Malaria: An Overview

Introduction

Malaria is named after the Italian term “mal’aria”, which means “bad air” to represent the association of the disease with marshy areas.¹ It is an endemic vector-borne parasitic disease caused by protozoan parasites of the genus Plasmodium in tropical and subtropical regions worldwide.² Plasmodium consists of over 200 species, infecting mammals, birds, and reptiles, and malaria parasites generally tend to be host-specific.³ Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, Plasmodium ovale, and Plasmodium knowlesi are the five known species of the genus Plasmodium that causes malaria in humans.^4,5 Of the five Plasmodium species that cause malaria in humans, P. falciparum causes severe malaria.⁶

The life cycle of the parasites is a complex process occurring in both vertebrate hosts and a mosquito vector. Besides, it undergoes both sexual and asexual reproduction mechanisms. This makes the development of drugs and vaccines challenging.⁷

The first trial to treat malaria dates back to the 2nd century before Christ (168 BC) when China used Qinghai (Latin Artemisia annua) for the treatment of fever and chills.⁸ The next documented trial was in the 16th century when the Spanish invaders in Peru discovered the Cinchona medications against malaria from the bark of the Cinchona tree (Latin Cinchona succirubra). The active ingredient of Cinchona succirubra which had been used for many years in chemoprophylaxis and treatment of malaria was isolated in 1820 by the French chemists Pierre Joseph, Pelletie, and Joseph Bienaimé Caventou. In 1970, a group of Chinese scientists led by Dr Youyou Tu isolated an active substance Artemisinin, a compound that has proven activity against malaria, from the plant Artemisia annua.⁹

Although artemisinin and its derivatives are potent treatments for malaria and are being widely used in combination therapies worldwide, resistance is emerging in some parts of the world. This calls for the need to discover new anti-malarial agents possessing high therapeutic value with minimal toxicity, and lower cost.¹⁰

Tackling malaria transmission by interrupting parasite transmission or by tackling the insect vector was started in the late 1800s after Laveran in Algeria discovered the cause of malaria and the Plasmodium parasite around 1880–1882. In the early 1900s, numerous effective local initiatives were reported in different parts of the world where malaria was common.¹¹ Later on, DDT (dichloro-diphenyl-trichloroethane), came to the picture. Even though it was synthesized in 1874, it was only in 1939 that its insecticide properties were discovered. It was used extensively during and after World War II. Due to its high efficacy on malaria vectors, it became the main tool of the malaria eradication campaign launched in 1955 under the auspices of the League of Nations. But due to failure to meet expectations, it was officially abandoned in 1978.¹² Afterwards, other and costlier insecticides were developed (pyrethroids) for use in the impregnation of bed nets. However, the impact of long-lasting impregnated nets (LLINs) has been compromised in recent years by several factors: the emergence of resistance to the insecticide in the mosquito populations, the diversity and changes in the behavior of Anopheles since some of the species are (or have become) exophagic/exophilic and early biters.¹²

Epidemiology of Malaria

Following the second world war, incredible success was achieved with the discoveries of DDT and chloroquine, lowering the global extents of both P. vivax and P. falciparum, and benefiting enormous parts of the Americas, Europe, and Asia.¹³ These tremendous gains in malaria control continued until the first decade of the 21st century, but the second decade appears to be a bit harder. Malaria incidence has been on the rise in several places since 2014.¹⁴ In 2019, an estimated 229 million cases were reported worldwide (from 87 malaria-endemic countries). About 94% of the reported cases (215 million cases) were recorded in the African region. The Southeast Asia region accounted for about 3% of the burden of malaria cases globally. In the same year, an estimated 409,000 deaths from malaria occurred worldwide of which 94% happened in Africa.¹⁵

Malaria affects majorly children under the age of 5 years; with 67% death from the total death in 2019. Underdeveloped immunity is thought to be the major reason that makes children under 5 years of age vulnerable to malaria. Aftereffects of fever and illness like reduced appetite, limited social life, and restricted play contribute to meager growth.¹⁶

Etiology of Malaria

Protozoan parasites of the genus Plasmodium originate from photosynthetic protozoa named Dinoflagellates. About 200 different species of protozoa have been identified so far and among them, at least 13 species are known to be pathogenic to humans.¹⁷ Five of the parasites namely P. falciparum, P. vivax, P. malariae, P. ovale (P. ovale curtisi and P. ovale wallikeri), and P. knowlesi are well-known etiologies of malaria in humans.¹⁸

