Plasmodium

File:Malaria blood.jpg
Plasmodium in blood. Image from University of Tubingen

Classification

Higher order taxa

Eukaryota; Alveolata; Apicomplexa; Haemosporida

Species

Plasmodium falciparum
Plasmodium ovale
Plasmodium vivax
Plasmodium malariae

Description and Significance

Plasmodium, the parasite responsible for human malaria, is among the most researched genera of parasites in the world. Despite extensive studies on possible control methods, infection in humans continues to grow in tropic and sub-tropic areas. Currently there are an estimated 500,000,000 infected persons, with 1-2 million dying annually. There are four types of Plasmodium which cause human malaria: Plasmodium falciparum, Plasmodium ovale, Plasmodium vivax, and Plasmodium malariae. All of these are transmitted to human hosts solely by way of Anophele mosquito vectors. Plasmodium is one of the oldest known parasites; its long history suggests a long, adaptive relationship with the human host. Today cases of the disease are increasing in non-malarious countries as more people travel to Africa, India, Brazil, and some Asian nations, where the mosquito vectors are most prevalent. Symptoms of the disease may go unnoticed or misdiagnosed; clinical signs include fever, chills, weakness, headache, vomiting, diarrhea, anemia, pulmonary and renal dysfunction, neurologic changes. Untreated malaria may result in death.

Genome Structure

The genome of the most common form of Plasmodium which causes human malaria, Plasmodium falciparum, has been sequenced completely, yielding 14 chromosomes and 5,300 genes--a large number of which are responsible for dodging the host's immunities. The average gene density is approximately 1 gene/4,338 base pairs. The mapping of this genome sequence provides new avenues for research on possible vaccines. Click here for a complete analysis of Plasmodium falciparum's genome sequence.

Cell Structure and Metabolism

While the four major species of Plasmodium differ in some ways from each other, they all share the same complex life cycle involving the insect (mosquito) vector and the human host. When an infected Anophele mosquito bites a human, sporozoites are injected with the saliva. The sporozoites are 10 -15 µm in length and about 1 µm in diameter. They have a thin outer membrane, a double inner membrane below which lies the subpelicular microtubules. They have 3 polar rings and the rhoptries are long, extending half the length of the body. The micronemes, convoluted elongate bodies, run forward to the anterior of the sporozoite entering a common duct with the rhoptries. Mitochondria are located at the posterior end. After entering the circulatory system, the sporozoites make quick work of invading liver cells using the apical organelles (characteristic of all apicomplexans; for more details, see Apicomplexa).

Inside the host's liver cell the Plasmodium cell undergoes asexual replication. The products of this replication, called merozoites, are released into the circulatory system. The merozoites invade erythrocites and become enlarged ring-shaped trophozoites. In this stage the cells ingest the host cytoplasm and proteolyze hemoglobin into amino acids. Several rounds of nuclear divison yield a schizont. From these schizonts merozoites bud, which are released after rupturing the erythrocites. More erythrocites are invaded, and the cycle is reinitiated.

Sometimes instead of schizogony (as the schizont cycle is known) the parasites will reproduce sexually into micro- or macrogametocytes. Through gametogenesis these micro- or macrogametocytes morph into micro- or macrogametes. This may only occur after the gametocytes have been ingested by a mosquito. After said ingestion, the microgametocyte undergoes three nuclear divisions; the resulting eight nuclei become associated with thrashing flagella (this process is called exflagellation). The highly motile microgametes fuse with macrogametes and produce a zygote, which then develops into an ookinete. Once reaching the space between the epithelial cells and the basal lamina of the host, the ookinete develops into an oocyst. Asexual replication results in the production of a large number of sporozoites, which are released into the body cavity of the mosquito vector upon the maturation of the oocyst. The sporozoites are able to recognize the salivary gland of the vector, and are injected into the vertebrate host during the mosquito's blood meal, thus beginning the process over again.

Life Cycle of Plasmodium

Image from Centers of Disease Control and Prevention

Anophele mosquito vector

Image from The Walter and Eliza Hall Institute of Medical Research

The four species of Plasmodium that affect humans are different morphologically, slightly in terms of their life cycles, in terms of their host erythrocite preferences, and varying clinical symptoms.

Plasmodium vivax '''ring stage

Plasmodium malariae''' ring stage

Plasmodium falciparum''' ring stage

Plasmodium ovale '''ring stage

Images from the College of Biological Medicine, Ohio State University

Ecology

Plasmodium is a parasite which is widely distributed all over the world. Because it requires warm, humid environments for replication in the insect vector, malaria-generating species of Plasmodium are generally limited to tropical and sub-tropical locations. Global warming and population migrations do have a bearing on Plasmodium's distribution. Plasmodium falciparum is the most widespread in tropical and sub-tropical areas. Plasmodium ovale is most prevalent in the west coast region of Africa. Plasmodium malariae has a widespread distribution area but is fairly scattered within this area. Plasmodium vivax, like falciparum, ranges over a wide area, but in relatively rare in African countries. A number of methods of control have been tested and some, including use of DDT, have proved worthwhile, but drug resistance and other health concerns make some of these methods undesirable.