The fifth Phylum of the Protist Kingdom, known as Apicomplexa, gathers several species of obligate intracellular protozoan parasites classified as Sporozoa or Sporozoans, because they form reproductive cells known as spores. Many sporozoans are parasitic and pathogenic species, such as Plasmodium (P. falciparum, P. malariae, P. vivax), Toxoplasma gondii, Pneumocysts carinii, Coccidian, Babesia, Cryptosporidum (C. parvum, C. muris), and Gregarian. The Sporozoa reproduction cycle has both asexual and sexual phases. The asexual phase is termed schizogony (from the Greek, meaning generation through division), in which merozoites (daughter cells) are produced through multiple nuclear fissions. The sexual phase is known as sporogony (i.e., generation of spores) and is followed by gametogony or the production of sexually reproductive cells termed gamonts. Each pair of gamonts form a gamontocyst where the division of both gamonts, preceded by repeated nuclear divisions, originates numerous gametes. Gametes fuse in pairs, forming zygotes that undergo meiosis (cell division), thus forming new sporozoites. When sporozoites invade new host cells, the life cycle starts again. This general description of Sporozoan life cycle has some variation among different species and groups.
Sporozoans have no flagellated extensions for locomotion, with most species presenting only gliding motility, except for male gametes in the sexual phase, which have a flagellated stage of motility. All Sporozoa have a cellular structure known as apical complex, which gave origin to the name of the Phylum, i.e., Apicomplexa. Sporozoa cellular organization consists of the apical complex, micropore, longitudinal microtubular cytoskeleton, and cortical alveoli. The apical complex consists of cytoskeletal and secretory structures forming a conoid (a small open cone), polar wings that fix the cytoskeletal microtubules, two apical rings, and secretory vesicles known as micronemes and rhoptries. The apical complex enables Sporozoans to invade the host cells.
Plasmodium species are the causing agents of malaria in humans and animals and affects approximately 300 million people around the world, with an estimative of one million new cases each year. They are transmitted by the female anopheles mosquito (infecting vector) that injects Plasmodium sporozoites present in the salivary glands of the mosquito into the host's blood stream. Once in the blood stream, Plasmodium sporozoites invade erythrocytes (red blood cells) and migrate to the liver to infect the hepatocytes, where their asexual reproductive phase starts. When the merozoite stage is reached, they are released into the circulation again, where they become ring-like trophozoites that undergo schizogony, forming new merozoites that invade the erythrocytes, thus repeating the reproductive cycle. Female anopheles mosquitoes ingest merozoites together with the host's blood. Ingested merozoites form zygotes in the guts of the vector mosquito, later developing into oocysts, from which new sporozoites will be formed and migrate to the anopheles' salivary gland, ready to contaminate the next host. Malaria can also be transmitted through infected blood transfusions.
The vectors for Babesia are ticks, causing fever, peripheral capillary hemorrhage, and anemia. Contaminated cats are Toxoplasma gondii direct vectors to humans, through the ingestion of oocysts present in cat feces. However, this parasite is also present in birds and other mammals, and humans can be infected by ingesting raw or poorly cooked contaminated meat. Pregnant women may have miscarriages when infected, or can transmit toxoplasmosis through the placenta to the fetus, leading to blindness and/or mental retardation of the child. Periodic fecal tests of the house cat and adequate treatment may prevent transmission, as well as avoidance of half-cooked meat in the diet.
Pneumocysts carinii causes interstitial plasma cell pneumonia when the cysts containing trophozoites are inhaled. Cryptosporidium parvum is usually transmitted through the ingestion of water or foods contaminated with its oocysts, causing intestinal infection and, in immunodepressed patients, diarrhea can be chronic, accompanied by fever. Coccidian species infect epithelial tissues of both vertebrates and invertebrates whereas Gregarian species are found in the body cavities of invertebrates, such as earthworms.
See also Gastroenteritis; Malaria and the physiology of parasitic infections; Microbial taxonomy
Sporozoa (spôr´əzō´ə), phylum of unicellular heterotrophic organisms of the kingdom Protista. Unlike most other protozoans, sporozoans have no cilia or flagella. All species are parasitic and have elaborate life cycles, often requiring more than one host. The best-known sporozoan is Plasmodium falciparum, the causative organism of malaria.