Plasmodia are malaria pathogens found in the saliva of the Anopheles mosquito, in whose bite they are transmitted to and multiply parasitically in the human host. Plasmodium ovale is one of a total of four malaria pathogens. Like Plasmodium vivax, the parasite causes malaria tertiana with mild progression.
What is Plasmodium ovale?
Plasmodia are unicellular parasites that belong to the sporozoa. Since the new systematics, they belong to the phylum Apicomplexa. All plasmodia live in the saliva of female Anopheles mosquitoes. They are all clinically relevant as causative agents of malaria. Malaria pathogens such as Plasmodium ovale colonize red blood cells in their hosts and feed on hemoglobin. The red blood pigment is converted to hemozoin by plasmodia such as Plasmodium ovale. In the colonized erythrocytes, this transformation appears as a brownish black pigment. The red blood cells disintegrate as a result of colonization, releasing toxic degradation products that have an effect on the patient’s central nervous system. Plasmodium ovale is one of four unicellular pathogens of Malaria Tertiana. In western regions, its distribution is low. In tropical regions, the pathogen is more common. Malaria tertiana is a benign form of the disease. The Plasmodium ovale pathogen is less commonly associated with cases of the infection than its relative Plasmodium vivax. The main distribution area of the pathogen is western Africa south of the Sahara. The pathogen can also be found in Thailand or Indonesia. The Anopheles species relevant for transmission are the species gambiae and funestus.
Occurrence, distribution, and characteristics
All plasmodia change from sexual to asexual reproduction and back again during their existence. They thus undergo generational changes that are accompanied by simultaneous host change. The pathogens migrate from the salivary glands of the transmitting mosquito into humans and are eventually reabsorbed from human blood by a mosquito. The circle closes. In humans, the pathogens initially live in a phase of schizogony. They enter the human organism as sporozoites and reach the tissue of the liver. There they colonize the hepatocytes, where they transform into schizonts. The schizonts disintegrate into merozoites, which advance from the liver into the blood. Once in the bloodstream, Plasmodium ovale infects red blood cells in its survivor forms. Within the cells, the pathogens develop into so-called blood schizonts, which again give rise to merozoites. A certain proportion of them do not turn into schizonts, but undergo differentiation into microgametocytes or macrogametocytes. Gamonts are transmitted back to the next mosquito that bites the infested host. In the intestinal tract of the mosquito, the gamonts mature. In an act of sexual reproduction, fusion occurs. This produces a zygote that infiltrates the intestinal wall of the infested mosquito. In a sequence, an oocyst is formed. From this point on, asexual division takes place. Up to 10,000 sporozoites are formed in this way. The individual sporozoites are released as soon as the oocysts burst. They reach the salivary glands of the infested mosquito and are thus transmitted to the next person. The cycle continues. As is common for plasmodia, Plasmodium ovale undergoes different stages of development. Liver schizonts are roundish or oval in shape and reach about 50 microns. The individual merozoites of the schizonts are over one micrometer in size. Single cells are infected by Plasmodium ovale, sometimes multiple times. Once trophozoites form, the red blood cells of the hosts swell. In addition to the increase in size, the typical coloration, Schüffner’s stippling, occurs.
Diseases and symptoms
Plasmodia of the species ovale are obligate human pathogens of malaria tertiana. After the mosquito bite, there is an incubation period during which patients do not show any symptoms. This period can extend over 18 days. Because affected individuals have often taken chemoprophylaxis based on medical recommendations, the incubation period can even extend for weeks or months. After the incubation period, affected individuals develop a cyclic fever. The fever episodes are interrupted by fever-free days. At the beginning of each fever attack is the so-called freezing phase, which lasts barely an hour.In the freezing phase, the body temperature begins to rise rapidly. The subsequent heat phase lasts about four hours and is characterized by agonizing burning of the skin, severe nausea, vomiting and exhaustion. The body temperature of those affected often reaches heights of 40 degrees Celsius. Sweating occurs in the third phase, which lasts for three hours and is accompanied by gradual normalization of temperature. Patients recover step by step during this third phase until another episode of fever sets in. Plasmodium ovale malaria tertiana only rarely leads to acutely life-threatening conditions. Preventive vaccinations are not yet available for forms of malaria. For prevention, travel plans to high-risk areas for malaria should be avoided if possible. Chemoprophylaxis is one possible preventive measure. In addition, travelers must carry at least antimalarial drugs in the relevant areas. Quinine is known as a drug against malaria and helps to kill the schizonts in the blood of infected persons. Quinine can improve the general condition of a malaria patient accordingly. Synthetic drugs are also available against malaria. However, malaria pathogens such as Plasmodium ovale are now immune to many synthetic antimalarial drugs. For this reason, quinine is resorted to all the more frequently these days.
