Plasmodium knowlesi
From MicrobeWiki, the student-edited microbiology resource
A Microbial Biorealm page on the genus Plasmodium knowlesi
Classification
- Eukaryota, Alveolata, Apicomplexa, Aconoidasida, Haemosporida, Plasmodium, knowlesi
Description and significance
- Plasmodium knowlesi is one of 5 known strains of malaria and is the most recently discovered strain of the group. Initially discovered in 1927 by Giuseppe, the parasite originally caused malaria in long-tailed and pig-tailed macaques. However, more recently P. knowlesi has been a popular source of malaria in humans due to urban development and increased interactions with forest fringes. In certain areas of South East Asia it accounts for up to 70% of all malaria cases. The virus can be transmitted to humans both directly, by the bite of an infected macaques, or indirectly through the bite of a mosquito carrying the virus. Plasmodium knowlesi has health, social and economic consequences for the regions affected by it. Although treatable, malaria continues to be the most important cause of fever and morbidity in tropical regions of the world. The virus undergoes asexual erythrocytic cycle replication in a 24 hour time period and therefore is known to cause sever and rapid cases of malaria in humans upon infection.
Genome structure
- There are currently 5 strains of Plasmodium knowlesi that have been archived. P. knowlesi has 14 chromosomes on which a total of 5,102 protein-encoding genes have been identified. Unlike other Plosmodium species, (G+C) - rich repeat regions containing intrachromosomal telomeric sequences (ITSs, containing the sequence GGGTT[T/C]A) are found at multiple sites in P. knowlesi chromosomes. These sequences may be templates for the recombinations that result in gene conversion among variant antigen genes. ITSs are also thought to play a role in transcriptional control. Another unique feature of P. knowlesi is that it contains five distinct gene families with 4–15 paralogous members of unknown function. P. knowlesi does not have any plasmids.
- Acquisition of host proteins, and thus the ability to mimic their function, has been observed in many bacterial and viral pathogens. Malaria parasites, such as P. knowlesi, are known to have a potential immunomodulatory role either by secreting functional homologues of host molecules or by binding to host antigen-presenting cells. P. knowlesi is the first strain observed in which a malaria protein has acquired host peptide sequences that are likely to be on the infected cell surface and thus may interact with the host. The mechanism by which the virus acquires these host sequences has yet to be discovered.
Cell structure, metabolism & life cycle
- The Plasmodium knowlesi parasite replicates and completes a blood stage life cycle in 24-hour cycles. This results in fairly high loads of parasite densities in a very short period of time. Rapid replication makes P. knowlesi a potentially extremely severe disease if it remains untreated in the body. Infection begins when sporozoites, the infective stages, are injected in to the body by a mosquito and are circulated throughout body until they invade liver hepatocytes. It is in the liver that sporozoites undergo exoerythrocytic schizogony - a phase of asexual multiplication –, which results in the production of many uninucleate merozoites. These merozoites exit the liver, enter the bloodstream, and then proceed to invade red blood cells. Once in the cells, P. knowlesi initiates a second phase of asexual multiplication called erythrocytic schizogony, which results in the production of about 8-16 merozoites that then invade new red blood cells. This phase is responsible for the disease malaria. Erythrocytic schizogony is cyclical and is repeated almost indefinitely. As the infection progresses, some young merozoites develop into male and female gametocytes that circulate in the peripheral blood until female mosquitoes take them up during feeding. Within the mosquito the gametocytes mature into male and female gametes, fertilization occurs and a motile zygote is formed within the mosquito gut, beginning a process known as sporogony. The motile zygote penetrates the gut wall and becomes a oocyst within which another phase of multiplication occurs resulting in the formation of sporozoites that migrate to the salivary glands of a mosquito and are injected when the mosquito feeds on a new host. These stages of Plasmodium knowlesi are microscopically indistinguishable from Plasmodium malariae and the early trophozoites are identical to those of Plasmodium falciparum.
Ecology (including pathogenesis)
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Interesting feature
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