In Africa, the most prevalent and pathogenic species is P. falciparum. However, malaria infection from most malaria-endemic regions of Africa shows the presence of multiple sympatric species and co-infection within an individual human host or mosquito vector.¹⁹ P. malariae is the species most commonly found in sympatry with P. falciparum in malaria-endemic regions of Africa.²⁰

In each endemic area, malaria is transmitted by a specific set of Anopheles species.²¹ So far, more than 400 different species of Anopheles mosquitoes have been identified. But only 30 of them are known to transmit malaria. All vectors of malaria undergo the bite between dusk and dawn.²²

Stability is observed in the distribution pattern of the mosquito species in malaria-endemic regions of the African continent. The complete disappearance of a given vector species from a region is unusual and when the non-indigenous vector is introduced to the area, it is a serious public health concern since it is known to result in devastating epidemics. Indigenous vectors are hard to eradicate with known vector eradication activities.²¹

The Life Cycle of Malaria Parasite

The life cycle of the malaria parasite is a complex process involving an Anopheles mosquito and a vertebrate host.²³ The first stage of the infection is the entrance of the sporozoites in mosquito saliva into the skin and bloodstream of the human host and then, it invades hepatocytes to undergo asexual replication.²⁴ During this phase (hepatic or pre-erythrocytic phase) the rupture of infected hepatocytes results in the release of thousands of merozoites.²⁵ In the case of P. vivax and P. ovale infections, some form dormant hypnozoites which remain within hepatocytes for periods of several months, and even as long as 4 years, before developing and multiplying to initiate a new episode of erythrocytic infection.²⁶

The erythrocytic infection involves the interaction of the merozoites with the red blood cells (RBC). The merozoites head orients and adjoin with the erythrocytes membrane by deforming the surface host cell. Then, through parasite-induced reorganization of the erythrocyte cytoskeleton, the parasite enters the erythrocyte to undergo the second asexual reproduction.²⁷ While younger erythrocytes are targeted favorably by P. vivax and P. ovale, erythrocytes of any age are invaded by P. falciparum and P. knowlesi. In contrast, P. malariae prefers senescent erythrocytes.²⁴ After invading RBC, merozoites reproduce into trophozoites and then into schizonts which erupt from the erythrocytes to release merozoites and reinvade new RBCs and continue the asexual replication cycle.²⁷

The sexual reproduction cycle of malaria starts when a portion of trophozoites matures to male and female sexual progeny or gametocytes.²⁸ The transmission of the malaria parasite from the mammalian host to the mosquito is mediated by these gametocytes. During the bite of an anopheles mosquito, the matured gametocytes will be taken to the midgut of the mosquito.²⁹ Inside the midgut, gametocytes get converted into fertile gametes and the next stage involves the conversion of zygotes into ookinetes which are motile and invasive.³⁰ The ookinetes in turn get converted into oocysts in the midgut basal lamina. The oocyst then matures releasing sporozoites, which migrate to the salivary gland of the mosquito. The parasite is transmitted to another mammalian host through an infected mosquito bite.³¹

Pathophysiology of Malaria

The pathophysiology of uncomplicated malaria is characterized by fever²⁵ secondary to the rupture of erythrocytes, macrophage ingestion of merozoites, and/or the presence of antigen-presenting trophozoites in the circulation or spleen which mediates the release of tumor necrosis factor α (TNF-α).³² Fever associated with malaria infection is known by its periodicity which differs among different species of the parasite. Tertian fever (“tertian malaria”) is expected in P. vivax and P. ovale malaria as a progeny of schizonts matures every 48 hours in these species. In contrast, P. malariae is attributed to quartan fever (“quartan malaria”) which occurs every 72 hours. However, the fever in falciparum malaria may occur every 48 hours, but is usually irregular, showing no distinct periodicity.²⁵

The binding of matured infected RBC to host endothelial cells (cytoadherence) is the major player in the pathogenesis of severe malaria.³³ The expression of genes that encode proteins involved in cytoadherence and immune evasion explains the virulence of P. falciparum when compared with other species. The P. falciparum erythrocyte membrane protein 1 (PfEMP1), rifin, and stevor proteins are encoded by members of the var, rif, and stevor gene families, respectively.³⁴ Var gene-encoded PfEMP1 is the best-characterized variant surface antigen which is expressed on the surface of infected erythrocytes where it mediates binding to endothelial receptors.³⁵

