of artemisinin resistance. The World Health Organization (WHO) recommends various ACTs, including artemether/lumefantrine, artesunate/amodiaquine, artesunate/mefloquine, dihydroartemisinin/piperaquine, artesunate/pyronaridine, and artesunate/sulfadoxine–pyrimethamine. Multiple drugs are employed for the prevention of malaria. Antimalarial drugs can be categorized as either single or combined therapies, as depicted in Table 1. Available antimalarial drugs can be classified into multiple classes, as illustrated in Table 1. Chloroquine, a 4-aminoquinoline, was considered the gold standard for the treatment of uncomplicated malaria for numerous years; however, due to the emergence of drug resistance, it is no longer suitable for the treatment of falciparum malaria in almost all regions. The treatment policy for P. vivax malaria was modified to include ACTs. Amodiaquine seems to be susceptible to the same resistance mechanisms as chloroquine; nonetheless, it exhibits improved potency and provides adequate efficacy against many chloroquine-resistant parasites. Consequently, it is a constituent of the widely utilized ACT, artesunate/amodiaquine.
Table 1
Classification of antimalarial drugs
Antimalarial drug resistance
The limited efficacies of numerous antimalarial drugs are constrained by the phenomenon of drug resistance, a phenomenon in which parasites develop resistance to the most recent agents. Recent evidence indicates that these parasites are indeed becoming resistant to these newer agents. The surveillance sites of the International Centers of Excellence for Malaria Research (ICEMR) are recognized as highly valuable tools in monitoring and studying this resistance phenomenon. Specifically, these surveillance sites provide a means to assess resistance through the conduct of clinical trials, which involve the comparison of antimalarial efficacies of various agents.
Resistance to quinine
The oldest antimalarial drug, was initially documented in Brazil and subsequently in Southeast Asia. This resistance to quinine is closely associated with genetic variations in various transporters. As previously mentioned, single nucleotide polymorphisms (SNPs) in pfmdr1, pfcrt, and pfmrp1 have been linked to decreased sensitivity towards quinine. In addition, amplification of the pfmdr1 gene can also contribute to the development of quinine resistance. Recent research has explored the relationship between polymorphisms in a pfnhe1 microsatellite, in vitro sensitivity of parasites, and clinical responses to different antimalarial drugs. However, the findings of these studies have been inconsistent. Nevertheless, it is suggested that these polymorphisms may have a modest impact on the sensitivity of parasites to quinine, as well as other drugs.
Resistance to malarone
Another antimalarial drug, is attributed to the presence of single-point mutations in the cytochrome b (Pfcytb) gene. Atovaquone, a key component of Malarone, functions as a potent inhibitor of electron transport. Studies have identified the quinone-binding sites of cytochrome as the primary target of atovaquone. It is worth noting that when malarone is used as a standalone treatment, resistance can develop rapidly and relapses after therapy are common. Specifically, mutations at positions 268S and 268N in the Pfcytb gene have been associated with treatment failure of Malarone. However, there have been reports of treatment failure even in the absence of these mutations.
Pregnancy
In the case of pregnant women who have not previously experienced malaria during pregnancy, it is unlikely that the infecting parasites will be detected by the immune response. This is because the immune system primarily targets surface-expressed antigens, and therefore if resistance to these antigens occurs, it does not necessarily have to be present in a variant subpopulation to ensure its survival. Interestingly, there is evidence to suggest that pregnant women are more attracted to mosquitoes, thereby increasing their risk of contracting malaria. While pregnant women are widely recommended for malaria prophylaxis, the only drugs considered safe for use during pregnancy are chloroquine, which is not effective against P. falciparum in most regions, and proguanil. As an alternative to prophylaxis, intermittent therapy (IPT) with sulfadoxine-pyrimethamine (SP) has gained popularity. This involves administering a single therapeutic dose of SP two- or three times during pregnancy. The selection of antimalarial resistance is facilitated by using drug concentrations that are sufficient to inhibit the multiplication of susceptible parasites but not resistant ones. It is important to note that the absorption of certain antimalarials, such as lumefantrine, halofantrine, atovaquone, and mefloquine, can vary due to their lipophilic and hydrophobic properties. 14
Malaria diagnostic methods
