Қан қоздырғыш плазмодий және обаның ең қауіпті түрі

Protozoan species of malaria parasite

Plasmodium falciparum is a unicellular protozoan parasite of humans, and the deadliest species of Plasmodium that causes malaria in humans. The parasite is transmitted through the bite of a female Anopheles mosquito and causes the disease's most dangerous form, falciparum malaria. It is responsible for around 50% of all malaria cases. P. falciparum is therefore regarded as the deadliest parasite in humans. It is also associated with the development of blood cancer (Burkitt's lymphoma) and is classified as a Group 2A (probable) carcinogen. The species originated from the malarial parasite Laverania found in gorillas, around 10,000 years ago. Alphonse Laveran was the first to identify the parasite in 1880, and named it Oscillaria malariae. Ronald Ross discovered its transmission by mosquito in 1897. Giovanni Battista Grassi elucidated the complete transmission from a female anopheline mosquito to humans in 1898. In 1897, William H. Welch created the name Plasmodium falciparum, which ICZN formally adopted in 1954. P. falciparum assumes several different forms during its life cycle. The human infective stage are sporozoites from the salivary gland of a mosquito. The sporozoites grow and multiply in the liver to become merozoites. These merozoites invade the erythrocytes (red blood cells) to form trophozoites, schizonts and gametocytes, during which the symptoms of malaria are produced. In the mosquito, the gametocytes undergo sexual reproduction to a zygote, which turns into ookinete. Ookinete forms oocytes from which sporozoites are formed. In 2022, some 249 million cases of malaria worldwide resulted in an estimated 608000 deaths, with 80 percent being 5 years old or less. Nearly all malarial deaths are caused by P. falciparum, and 95% of such cases occur in Africa. In Sub Saharan Africa, almost 100% of cases were due to P. falciparum, whereas in most other malarial countries, other, less virulent plasmodial species predominate.

Falciparum malaria was familiar to the ancient Greeks, who gave the general name πυρετός (pyretós) "fever". Hippocrates (c. 460–370 BCE) gave several descriptions on tertian fever and quartan fever. It was prevalent throughout the ancient Egyptian and Roman civilizations. It was the Romans who named the disease "malaria"—mala for bad, and aria for air, as they believed that the disease was spread by contaminated air, or miasma. However, his discovery was received with skepticism, mainly because by that time, leading physicians such as Theodor Albrecht Edwin Klebs and Corrado Tommasi Crudeli claimed that they had discovered a bacterium (which they called Bacillus malariae) as the pathogen of malaria. Laveran's discovery was only widely accepted after five years when Camillo Golgi confirmed the parasite using better microscopes and staining techniques. Laveran was awarded the Nobel Prize in Physiology or Medicine in 1907 for his work. In 1900, the Italian zoologist Giovanni Battista Grassi categorized Plasmodium species based on the timing of fever in the patient; malignant tertian malaria was caused by Laverania malariae (now P. falciparum), benign tertian malaria by Haemamoeba vivax (now P. vivax), and quartan malaria by Haemamoeba malariae (now P. malariae). The British physician Patrick Manson formulated the mosquito malaria theory in 1894; until that time, malarial parasites were believed to be spread in air as miasma, a Greek word for pollution. Ross, Manson and Grassi were nominated for the Nobel Prize in Physiology or Medicine in 1902. Under controversial circumstances, only Ross was selected for the award. There was a long debate on the taxonomy. It was only in 1954 the International Commission on Zoological Nomenclature officially approved the binominal Plasmodium falciparum. The valid genus Plasmodium was created by two Italian physicians Ettore Marchiafava and Angelo Celli in 1885. The Greek word plasma means "mould" or "form"; oeidēs meaning "to see" or "to know." The species name was introduced by an American physician William Henry Welch in 1897. It is derived from the Latin falx, meaning "sickle" and parum meaning "like or equal to another". Genetic diversity indicates that the human protozoan emerged around 10,000 years ago. These two species are closely related to the chimpanzee parasite P. reichenowi, which was previously thought to be the closest relative of P. falciparum. P. falciparum was also once thought to originate from a parasite of birds. Levels of genetic polymorphism are extremely low within the P. falciparum genome compared to that of closely related, ape infecting species of Plasmodium (including P. praefalciparum).