The PfEMP1 family forms electron-dense protrusions named knobs on the membrane of parasitized RBC (pRBC) by getting inserted into and protruding from the erythrocyte membrane. Knobs serve as a site by which parasitized erythrocyte binds to other cell surfaces like normal RBC and endothelium.³⁶

The adhesion of parasitized erythrocytes to vascular endothelium leads to sequestration, the phenomenon by which infected RBCs translocated from the peripheral circulation by getting bound to the vascular endothelium, in the deep microvasculature of various tissues and organs.³⁷ Host molecules like cluster of differentiation 36 (CD36), intercellular adhesion molecule-1 (ICAM1), thrombospondin (TSP), P-selectin, chondroitin sulfate A (CSA), and protein C receptor have been identified as receptor binding for pRBC to the endothelium.³⁸ For instance, when PfEMP1 on infected RBCs binds to host receptors such as ICAM-1 and CD36 on brain endothelial cells, it mediates sequestration to cause cerebral malaria.³⁹

On the other hand, pRBCs can bind to uninfected RBCs and impair microcirculation then cause hypoxia. The phenomenon is called rosettes, the spontaneous binding of normal RBCs to malaria-infected RBCs. Blood group antigens A and B, CD36, complement receptor 1 (CR1), and heparan sulfate-like glycosaminoglycans (HS-GAGs) are the five identified receptors on RBCs implicated in the process of rosettes.³⁷

Parasite-derived molecules called toxins are also implicated in the pathogenesis of severe malaria.⁴⁰ Glycolipids named glycosylphosphatidylinositol (GPI), coupled with protein or free form, induce the overproduction of cytokines: TNF and interleukin I (ILI) by macrophages.⁴¹ Although cytokines have a physiological role in defending microorganisms including the malaria parasite when produced in lower amounts,⁴² overproduction causes high-grade fever, upregulation of endothelial receptor expression, and upregulation of nitric oxide production, this in turn may cause local damage and suppression of erythrocyte production in the bone marrow.³⁷

Diagnosis of Malaria

Diagnosis of malaria can be done through clinical observation of the signs and symptoms of the disease. However, clinical diagnosis of malaria has poor accuracy due to the resemblance of the clinical symptoms with other tropical diseases and the possibility of incidence of coinfection.⁴³ Other diagnostic techniques used to diagnose malaria are the microscopic detection of parasites from blood smears and antigen-based rapid diagnostic tests. The latter is based on immunologically detecting different malaria antigens such as lactate dehydrogenase (LDH), aldolase, and histidine-rich protein-2 (HRP-2) in a small amount of blood.⁴⁴

Although microscopy and rapid diagnostic test (RDTs) are being used widely, they are less sensitive and less specific to malaria parasites. The shortcomings of the conventional techniques necessitate the development of molecular and biosensing-based methods which are more accurate, easy to quantify, and allow point-of-care (POC) application. Thus, newly developed techniques like dielectrophoretic and magnetophoretic detection are emerging.⁴⁵ However, polymerase chain reaction (PCR)-based nucleic acid detection methods that are highly sensitive are applicable only in research laboratories because of their high running and maintenance costs.⁴⁴

Management of Malaria

The management of malaria involves preventive and/or curative approaches. Since untreated uncomplicated malaria can progress to severe malaria, early diagnosis and effective rational treatment are the first core principles in the management of malaria. To prevent or delay the spread of antimalarial drug resistance, WHO recommends the use of combination therapy for all episodes of malaria with at least two effective antimalarial agents having a different mechanism of action.⁴⁶

Pharmacological Treatment of Malaria

Antimalarial agents can be grouped as quinoline derivatives, antifolates, and artemisinin derivatives based on their chemical structures and/or mechanism of action.⁴⁷ Quinoline derivatives that comprise chloroquine, amodiaquine, quinine, quinidine, mefloquine, primaquine, lumefantrine, and halofantrine are active against the erythrocytic stage of the parasite.⁴⁸ Among them, primaquine is active against the hepatic stage of the parasite and gametocytes.⁴⁹

The antimalarial mechanism of quinolone derivatives is proposed to be the result of the following two-step activities; the first step involves retarding deposition of heme onto the crystal surface by capping the growing hemozoin crystals and the second step involves complexing with free heme in the lumen of the digestive vacuole. The overall outcome of both steps is killing the parasite by halting heme crystallization after being released from the hemoglobin.⁵⁰