The process of diagnosing malaria encompasses the act of identifying the presence of malaria parasites or antigens within the bloodstream of the infected individual. Various methodologies can be implemented to accomplish this crucial task, ranging from the conventional employment of microscopy to the utilization of rapid diagnostic tests (RDTs), loop-mediated isothermal amplification (LAMP), and polymerase chain reaction (PCR). The process of diagnosing malaria entails the identification of malaria parasites or antigens/products present in the blood of the patient. Malaria, being a potentially perilous medical emergency, necessitates prompt and appropriate treatment. It is imperative to note that any delays in the diagnosis and subsequent treatment of malaria serve as primary contributors to the alarming rates of mortality observed within numerous countries.15
Rapid diagnostic tests (RDT): Since the recognition by the World Health Organization (WHO) of the immediate necessity for novel, uncomplicated, expeditious, precise, and cost-efficient diagnostic examinations for the determination of the existence of malaria-causing parasites, to overcome the deficiencies inherent in light microscopy, a multitude of new techniques for malaria diagnosis have been formulated. This subsequent development has resulted in an escalation in the utilization of rapid diagnostic tests (RDTs) for the identification of malaria, given their speed and ease of execution, as well as their lack of dependence on electricity or specialized equipment. To combat the limitations of both microscopy and PCR-based techniques, alternative methods are currently being explored. To be deemed reliable, these methods must be compared against microscopy or PCR analysis, as well as reference strains, to establish their validity. For instance, according to the WHO (1999), RDTs must have the capability to detect 100 parasites per milliliter, which is equivalent to 0.002% parasitemia, and must possess a minimum sensitivity of at least 95%, in comparison to microscopy, as well as a minimum specificity of no less than 90% for all malaria species. It was during the mid-1990s that RDTs for malaria were first developed. These rapid diagnostic tests are immune-chromatographic lateral flow devices that are extensively utilized for the diagnosis of malaria, as well as the estimation of its prevalence, founded on the principle of detecting malaria antigens within the blood. The technique involves the application of a blood specimen to the test card or cassette, depending on the manufacturer, followed by the addition of a buffer reagent, typically consisting of 3 to 5 drops. This method employs three distinct types of antigens, namely Plasmodium histidine-rich protein 2 (pHRP-2), Plasmodium lactate dehydrogenase (pLDH), and Plasmodium aldolase. While pHRP-2 is specific to P. falciparum, both pLDH and Plasmodium aldolase can be found in all species. It should be noted that more than 90% of commercially available RDTs target pHRP-2. The immobilized antibodies on the surface of the test strip detect these parasitic antigens as the blood migrates across it. Each RDT contains a positive control, which serves to indicate the reliability of the test. At present, RDTs that are specific to particular species of malaria, such as P. falciparum and P. vivax, are readily accessible. However, for other species, the RDT can only identify the presence of the parasite itself, without providing any information regarding its species. Nevertheless, RDTs continue to serve as a valuable and widely employed diagnostic tool for both the surveillance and control of malaria.
Loop-mediated Isothermal Amplification (LAMP): The LAMP technique has been asserted as a molecular malaria diagnostic test that is both simple and cost-effective. Its ability to detect the conserved 18S ribosome RNA gene of P. falciparum has been documented. Various studies have demonstrated the test's high sensitivity and specificity, not only for P. falciparum but also for P. vivax, P. ovale, and P. malaria. Molecular methods for malaria diagnosis, including polymerase chain reaction (PCR) and loop-mediated isothermal amplification (LAMP), exhibit significant potential in regions with a low density of infections that may be overlooked by rapid diagnostic tests (RDTs). LAMP, a relatively recent technique for nucleic-acid amplification, was initially described in 2000 and subsequently modified to visualize amplified products using fluorescent or colorimetric dyes such as calcein and hydroxy naphthol blue (HNB) respectively. LAMP was employed to amplify Plasmodium DNA and determine the presence of Plasmodium DNA, which serves as an indicator of malaria infection, in the blood samples. The same 115 DNA samples used for PCR were also subjected to LAMP. A total of 15μl of DNA template was mixed with pan primers (FIP and BIP consisting of F1, F2, B1, and B2 priming sites, as well as two "Displacement primers" F3 and B3), along with the Bst polymerase (derived from Bacillus stearothermophilus) that is provided in a dried form within the LAMP tube caps. The LAMP tubes were inverted five times to ensure proper mixing of the DNA template with the reagents in the LAMP tube cap. A thermocycler was employed to amplify the DNA at a temperature of 65˚C for a duration of 45 minutes. The results were visualized by comparing the change in turbidity, using the negative and positive controls as references, through fluorescence under UV light on a tabletop.
