Plasmodium ovale

Plasmodium ovale trophozoite in Giemsa - stained blood smear

Plasmodium ovale is a protozoan parasite of the genus Plasmodium and a pathogen of tertian malaria. Infection with this parasite are relatively rare and usually run comparatively benign. Like other malaria parasite is transmitted by Anopheles mosquitoes Plasmodium ovale. The parasite is found in many tropical countries.

Discovery and description

History

Plasmodium ovale was the last of the classic malaria parasites of man, who was discovered. John Stephens of the Liverpool School of Tropical Medicine identified the parasites in a blood smear from an East African malaria patients. The term oval stems from an oval shape of infected erythrocytes in the blood smear, a peculiarity which is not observed in the other malaria parasites.

Description

As with all plasmodia and P. ovale occurs in various stages of development. Leberschizonten often have an oval shape and can reach a size of about 50 x 80 microns and contain hundreds of merozoites. With the proliferation of the parasite in erythrocytes these are usually significantly enlarged and have a characteristic Schüffnersche stippling, which is more prominent than in Plasmodium vivax. Maturity Blutschizonten contain 8 to 20 merozoites. Immature gametocytes are microscopically difficult to distinguish from the asexual forms. The mature gametocytes fill the host cell from complete; the parasites are surrounded by a clear ring of pits. Macrogametocyte show after staining medium blue cytoplasm with a prominent, situated at the edge of the core and in granules such as pearl necklaces divided pigment, while the microgametocyte show a slightly blue cytoplasm with a large core.

System

Plasmodium ovale is classified as most other primates infecting plasmodia in the subgenus Plasmodium. The closer relationships to other species are unclear. Although P. vivax is oval in many respects similar to Plasmodium, molecular studies show no particularly close relationship. Also exists with Plasmodium schwetzi a kind, found in chimpanzees and gorillas, the microscopic and the disease course her great resemblance to P.ovale shows there are for P. schwetzi before but no molecular data for comparison. Some phylogenetic analyzes place P. ovale near members of the genus Hepatocystis; the status of these species is itself unclear, since the genus Plasmodium is paraphyletic Hepatocystis.

Distribution and host animals

As a distribution area for Plasmodium ovale are mainly sub-Saharan Africa and some islands in the western Pacific, particularly New Guinea stated. From Southeast Asia, there are only isolated reports, including those from Indonesia, Thailand and Vietnam. Whether the parasite has become established there seems unclear. The reasons for this restricted area of ​​distribution are unknown, appropriate hosts and vectors are also found in other tropical regions

Humans are the only known reservoir host for Plasmodium ovale. Chimpanzees can be infected under experimental conditions, in macaques and New World monkeys, this was not possible. A number of Anopheles species are susceptible to infection with P. ovale and come as vectors in question. Anopheles gambiae and Anopheles funestus are considered important natural transmitters.

Life cycle

The life cycle of P. ovale is similar to the other plasmodia substantially. The parasite shows an obligate host alternation. The sporozoites enter by infected mosquitoes in the bloodstream of people who migrate from there to the liver and invade hepatocytes one in which they reproduce asexually. The incubation period of the liver phase is approximately 9 days. The Leberschizonten each produce hundreds of merozoites, which are released and infect red blood cells, where another asexual reproduction takes place. The generation time in the multiplication in the erythrocytes is on average 49 hours. Since the development is synchronous, there is at the end of each cycle of propagation to a massive release of new parasites, which is connected with a fever. From the periodicity of the fever, the term derives from tertian malaria.

A few plasmodia developing in the erythrocytes to generation forms. This microgametocyte and macrogametocyte can be taken up by mosquitoes during a blood meal, and in the intestines of the insect a new development cycle in motion. After a fusion of the gametes new sporozoites are formed in the intestine, which migrate to the salivary glands, from where they can be transferred to a new host. The development time in the mosquito is approximately 14 to 16 days.

Not all liver parasites are released into the bloodstream. There remain calm forms, so-called Hypnozoiten, in the liver that can lead to relapse also called relapses after weeks or months. These hypnozoites must be eliminated by treatment with agents such as primaquine.