Antifolate antimalarial agents can be grouped as class I and class II, based on their mode of action. Class I antifolate agents act by inhibiting the production of dihydrofolic acid through inhibition of the enzyme dihydropteroate synthase (DHPS) and hence the synthesis of nucleic acid.⁵¹ Class II antifolate agents block the reduction of dihydrofolate to tetrahydrofolate by inhibiting the enzyme dihydrofolate reductase (DHFR) in the parasite. Tetrahydrofolate is important for the production of nucleic acid and amino acids. Class II agents have schizonticidal activity and they act on the asexual form of the parasite.⁵² Sulfadoxine is among the class I antifolate agents while proguanil and pyrimethamine belong to Class II antifolate agents.⁵¹

Artemisinin and its derivatives like artesunate, artemether, arteether, and dihydro-artemisinin are of natural origin.⁵³ The generation of free radicals was the first suggested mechanism of action of artemisinin and its derivatives. The malaria parasite is known to be rich in heme since it causes proteolysis of the host cell hemoglobin. Therefore, artemisinin interacts with intraparasitic heme and gets activated into toxic free radicals through the process. A resulting carbon-centered free radical then kills the parasite by alkylating and denaturing one or more essential malarial proteins. The fact that artemisinin is selectively toxic to the parasite is attributed to its selective interaction with intraparasitic heme.⁵⁴

Prevention of Malaria

Malaria Chemoprophylaxis

Casual prophylaxis is administered as a drug active against the pre-erythrocytic (liver stage) malaria parasite. These drugs can be discontinued after leaving the malaria-endemic area. Whereas, suppressive prophylaxis represents administering drugs that act against asexual blood-stage (erythrocytic) parasites. These drugs must be taken for at least 4 weeks after leaving the area to eliminate asexual parasites emerging from the liver weeks after exposure unlike casual prophylaxis.⁴⁶ In areas where P. falciparum malaria is prevalent, for instance in sub-Saharan Africa, suppressive prophylaxis is indicated. Whereas, in areas where P. vivax coexists with P. falciparum or alone, causal prophylaxis is recommended.⁵⁵

The Centers for Disease Control emphasizes that there is no antimalarial agent that can prevent malaria 100%. Therefore, prophylaxis shall be augmented with the use of personal protective measures. Currently, there are four drugs approved to be taken for chemoprophylaxis against malaria, namely atovaquone/proguanil, chloroquine, doxycycline, and mefloquine. Selection is based on client factors (pregnancy, disease conditions like renal impairment and cardiac conduction abnormalities), cost, preference on the frequency of administration, tolerability, resistance profile of the area, and the like.⁵⁶

Vector Control

Insecticide-treated nets (ITNs) and indoor residual spraying (IRS) are the two currently applicable malaria vector control methods recommended by WHO.⁵⁷ Whether treated with insecticide or not, bed nets provide a physical impediment against insects. When treated with insecticide, it provides further protection by killing insects coming in contact with the net. Pyrethroids, like permethrin and deltamethrin, were the only insecticides used to impregnate bed nets.⁵⁸

The emergence of pyrethroid-resistant anopheles necessitates the discovery of pyrethroid–piperonyl butoxide (PBO). PBO works in synergy with pyrethroid by inhibiting parasitic metabolic enzymes like mixed-function oxidases that quench insecticide`s action by sequestering and detoxifying.⁵⁹

IRS is employed to prevent the entry of mosquitos by covering the walls and floors of a house with insecticide. The effect of insecticide lasts for several months.⁶⁰ According to WHO, five chemical classes that meet the safety and efficacy level stated by the WHO prequalification are advised to be used for IRS: pyrethroids, organochlorines, carbamates, organophosphates, and neonicotinoids. Nevertheless, the organochlorine insecticide, DDT, is not included in the prequalified list.⁶¹

Malaria Vaccine

Resistance of the parasite to antimalarial agents and toxicity associated with chemoprophylaxis arose the need for the development of an effective vaccine against malaria. Recently, researchers are focusing on designing vaccines and so far, one candidate has emerged to reach a large Phase III trial. In addition, other promising candidates are also under investigation. In general, malaria vaccines can be grouped as pre-erythrocytic, erythrocytic, and transmission-blocking vaccines based on their target on the malaria parasite lifecycle.⁶²

