Plasmodium falciparum biology

''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 (i.e. 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.

As of the World Health Organization ''World Malaria Report 2021'', there were 241 million cases of malaria worldwide in 2020, resulting in an estimated 627,000 deaths. Nearly all malarial deaths are caused by ''P. falciparum'', and 95% of such cases occur in Africa. Children under five years of age are most affected, accounting for 80% of the total deaths. 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.

History

Falciparum malaria was familiar to the ancient Greeks, who gave the general name (''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.

Discovery

A German physician, Johann Friedrich Meckel, must have been the first to see ''P. falciparum'' but without knowing what it was. In 1847, he reported the presence of black pigment granules from the blood and spleen of a patient who died of malaria. The French Army physician Charles Louis Alphonse Laveran, while working at Bône Hospital (now Annaba in Algeria), correctly identified the parasite as a causative pathogen of malaria in 1880. He presented his discovery before the French Academy of Medicine in Paris, and published it in ''The Lancet'' in 1881. He gave it the scientific name ''Oscillaria malariae''. 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. His colleague Ronald Ross of the Indian Medical Service validated the theory while working in India. Ross discovered in 1897 that malarial parasites lived in certain mosquitoes. The next year, he demonstrated that a malarial parasite of birds could be transmitted by mosquitoes from one bird to another. Around the same time, Grassi demonstrated that ''P. falciparum'' was transmitted in humans only by female anopheline mosquito (in his case ''Anopheles claviger''). 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".

Origin and evolution

''P. falciparum'' is now generally accepted to have evolved from ''Laverania'' (a subgenus of ''Plasmodium'' found in apes) species present in gorilla in Western Africa. Genetic diversity indicates that the human protozoan emerged around 10,000 years ago. The closest relative of ''P. falciparum'' is ''P. praefalciparum'', a parasite of gorillas, as supported by mitochondrial, apicoplastic and nuclear DNA sequences. 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''). This suggests that the origin of ''P. falciparum'' in humans is recent, as a single ''P. praefalciparum'' strain became capable of infecting humans. The genetic information of ''P. falciparum'' has signaled a recent expansion that coincides with the agricultural revolution. It is likely that the development of extensive agriculture increased mosquito population densities by giving rise to more breeding sites, which may have triggered the evolution and expansion of ''P. falciparum''.

Structure

''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 trophozoites feed 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. As an apicomplexan, it harbours a plastid, an apicoplast, similar to plant chloroplasts, which they probably acquired by engulfing (or being invaded by) a eukaryotic alga and retaining the algal plastid as a distinctive organelle encased within four membranes. The apicoplast is involved in the synthesis of lipids and several other compounds and provides an attractive drug target. During the asexual blood stage of infection, an essential function of the apicoplast is to produce the isoprenoid precursors isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) via the MEP (non-mevalonate) pathway.

Genome

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.

Life cycle

Humans are the intermediate hosts in which asexual reproduction occurs, and female anopheline mosquitos are the definitive hosts harbouring the sexual reproduction stage.

In humans

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.

Liver stage or exo-erythrocytic schizogony

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 and meiosis 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.

Blood stage or erythrocytic schizogony

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.

The parasite can also alter the morphology of the erythrocyte, causing knobs on the erythrocyte membrane. Infected erythrocytes are often sequestered in various human tissues or organs, such as the heart, liver and brain. This is caused by parasite-derived cell surface proteins being present on the erythrocyte membrane, and it is these proteins that bind to receptors on human cells. Sequestration in the brain causes cerebral malaria, a very severe form of the disease, which increases the victim's likelihood of death.

=Trophozoite

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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 erythrocyte, digesting its proteins and converting (by biocrystallization) the remaining heme into an 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.

=Schizont

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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. The red blood cells are ruptured by the merozoites. The liberated merozoites invade fresh erythrocytes. A free merozoite is in the bloodstream for roughly 60 seconds before it enters another erythrocyte.

The duration of one complete erythrocytic schizogony is approximately 48 hours. This gives rise to the characteristic clinical manifestations of falciparum malaria, such as fever and chills, corresponding to the synchronous rupture of the infected erythrocytes.

=Gametocyte

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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.

Incubation period

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, but may range from 9 to 30 days. In isolated cases, prolonged incubation periods as long as 2, 3 or even 8 years have been recorded. 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).

In mosquitoes

Within the mosquito midgut, the female gamete maturation process entails slight morphological changes, becoming more enlarged and spherical. The male gametocyte undergoes a rapid nuclear division within 15 minutes, producing eight flagellated microgamete