P. falciparum does not have a fixed structure but undergoes continuous change during the course of its life cycle. A sporozoite is spindle shaped and 10–15 μm long. In the liver it grows into an ovoid schizont of 30–70 μm in diameter. Each schizont produces merozoites, each of which is roughly 1.5 μm in length and 1 μm in diameter. In the erythrocyte the merozoite form a ring like structure, becoming a trophozoite. A trophozoite feeds on the haemoglobin and forms a granular pigment called haemozoin. Unlike those of other Plasmodium species, the gametocytes of P. falciparum are elongated and crescent shaped, by which they are sometimes identified. A mature gametocyte is 8–12 μm long and 3–6 μm wide. The ookinete is also elongated measuring about 18–24 μm. An oocyst is rounded and can grow up to 80 μm in diameter. Microscopic examination of a blood film reveals only early (ring form) trophozoites and gametocytes that are in the peripheral blood. Mature trophozoites or schizonts in peripheral blood smears, as these are usually sequestered in the tissues. On occasion, faint, comma shaped, red dots are seen on the erythrocyte surface. These dots are Maurer's cleft and are secretory organelles that produce proteins and enzymes essential for nutrient uptake and immune evasion processes. The apical complex, which is actually a combination of organelles, is an important structure. It contains secretory organelles called rhoptries and micronemes, which are vital for mobility, adhesion, host cell invasion, and parasitophorous vacuole formation.

In 1995 the Malaria Genome Project was set up to sequence the genome of P. falciparum. The genome of its mitochondrion was reported in 1995, that of the nonphotosynthetic plastid known as the apicoplast in 1996, and the sequence of the first nuclear chromosome (chromosome 2) in 1998. The sequence of chromosome 3 was reported in 1999 and the entire genome was reported on 3 October 2002. The roughly 24 megabase genome is extremely AT rich (about 80%) and is organised into 14 chromosomes. Just over 5,300 genes were described. Many genes involved in antigenic variation are located in the subtelomeric regions of the chromosomes. These are divided into the var, rif, and stevor families. Within the genome, there exist 59 var, 149 rif, and 28 stevor genes, along with multiple pseudogenes and truncations. It is estimated that 551, or roughly 10%, of the predicted nuclear encoded proteins are targeted to the apicoplast, while 4.7% of the proteome is targeted to the mitochondria.

Infection in humans begins with the bite of an infected female Anopheles mosquito. Out of about 460 species of Anopheles mosquito, more than 70 species transmit falciparum malaria. Anopheles gambiae is one of the best known and most prevalent vectors, particularly in Africa. The infective stage called the sporozoite is released from the salivary glands through the proboscis of the mosquito to enter through the skin during feeding. The mosquito saliva contains antihaemostatic and anti inflammatory enzymes that disrupt blood clotting and inhibit the pain reaction. Typically, each infected bite contains 20–200 sporozoites. A proportion of sporozoites invade liver cells (hepatocytes). The sporozoites move in the bloodstream by gliding, which is driven by a motor made up of the proteins actin and myosin beneath their plasma membrane.

Entering the hepatocytes, the parasite loses its apical complex and surface coat, and transforms into a trophozoite. Within the parasitophorous vacuole of the hepatocyte, it undergoes 13–14 rounds of mitosis which produce a syncytial cell (coenocyte) called a schizont. This process is called schizogony. A schizont contains tens of thousands of nuclei. From the surface of the schizont, tens of thousands of haploid (1n) daughter cells called merozoites emerge. The liver stage can produce up to 90,000 merozoites, which are eventually released into the bloodstream in parasite filled vesicles called merosomes.

Merozoites use the apicomplexan invasion organelles (apical complex, pellicle and surface coat) to recognize and enter the host erythrocyte (red blood cell). The merozoites first bind to the erythrocyte in a random orientation. It then reorients such that the apical complex is in proximity to the erythrocyte membrane. The parasite forms a parasitophorous vacuole, to allow for its development inside the erythrocyte. This infection cycle occurs in a highly synchronous fashion, with roughly all of the parasites throughout the blood in the same stage of development. This precise clocking mechanism has been shown to be dependent on the human host's own circadian rhythm. Within the erythrocyte, the parasite metabolism depends on the digestion of haemoglobin. The clinical symptoms of malaria such as fever, anemia, and neurological disorder are produced during the blood stage.

