The single-celled parasite, Plasmodium falciparum, causes malaria, was responsible for 214 million cases and 438,000 deaths globally in 2015, of which 88% of cases and 90% of deaths occurred in sub-Saharan Africa.
The morphology of P. falciparum is characterized by numerous rings (young trophozoites) and the absence of mature trophozoites and schizonts. In a P. falciparum blood smear, multiple infected erythrocytes appear more often than in other species and the rings of the parasite in the blood smear tend to be smaller than other species. Distinctive crescent-shaped gametocytes appear late in the infection.
In vitro cultivation of P. falciparum has shown that the optimal oxygen (O2) and carbon dioxide (CO2) requirements for the parasites occur at 3% O2 and 1-2% CO2, and these concentrations allow the culture to be self-perpetuating . Optimal living conditions, in which P. falciparum can be maintained in continuous culture in human erythrocytes, include the use of RPMI 1640 Medium, in an atmosphere of 7% CO2 and low oxygen .
P. falciparum produces a parasitiphorous vacuole (PV), a bubble-like compartment comprised of a plasma membrane, containing cytoplasm and the parasite. This PV allows the parasite to be protected from the cell defence mechanisms of the host, while allowing the existence and growth of the parasite within the cell .
The female Anopheles mosquito transmits the parasite, with the Anopheles gambei species being the most common vector for transmission of the most dangerous parasite, P. falciparum. Most species bite between dusk and dawn, and the intensity of transmission depends on numerous factors involving to the vector, the parasite, the host and the environment . The incubation period for the disease is usually between ten days to four weeks after being bitten, although a person may feel unwell as early as seven days and up to a year after infection .
The life cycle of the P. falciparum parasite encompasses two hosts. While obtaining a blood meal, a mosquito which is infected with malaria introduces sporozoites into the human hosts which replicate in the liver to form schizonts. These then rupture and release merozoites, which infect red blood cells. It is at this stage that the disease manifests itself . Infected red blood cells do not remain in circulation for long. After about 24-32 hours, during a period called sequestration, infected red blood cells adhere to the endothelial cells in the microcirculation of various organs, and this is thought to occur in order that the parasite can avoid being removed by the spleen .
Asexual reproduction occurs in the erythrocytes and the gametocytes are then ingested by the mosquito during a blood meal. Whilst in the stomach of the mosquito, the male and female gametocytes join to form zygotes, which in turn become elongated and motile ookinetes. These develop in the midgut wall of the mosquito and develop into oocysts, which later rupture and release sporozoites. These make their way into the saliva of the mosquito and are inoculated into a new human host, perpetuating the cycle .
Symptoms of malaria include fevers and chills, headaches, joint and muscle pain, and vomiting and diarrhea. These can also be accompanied by enlargement of the spleen, anemia, neurological complications, hypoglycemia, blood platelet deficiency and pulmonary and renal dysfunction . P. falciparum infections account for most fatalities due to the parasite entering the brain, or causing severe anemia . Treatment consists of an initial diagnosis of the parasite by means of diagnostic testing and then treatment with artemisinin-based combination therapy (ACT), which involves treatment with the antimalarial drug, artemisinin in combination with other different classes of antimalarial drugs. This therapy is used to combat the increasing resistance shown by P. falciparum to other drugs, such as chloroquine and sulfadoxine-pyrimethamine which have been used to treat P. falciparum malaria in the past.
Describe the optimum living conditions for P. falciparum. (Paragraph 3)
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