Plasmodium is a single-celled, cell-wallless parasite that can affect mammals, birds and reptiles and is considered a member of the genus Apicomplexa (formerly Sporozoa). Of the approximately 200 known species, 4 are relevant for humans as pathogens of malaria. All plasmodium species have in common that they undergo an obligatory host change between mosquito and vertebrate, which at the same time includes a change between sexual and asexual reproduction.

What are plasmodia?

Infogram on the transmission cycle of malaria by the Anopheles mosquito. Click to enlarge.

Plasmodium, which has no cell wall, is a single-celled parasite with cell nucleus and is therefore expected to be eukaryotes (formerly also eukaryotes). The name plasmodium is due to the fact that in plasmodia after dividing two nuclei are present, but the cytoplasm of both cells is not separated, but forms a contiguous plasma space.

Of about 200 known Plasmodium species take 4 as malaria parasite of humans a special position. All Plasmodium species undergo an obligatory host change between mosquito and vertebrate. At the same time, the change of hosts involves a change between sexual and asexual reproduction.

In humans acting as an intermediate host, the malaria carrier is the female Anopheles mosquito. The Anopheles mosquito transmits the pathogen in the form of sporozoites, which are in their saliva. On the mosquito side, the sporozoites represent the end stage of the gametocytes, with which the mosquito had previously infected itself with the absorbed human blood.

The four plasmodium species that cause malaria in humans are Plasmodium falciparum (Malaria tropica), Plasmodium vivax, (Malaria fertiana), Plasmodium ovale (Malaria tertiana) and Plasmodium malariae (Malaria quartana). It is currently being discussed whether Plasmodium knowlesi, which can be found in Southeast Asia, is also one of the dangerous malaria pathogens. The Plasmodium knowlesi was previously known as the cause of malaria in macaques.

The malaria develops flu-like symptoms with fever episodes and shows in the case of malaria tropica untreated, a serious course. The individual Plasmodienarten are with respect to intermediate carrier (mosquito) and Endwirt (vertebrate) mostly specific and "artentreu".

Occurrence, distribution & characteristics

Plasmodia are native to all continents with the exception of the Antarctic. However, the incidence of human-relevant malaria is limited to tropical and subtropical areas. Until the 19th century, malaria-causing plasmodia were also found in southern European countries and North America.

In tropical and subtropical regions, the annual mortality rate ranges from 1.0 to 1.5 million. The estimates of the worldwide malaria sufferers vary widely and are therefore from 250 to 500 million. The plasmodia are transmitted exclusively by the Anopheles mosquito. Direct human-to-human transmission is virtually impossible because the sexual part of the development cycle that takes place in the mosquito is missing. However, a few cases are known in which contaminated blood transfusion needles caused a direct transmission of the pathogen.

Although the development cycle of the individual Plasmodienarten runs a little differently, but basically follows the following development scheme: The Anopheles mosquito transmits the Plasmodia in the form of sporozoites, which are first washed with the blood in the liver and there accumulate in liver cells. In the liver cells, they grow into schizonts through asexual division processes, which at a later stage differentiate into a large number of further diploid merozoites, which attack the erythrocytes (red blood cells) and proliferate there through further division.

The time in which the sporozoites have lodged in the liver cells is usually asymptomatic. Some of the diploid merozoites develop by meiosis into haploid micro- and macrogametocytes, which can be taken up by a blood-sucking Anopheles mosquito over their Stechrüssel. In the intestine of the mosquito, the union of gametocytes differentiated into complete gametes leads to a diploid zygote. In the intestinal wall of the mosquito, this grows into an oocyst in which, through mitotic division, grow up to 10, 000 infectious diploid sporozoites. After the bursting of the oocyst, part of the sporozoites gets into the saliva of the mosquito, forming a new infectious reservoir.

The incubation period from the infection with sporozoites to the onset of malaria is about 7 to 50 days, depending on the pathogen and without malaria prophylaxis.

Diseases & complaints

Except for the malaria tropica, in which the fever episodes occur at irregular intervals, set in other pathogens a clear rhythm. In the case of malaria quartana, this is four-fold. A day with a fever episode is followed by two days without fever, before another 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 causative agents of malaria tertiana, can produce hypnozoites during their liver stages, which can go unnoticed and symptomless for several months - in some cases even several decades - before another malaria spasm is triggered.

The best protection against malaria, in addition to a chemical prophylaxis, which should be tailored to the prevalent pathogens in the region, in protection against the female Anopheles mosquito. At night, a mosquito net over the bed can provide effective protection, and during the day a long-sleeved dress and long pants are recommended, impregnated with permithrin or other mosquito-repellent substance. The uncovered body parts should be treated with creams or sprays, which also have a mosquito-repellent effect.