After invading the erythrocyte, the parasite loses its specific invasion organelles (apical complex and surface coat) and de differentiates into a round trophozoite located within a parasitophorous vacuole. The trophozoite feeds on the haemoglobin of the erythrocyte, digesting its proteins and converting (by biocrystallization) the remaining heme into insoluble and chemically inert β hematin crystals called haemozoin. The young trophozoite (or "ring" stage, because of its morphology on stained blood films) grows substantially before undergoing multiplication.

At the schizont stage, the parasite replicates its DNA multiple times and multiple mitotic divisions occur asynchronously. Cell division and multiplication in the erythrocyte is called erythrocytic schizogony. Each schizont forms 16 18 merozoites.

Some merozoites differentiate into sexual forms, male and female gametocytes. These gametocytes take roughly 7–15 days to reach full maturity, through the process called gametocytogenesis. These are then taken up by a female Anopheles mosquito during a blood meal.

The time of appearance of the symptoms from infection (called incubation period) is shortest for P. falciparum among Plasmodium species. An average incubation period is 11 days, Pregnancy and co infection with HIV are important conditions for delayed symptoms. Parasites can be detected from blood samples by the 10th day after infection (pre patent period). The flagellated microgamete fertilizes the female macrogamete to produce a diploid cell called a zygote. The zygote then develops into an ookinete. The ookinete is a motile cell, capable of invading other organs of the mosquito. It traverses the peritrophic membrane of the mosquito midgut and crosses the midgut epithelium. Once through the epithelium, the ookinete enters the basal lamina, and settles to an immotile oocyst. For several days, the oocyst undergoes 10 to 11 rounds of cell division to create a syncytial cell (sporoblast) containing thousands of nuclei. Meiosis takes place inside the sporoblast to produce over 3,000 haploid daughter cells called sporozoites on the surface of the mother cell. Immature sporozoites break through the oocyst wall into the haemolymph. They migrate to the mosquito salivary glands where they undergo further development and become infective to humans. The zygote can undergo another round of chromosome replication to form a ookinete (4N) (see Figure: Life cycle of plasmodium). The ookinete that differentiates from the zygote is a highly mobile stage that invades the mosquito midgut. The ookinetes can undergo meiosis involving two meiotic divisions leading to the release of haploid sporozoites (see Figure). The sporozoites do not enter the blood stream directly, but rather remain in the skin for two to three hours. About 15–20% of the sporozoites enter the lymphatic system, where they activate dendritic cells, which send them for destruction by T lymphocytes (CD8+ T cells). At 48 hours after infection, Plasmodium specific CD8+ T cells can be detected in the lymph nodes connected to the skin cells. Most of the sporozoites remaining in the skin tissue are subsequently killed by the innate immune system. The sporozoite glycoprotein specifically activates mast cells. The mast cells then produce signaling molecules such as TNFα and MIP 2, which activate cell eaters (professional phagocytes) such as neutrophils and macrophages. Only a small number (0.5 5%) of sporozoites enter the blood stream into the liver. In the liver, the activated CD8+ T cells from the lymph bind the sporozoites through the circumsporozoite protein (CSP). Even at the height of the infection, when red blood cells (RBCs) are ruptured, the immune signals are not strong enough to activate macrophages or natural killer cells.

Although P. falciparum is easily recognized by the human immune system while in the bloodstream, it evades immunity by producing over 2,000 cell membrane antigens. The initial infective stage sporozoites produce circumsporozoite protein (CSP), which binds to hepatocytes. Binding to and entering into the hepatocytes is aided by thrombospondin related anonymous protein (TRAP). TRAP and other secretory proteins (including sporozoite microneme protein essential for cell traversal 1, SPECT1 and SPECT2) from microneme allow the sporozoite to move through the blood, avoiding immune cells and penetrating hepatocytes. During erythrocyte invasion, merozoites release merozoite cap protein 1 (MCP1), apical membrane antigen 1 (AMA1), erythrocyte binding antigens (EBA), myosin A tail domain interacting protein (MTIP), and merozoite surface proteins (MSPs). The virulence of P. falciparum is mediated by erythrocyte membrane proteins, which are produced by the schizonts and trophozoites inside the erythrocytes and are displayed on the erythrocyte membrane. PfEMP1 is the most important, capable of acting as both an antigen and an adhesion molecule.