RTS, S/AS01 is a monovalent recombinant protein vaccine that successfully passed to advanced clinical trials and studied well in the blockage of P. falciparum sporozoite. It initiates an immune response against a protein covering the surface of sporozoite named circumsporozoite protein (PfCSP).⁶³ Thus, it promotes immunoglobulin G (IgG) antibody response towards the region of the citrate synthase (CS) protein and potent T-cell (CD4+) response.⁶²

RTS, S/AS01 is currently recommended by WHO for use on children in sub-Saharan Africa and other regions of the world with moderate-to-high transmission of P. falciparum. It should be administered in a schedule of 4 doses in children starting from the age of 5 months. This decision is made based on the result observed on the ongoing pilot program in Ghana, Kenya, and Malawi, which covered 800,000 children since 2019. The pilot program in these three countries will continue to uncover the advantage of administering the 4th dose and the long-term outcome on child deaths.⁶⁴ Similarly, PAMAVAC is a promising blood-stage malaria vaccine among vaccines in the pipeline.⁶⁵

Resistance to Anti-Malaria Agents

Though artemisinin and its derivatives are potent treatments for malaria and are being widely used in combination therapies worldwide, resistance is emerging in some parts of the world. This calls for the need to discover new anti-malarial agents possessing high therapeutic value, minimal toxicity, and low cost.⁶⁶

Antimalarial drug resistance has been defined as

the ability of a parasite strain to survive and/or multiply despite the administration and absorption of a drug given in doses equal to or higher than those usually recommended but within the limits of tolerance of the subject, given that the drug in question got access to the parasite or the infected red blood cell for the duration of the time necessary for its normal action.⁶⁷

In 1978 and 1995 the first case of chloroquine resistance by P. falciparum was observed in Africa and Ethiopia, respectively. Similarly, chloroquine treatment failure against P. vivax has been reported in Debre Zeit, Ethiopia in 1995.⁶⁸ The mechanism of resistance to quinolones is primarily associated with an elevated level of drug efflux. Overexpression of p-glycoprotein transporter, P. falciparum multidrug resistance 1 (Pfmdr1) has been implicated in reduced responsiveness of the parasite to chloroquine and other quinolone antimalarial agents.⁶⁹

Resistance to antifolate antimalarial agents is found to be a result of a mutation in the target enzyme DHFR. A study conducted by Sirawaraporn and Yuthavong,⁷⁰ using a partially purified DHFR obtained from a cloned strain of pyrimethamine-sensitive P. chabaudi and its derived drug-resistant strain, showed a significant decrease in the affinity of binding of pyrimethamine to the enzyme from the resistant clone. Likewise, alterations in kinetic and other properties were also observed. This supports the claim that resistance is a result of a genetic change that further leads to a structurally different enzyme.

Artemisinin-resistant strains of malaria were first reported in 2008 and then spread in South East Asia (Greater Mekong) but not significantly in Africa so far.⁷¹ In Africa, due to the high prevalence of p. falciparum, there is repeated exposure of the host to malaria resulting in a higher degree of acquired immunity which in turn enables the host to defend against artemisinin-resistant infections. P. falciparum Kelch 13 (PfKelch13) is a molecular marker used to map the geographical distribution of artemisinin resistance. It is a substrate adapter for cullin E3 ligase, with a putative substrate of P. falciparum phosphatidylinositol 3-kinase (PfPI3K) and a redox sensor. Mutant K13 results in lowered artemisinin interactions with PfPI3K.⁷²

The emergence of resistant strains of malaria and the spread of the disease urges a relentless search for antimalarial agents of a new mechanism of action with better safety and efficacy profile. Therefore, studies to develop new antimalarial agents having a distinct target from the conventional agents, with well-characterized safety, efficacy, and toxicity profile have to be one of the priorities of health science.⁷³

Conclusion

One of the most prevalent and easily avoidable causes of death worldwide is malaria. Although the incidence of malaria as well as the rate of malaria-related deaths has been declining for decades, the progress appears to be stagnating. Malaria incidence has been on the rise in several places since 2014.

For over a century after Laveran in Algeria discovered the cause of malaria and the Plasmodium parasite in the late 1800s different approaches have been tried to control and eradicate malaria from the face of the earth with different magnitude of success. Yet even though there have been some tremendous successes in controlling malaria we are not remotely close to eradicating it. With the recent emergence of resistance to current front-line artemisinin-based combination therapy, the need for the discovery of new antimalarials that can act through novel mechanisms of action has been pushed firmly to the top of the development agenda.^74,75 However, more needs to be done, particularly in light of the rising drug and insecticide resistance.