Plasmodium is a unicellular, cell-wall-less parasite that can infect mammals, birds, and reptiles and belongs to the class Apicomplexa (formerly Sporozoa). Of the approximately 200 known species, 4 are relevant to humans as causative agents of malaria. All Plasmodia species have in common that they undergo an obligate host switch between mosquito and vertebrate, which simultaneously involves a switch between sexual and asexual reproduction.
What are plasmodia?
Infographic on the transmission cycle of malaria by the Anopheles mosquito. Click to enlarge. Plasmodium, which has no cell wall, is a unicellular parasite with a nucleus and is therefore classified as a eukaryote (formerly also eukaryotes). The name Plasmodium is due to the fact that in Plasmodia, although there are two nuclei after a division, the cytoplasm of both cells is not separated from each other, but forms a contiguous plasma space. Of about 200 known plasmodia species, 4 occupy a special position as human malaria pathogens. All plasmodia species undergo an obligate host switch between mosquito and vertebrate. The host switch simultaneously involves a switch between sexual and asexual reproduction. In humans, who act as intermediate hosts, the malaria vector is the female Anopheles mosquito. The Anopheles mosquito transmits the pathogen in the form of sporozoites found in its saliva. On the mosquito side, the sporozoites represent the final stage of the gametocytes with which the mosquito has previously infected itself with ingested human blood. The four species of plasmodia that cause malaria in humans are Plasmodium falciparum (Malaria tropica), Plasmodium vivax, (Malaria fertiana), Plasmodium ovale (Malaria tertiana), and Plasmodium malariae (Malaria quartana). Currently, there is debate as to whether Plasmodium knowlesi, which is found in Southeast Asia, should also be counted among the malaria pathogens that are dangerous to humans. Plasmodium knowlesi was previously known to cause malaria in macaques. Malaria develops flu-like symptoms with fever episodes and, in the case of malaria tropica, shows a severe course if left untreated. The individual plasmodial species are mostly specific and “species faithful” with respect to intermediate carrier (mosquito) and final host (vertebrate).
Occurrence, distribution, and characteristics
Plasmodia are native to all continents except Antarctica. However, the occurrence of malaria agents relevant to humans is now restricted to tropical and subtropical areas. Until the 19th century, the malaria-causing plasmodia were also found in southern countries of Europe and North America. In tropical and subtropical regions, the annual death rate is 1.0 to 1.5 million. Estimates of the number of people suffering from malaria worldwide vary widely, ranging from 250 to 500 million. Plasmodia are transmitted exclusively by the Anopheles mosquito. Direct transmission from person to person is virtually impossible because the sexual part of the development cycle, which takes place in the mosquito, is absent. However, a few cases are known in which contaminated blood transfusion needles caused direct transmission of the pathogen. Although the development cycle of the individual plasmodia species differs somewhat, it basically follows the following developmental scheme: the Anopheles mosquito transmits the plasmodia in the form of sporozoites, which are initially washed into the liver with the blood and attach themselves to liver cells. In the liver cells, they grow by asexual division processes into schizonts, which in a later stage differentiate into a large number of still diploid merozoites that infect the erythrocytes (red blood cells), where they multiply greatly by further divisions. The period during which the sporozoites have established themselves in the liver cells is usually asymptomatic. Some of the diploid merozoites develop by meiosis into haploid micro- and macrogametocytes, which can be ingested by a bloodsucking Anopheles mosquito via its proboscis. In the mosquito’s intestine, the union of the gametocytes, which are differentiated into complete gametes, takes place to form a diploid zygote. In the mosquito’s intestinal wall, the zygote grows into an oocyst, in which up to 10,000 infectious diploid sporozoites grow through mitotic divisions.After the oocyst bursts, some of the sporozoites enter the saliva of the mosquito, thus forming a new reservoir of infection. The incubation period from sporozoite infection to malaria outbreak is approximately 7 to 50 days, depending on the pathogen and without malaria prophylaxis.
Diseases and symptoms
Except for malaria tropica, in which febrile episodes occur at irregular intervals, other pathogens establish a clear rhythm. In malaria quartana, this rhythm is four days long. A day with an episode of fever is followed by two fever-free days before fever sets in again. The regular fever episodes are due to the development of plasmodia in the erythrocytes, which virtually simultaneously flood the body and cause the symptoms. Plasmodium ovale and Plasmodium vivax, both of which are the causative agents of malaria tertiana, can form hypnozoites during their liver stage, which can persist unnoticed and without symptoms for several months – in individual cases even several decades – before another malaria episode is triggered. In addition to chemical prophylaxis, which should be tailored to the prevailing pathogens in the region concerned, the best protection against malaria is protection against the female Anopheles mosquito. At night, a mosquito net over the bed can provide effective protection, and during the day, clothing with long sleeves and long pant legs impregnated with permithrin or another mosquito-repellent substance is recommended. Uncovered areas of the body should be treated with creams or sprays that also have a mosquito-repellent effect.