The clinical symptoms of falciparum malaria are produced by the rupture and destruction of erythrocytes by the merozoites. High fever, called paroxysm, is the most basic indication. The fever has a characteristic cycle of hot stage, cold stage and sweating stages. Since each erythrocytic schizogony takes a cycle of 48 hours, i. e., two days, the febrile symptom appears every third day. This is the reason the infection is classically named tertian malignant fever (tertian, a derivative of Latin word that means "third"). The most common symptoms are fever (>92% of cases), chills (79%), headaches (70%), and sweating (64%). Dizziness, malaise, muscle pain, abdominal pain, nausea, vomiting, mild diarrhea, and dry cough are also generally associated. High heartrate, jaundice, pallor, orthostatic hypotension, enlarged liver, and enlarged spleen are also diagnosed. The phagocytes can ingest free haemozoins liberated after the rupture of RBCs by which they are induced to initiate chains of inflammatory reaction that results in increased fever. It is the haemozoin that is deposited in body organs such as the spleen and liver, as well as in kidneys and lungs, to cause their enlargement and discolouration. Because of this, haemozoin is also known as malarial pigment. Unlike other malarias, which show regular periodicity of fever, falciparum, though exhibiting a 48 hour cycle, usually presents as irregular bouts of fever. This difference is due the ability of P. falciparum merozoites to invade a large number of RBCs sequentially without coordinated intervals, which is not seen in other malarial parasites. Women become susceptible to severe malaria during their first pregnancy. Susceptibility to severe malaria is reduced in subsequent pregnancies due to increased antibody levels against variant surface antigens that appear on infected erythrocytes. But increased immunity in the mother increases susceptibility to malaria in newborn babies. The mature schizonts change the surface properties of infected erythrocytes, causing them to stick to blood vessel walls (cytoadherence). This leads to obstruction of the microcirculation and results in dysfunction of multiple organs, such as the brain in cerebral malaria. Cerebral malaria is the most dangerous condition of any malarial infection and the most severe form of neurological disorders. According to the WHO definition, the clinical symptom is indicated by coma and diagnosis by high level of merozoites in the peripheral blood samples. It is the deadliest form of malaria, and to it are attributed to 0.2 million to over a million annual deaths throughout the ages. Most deaths are of children of below 5 years of age. It occurs when the merozoites invade the brain and causes brain damage of varying degrees. Death is caused by oxygen deprivation (hypoxia) due to inflammatory cytokine production and vascular leakage induced by the merozoites. Among the surviving individuals, persistent medical conditions such as neurological impairment, intellectual disability, and behavioural problems exist. Among them, epilepsy is the most common condition, and cerebral malaria is the leading cause of acquired epilepsy among African children. The reappearance of falciparum symptom, a phenomenon called recrudescence, is often seen in survivors. Recrudescence can occur even after successful antimalarial medication. It may take a few months or even several years. In some individuals, it takes as along as three years. In isolated cases, the duration can reach or exceed 10 years. It is also a common incident among pregnant women.

P. falciparum is endemic in 84 countries, and is found in all continents except Europe. Historically, it was present in most European countries, but improved health conditions led to the disappearance in the early 20th century. The only European country where it used to be historically prevalent, and from where we got the name malaria, Italy had been declared malaria eradicated country. In 1947, the Italian government launched the National Malaria Eradication Program, and following, an anti mosquito campaign was implemented using DDT. The WHO declared Italy free of malaria in 1970. There were 247 million cases of malaria worldwide in 2021, resulting in an estimated 619,000 deaths. It is estimated that approximately 2.4 billion people are at constant risk of infection.