PCR technique: PCR-based methods have emerged as a recent advancement in the field of molecular diagnosis for malaria. These techniques have proven to be highly specific and sensitive, making them one of the most effective diagnostic methods, especially for cases of malaria with low parasitemia or mixed infection 15. By amplifying the parasite's DNA, the PCR method achieves a high level of sensitivity, with the ability to detect as little as 0.004 parasites/μl 15. In comparison to microscopy, PCR-based assays exhibit a sensitivity that is 100 times higher, particularly for infections with low parasitemia. These methods work by identifying the presence of specific malaria target genes in a blood sample.16 However, it is important to note that the PCR method does not provide a straightforward means of estimating the parasite burden, which is frequently relied upon by clinicians 17. Despite overcoming the challenges of sensitivity and specificity in malaria diagnosis, the utility of PCR is hindered by its complex methodologies, high cost, and the requirement for trained technicians. Consequently, PCR is not commonly used in developing countries due to the complexity of the testing process and limited resources for conducting these tests effectively and routinely. Moreover, the PCR technique necessitates quality control and equipment maintenance, making it unsuitable for malaria diagnosis in remote rural areas or even in routine clinical settings 15, 18.
Transmission Dynamics
Vector ecology
Factors like climate, temperature, and humidity significantly impact mosquito ecology, affecting malaria transmission. Studies show that temperature and rainfall influence mosquito development, with temperature constraints on parasite and vector development. High temperatures can lead to smaller, less fecund mosquitoes, impacting transmission intensity. Additionally, rainfall creates breeding sites, increasing vector population and malaria transmission.
Human-vector interaction
Changes in human behavior and land use, such as urbanization and deforestation, can influence the spread of malaria. Urbanization alters local environments, affecting mosquito habitats and human exposure to mosquitoes. Deforestation can lead to increased malaria transmission due to changes in vector populations and breeding sites. Understanding these interactions is crucial for effective malaria control strategies.19, 20
Current Strategies in Prevention and Control
Malaria prevention and control strategies involve a combination of approaches that aim to prevent transmission of the disease and control the vector. These strategies have evolved over time, with advancements in scientific understanding and technological developments.
Vector control remains a critical aspect of malaria prevention. Insecticide-treated mosquito nets (ITNs) and indoor residual spraying (IRS) are mainstay prevention methods. However, the emergence of insecticide resistance in mosquitoes has posed challenges to these interventions, necessitating the development of new vector management strategies. Research is ongoing to identify new vector control tools and improve existing ones, such as the use of larvicides, larvivorous fish, and large-scale engineering works.
Vaccination is another crucial component of malaria control. Although no licensed vaccine against malaria currently exists, significant efforts are being made to develop one. Several candidate vaccines are in development, targeting various life cycle stages of the malaria parasite. Additionally, transmission-blocking vaccines are being explored, which work by blocking transmission of the malaria parasite to the mosquito vector.
Drug treatment and resistance are also important considerations in malaria control. Artemisinin-based combination therapies (ACTs) are the treatment backbone, but resistance is emerging, necessitating ongoing surveillance and the development of new drugs.
Effective and efficient scale-up of existing interventions is required, including the early treatment of malaria cases with ACTs, intermittent preventive treatment for pregnant women (IPTp), and interventions that reduce human-vector contact, such as IRS or use of long-lasting insecticide-treated bed nets (LLINs).
Innovative solutions, such as harnessing innovation and expanding research, are essential to respond to the spread of insecticide resistance and residual transmission, and to target the hypnozoite reservoirs of P. vivax. Strong political commitment, robust financing, and increased multisectoral collaboration are also key factors for further progress21, 22.
Recent Advances and Innovations
Novel vaccine developments
Recent advancements in malaria control include the development of newer vaccine candidates at various stages aiming to enhance efficacy and durability. These novel vaccine developments are crucial in the fight against malaria, with a focus on improving protection against Plasmodium falciparum and P. vivax. The ongoing research and innovation in vaccine development offer hope for more effective and long-lasting protection against malaria.
Genetic and genomic strategies
Genetic and genomic strategies, particularly gene drive technologies, are being explored to reduce malaria transmission by genetically modifying mosquito populations. These innovative approaches aim to disrupt the ability of mosquitoes to transmit the malaria parasite, thereby reducing the overall burden of the disease. By targeting mosquito populations, these strategies have the potential to significantly impact malaria transmission dynamics and contribute to malaria control efforts.
