Protozoan infections are parasitic diseases caused by organisms formerly classified in the kingdom Protozoa. These organisms are now classified in the supergroups Excavata, Amoebozoa, Harosa (SAR supergroup), and Archaeplastida. Species classified as "protozoa" are eukaryotic organisms that are usually unicellular and motile, but they are not closely related to one another. While the terms protozoan and protist are usually discouraged in modern biosciences, this terminology is still encountered in medicine due to the conservative character of medical classification and the necessity of classifying organisms based on morphology. Protozoan infections are usually transmitted either by an insect vector or by contact with an infected substance or surface. are usually contracted by either an insect vector or by contact with an infected substance or surface. Protozoans are responsible for diseases affecting a wide range of organisms, including plants, animals, and some marine life. Many of the most prevalent and deadly human diseases are caused by a protozoan infection, including African sleeping sickness, amoebic dysentery, and malaria. Protists are an artificial, polyphyletic grouping of over 64,000 different single-celled life forms, making them difficult to define due to their extreme uniqueness. This means that it is difficult to define protists due to their extreme differences and uniqueness. They grow in a wide variety of moist habitats and a majority of them are free-living organisms. In these moist environments, plankton and terrestrial forms can also be found. Protozoa are a subgroup of protists that obtain nutrients by breaking down organic compounds, either by decomposing dead organic matter, absorbing soluble products, or absorbing nutrients by phagocytosis. Encystment occurs when a protist becomes a dormant cyst with a cell wall; during encystment, the cyst has decreased complexity and metabolic activity relative to the protist. Encystment protects the protist from environmental changes; the cyst can complete nuclear reorganization and cell division, and it can act as a host cell in order to transfer parasitic species. Excystment is when a return to favorable conditions may cause a cyst to return to its original state. In parasitic protists, excystment may occur when the cyst is ingested by a new host. Protists reproduce asexually or sexually. If the protists reproduce asexually, they do so through binary fission, multiple fission, budding, and fragmentation. If the protists reproduce sexually, they do so through a syngamy process where there is a fusion of the gametes. If this occurs in an individual it is recognized as autogamy. If this occurs between individuals, it is known as conjugation.
Supergroup Excavata
Excavata are considered primitive eukaryotes. They are characterized by a feeding groove with a posteriorly located flagella, which allows them to create a current that captures small food particles. The cytostome is the specialized structure that allows the protists this function. This supergroup Excavata includes the subgroups Diplomonads (Fornicata), Parabasalids, and Euglenozoans.
Diplomonads Diplomonads used to be defined as Fornicata, but their characteristics remain the same despite their renaming. They are microaerophilic protists. Diplomonads were previously defined by the lack of a mitochondrion, but recent studies have found that they have a nonfunctional, mitochondrial remnant organelle called a mitosome. Most are harmless except for Giardia, Hexamita salmonis, and Histomonas meleagridis. Giardia causes diarrhea, Hexamita salmonis is a fish parasite, and Histomonas meleagridis is a turkey pathogen. Giardia intestinalis is a human pathogen, which is transmitted by cyst-contaminated water. It causes epidemic diarrhea from contaminated water. One can tell one may be infected by the observation of cysts or trophozoites in stools and ELISA (enzyme-linked immunosorbent assay) test. To prevent contamination, avoid any possibly contaminated water, and if contaminated water is the only thing available to drink, a slow sand filter should be used. A study found that the chlorination of water and nutritional intervention had no effect on childhood giardia infection. Only handwashing and hygienic sanitation interventions reduced infection rates in children. Hexamita salmonis is a common flagellated fish pathogen. Infected fish are weak and emaciated, and typically swim on their sides. Histomonas meleagridis is a common bird pathogen that causes histomoniasis. Signs of histomoniasis include reduced appetite, drooping wings, unkempt feathers, and yellow fecal droppings.
Parabasilia Most Parabasalia are flagellated endosymbionts of animals. They lack a distinct cytostome, which means they must use phagocytosis to engulf food. There are two subgroups: Trichonympha and Trichomonadida. Trichonympha are obligate mutualists of wood-eating insects such as termites. They secrete cellulase, which is used for digesting wood. The next subgroup, Trichomonadida, does not require oxygen and possesses hydrogenosomes. They only reproduce through asexual reproduction and some strains are human pathogens. There are three types of pathogenic parabasalia: Trichomonas foetus, Dientamoeba fragilis, and Trichomonas vaginalis. Trichomonas foetus causes spontaneous abortion in cattle, Dientamoeba fragilis causes diarrhea in humans, and Trichomonas vaginalis is a sexually transmitted disease.
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