In 1640, Huan del Vego first employed the tincture of the cinchona bark for treating malaria; the native Indians of Peru and Ecuador had been using it even earlier for treating fevers. Thompson (1650) introduced this "Jesuits' bark" to England. Its first recorded use there was by John Metford of Northampton in 1656. Morton (1696) presented the first detailed description of the clinical picture of malaria and of its treatment with cinchona. Gize (1816) studied the extraction of crystalline quinine from the cinchona bark and Pelletier and Caventou (1820) in France extracted pure quinine alkaloids, which they named quinine and cinchonine. The total synthesis of quinine was achieved by American chemists R. B. Woodward and W. E. Doering in 1944. Woodward received the Nobel Prize in Chemistry in 1965. Attempts to make synthetic antimalarials began in 1891. Atabrine, developed in 1933, was used widely throughout the Pacific in World War II, but was unpopular because of its adverse effects. In the late 1930s, the Germans developed chloroquine, which went into use in the North African campaigns. Creating a secret military project called Project 523, Mao Zedong encouraged Chinese scientists to find new antimalarials after seeing the casualties in the Vietnam War. Tu Youyou discovered artemisinin in the 1970s from sweet wormwood (Artemisia annua). This drug became known to Western scientists in the late 1980s and early 1990s and is now a standard treatment. Tu won the Nobel Prize in Physiology or Medicine in 2015.

According to WHO guidelines 2010, artemisinin based combination therapies (ACTs) are the recommended first line antimalarial treatments for uncomplicated malaria caused by P. falciparum. WHO recommends combinations such as artemether/lumefantrine, artesunate/amodiaquine, artesunate/mefloquine, artesunate/sulfadoxine/pyrimethamine, and dihydroartemisinin/piperaquine. Analysis of the results of the phase III trial (conducted between 2011 and 2016) revealed a rather low efficacy (20 39% depending on age, with up to 50% in 5–17 month aged babies), indicating that the vaccine will not lead to full protection and eradication. On October 6, 2021, the World Health Organization recommended malaria vaccination for children at risk.

The International Agency for Research on Cancer (IARC) has classified malaria due to P. falciparum as Group 2A carcinogen, meaning that the parasite is probably a cancer causing agent in humans. Its association with a blood cell (lymphocyte) cancer called Burkitt's lymphoma is established. Burkitt's lymphoma was discovered by Denis Burkitt in 1958 from African children, and he later speculated that the cancer was likely due to certain infectious diseases. In 1964, a virus, later called Epstein–Barr virus (EBV) after the discoverers, was identified from the cancer cells. The virus was subsequently proved to be the direct cancer agent, and is now classified as Group 1 carcinogen. In 1989, it was realised that EBV requires other infections such as with malaria to cause lymphocyte transformation. It was reported that the incidence of Burkitt's lymphoma decreased with effective treatment of malaria over several years. The actual role played by P. falciparum remained unclear for the next two and half decades. EBV had been known to induce lymphocytes to become cancerous using its viral proteins (antigens such as EBNA 1, EBNA 2, LMP1, and LMP2A). From 2014, it became clear that P. falciparum contributes to the development of the lymphoma. P. falciparum infected erythrocytes directly bind to B lymphocytes through the CIDR1α domain of PfEMP1. This binding activates toll like receptors (TLR7 and TLR10) causing continuous activation of lymphocytes to undergo proliferation and differentiation into plasma cells, thereby increasing the secretion of IgM and cytokines. This in turn activates an enzyme called activation induced cytidine deaminase (AID), which tends to cause mutation in the DNA (by double strand break) of an EBV infected lymphocytes. The damaged DNA undergoes uncontrolled replication, thus making the cell cancerous.

The high mortality and morbidity caused by P. falciparum has placed great selective pressure on the human genome. Several genetic factors provide some resistance to Plasmodium infection, including sickle cell trait, thalassaemia traits, glucose 6 phosphate dehydrogenase deficiency, and the absence of Duffy antigens on red blood cells. E. A. Beet, a doctor working in Southern Rhodesia (now Zimbabwe) had observed in 1948 that sickle cell disease was related to a lower rate of malaria infections. This suggestion was reiterated by J. B. S. Haldane in 1948, who suggested that thalassaemia might provide similar protection. This hypothesis has since been confirmed and extended to hemoglobin E and hemoglobin C.