Surveillance and diagnostic tools
Advancements in surveillance and diagnostic tools are revolutionizing malaria control efforts. Improved rapid diagnostic tests and real-time data collection technologies are enhancing surveillance capabilities, allowing for more accurate and timely detection of malaria cases. These tools play a crucial role in monitoring the spread of the disease, identifying hotspots, and guiding targeted interventions to prevent malaria transmission. By strengthening surveillance and diagnostic capabilities, these innovations are instrumental in the global fight against malaria.23, 24
Challenges and Future Directions
Malaria is a significant global health issue, with millions of cases and deaths each year, primarily in developing countries in Africa and Asia. Public health strategies for malaria in endemic countries aim to prevent transmission of the disease and control the vector. This historical analysis considers the strategies for vector control developed during the first four decades of the twentieth century and shows that the principles for many of the present public health strategies for malaria have nearly all been established, except for large-scale usage of pesticides, which came later at the end of the Second World War.
Malaria control strategies include early case detection and prompt treatment (EDPT), vector control, community participation, environmental management and source reduction methods, and monitoring and evaluation of the program. Vector control measures include the use of insecticides, such as indoor residual spraying (IRS) and chemical larvicides, biological control methods, such as larvivorous fish and biocides, and personal protective measures, such as mosquito repellent creams, liquids, coils, mats, and bednets treated with insecticide.
New and improved diagnostics are essential for the effective control of malaria. Currently, the most reliable technique for diagnosing malaria is labor-intensive, relying on highly trained technicians using microscopes to analyze blood smears. However, such microscopic analysis is time-consuming, variable in quality, difficult to use in resource-poor field settings, and cannot detect drug resistance. Therefore, research is ongoing to develop easy-to-use tests that diagnose the malaria parasite causing an infection and identify its drug resistance.
NIAID-supported researchers are seeking to understand the molecular biology of the Plasmodium parasite and how it interacts with its human host at each stage in that cycle. Using that information, scientists hope to develop new drugs that block different molecular processes required for parasite survival and identify new targets for malaria control.
In recent years, there have been significant advances in the prevention and treatment of malaria. These include the development of prevention and treatment strategies that aim to reduce malaria cases worldwide and put into perspective future directions in the field. New blood schizonticides in phase II, transmission blocking, low-dose primaquine and beyond, tafenoquine, and seasonal malaria chemoprevention (SMC) are some of the tools and strategies being used to control and eliminate malaria.
However, detection and response remain serious operational challenges in resource-constrained settings plagued by weak health and surveillance systems. Effective and efficient scale-up of existing interventions is required, including the early treatment of malaria cases with ACTs, intermittent preventive treatment for pregnant women (IPTp), and interventions that reduce human-vector contact, such as IRS or use of long-lasting insecticide-treated bed nets (LLINs).25, 26
Conclusion
Malaria, a disease that has afflicted humans for centuries, continues to be a significant cause of illness and mortality among both children and adults in regions where it is endemic. The presence of Falciparum and vivax malaria poses a substantial challenge to the overall health of the affected communities. Various products, although they may have adverse effects, are commonly employed for the prevention and treatment of malaria caused by species other than P. falciparum. Irrespective of the particular plasmodial species involved, the drugs that can be utilized are the same. Notably, medications like chloroquine and proguanil are seldom associated with severe adverse effects when administered at recommended dosages. Unfortunately, in certain areas of the world, these products have lost their effectiveness as prophylactic agents. Although the current diagnostic methods in use are not without flaws, they continue to play crucial roles in addressing the current global malaria situation and reducing the incidence of the disease. The full potential of rapid diagnostic tests (RDTs) and polymerase chain reaction (PCR) is not being realized due to various barriers and limitations, including cost, availability of trained personnel, access to necessary equipment, and unreliable electricity supply. RDTs serve as a valuable source of DNA for PCR, loop-mediated isothermal amplification (LAMP), and other molecular techniques. The excessive use and prescription of antimalarial drugs are likely contributing to the emergence of drug-resistant Plasmodium parasites, which pose a significant threat in the fight against malaria. If not addressed promptly, this issue could undermine the effectiveness of most malaria control programs. Artemisinin-based combination therapy (ACT) appears to be effective in the majority of cases. Establishing trust between communities and healthcare providers, as well as ensuring the adherence of hospitals and pharmacy retailers to recommendations based on test results, will be of utmost importance. Achieving this will necessitate a multi-sectoral approach and the political will to advocate for the implementation of control and elimination strategies.
Article DOI
