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Baron S, editor. Clinical Microbiology. Fourth edition. Galveston (TX): college of Texas medical Branch at Galveston; 1996.



Protozoa are one-celled animals found an international in most habitats. Most varieties are cost-free living, but all higher animals room infected through one or more species of protozoa. Infections selection from asymptomatic to life threatening, depending upon the species and strain of the parasite and also the resistance of the host.


Protozoa are microscopic unicellular eukaryotes that have a relatively complex internal structure and carry out complex metabolic activities. Some protozoa have actually structures for propulsion or other varieties of movement.


On the basis of light and electron microscopic morphology, the protozoa are at this time classified into six phyla. Most types causing human disease are members that the phyla Sacromastigophora and also Apicomplexa.

Life cycle Stages

The stages of parasitic protozoa that actively feed and multiply are commonly called trophozoites; in part protozoa, other terms are used for these stages. Cysts room stages v a safety membrane or thickened wall. Protozoan cysts that should survive outside the organize usually have much more resistant wall surfaces than cysts that kind in tissues.


Binary fission, the most common form of reproduction, is asexual; many asexual division occurs in part forms. Both sexual and also asexual reproduction occur in the Apicomplexa.


All parasitic protozoa need preformed essential substances—that is, nutrition is holozoic as in greater animals.


The Protozoa are considered to it is in a subkingdom the the kingdom Protista, back in the classical system lock were inserted in the kingdom Animalia. More than 50,000 species have been described, many of which room free-living organisms; protozoa are uncovered in almost every possible habitat. The fossil record in the form of shells in sedimentary rocks shows that protozoa were present in the Pre-cambrian era. Anton van Leeuwenhoek to be the first person to check out protozoa, making use of microscopes he built with straightforward lenses. Between 1674 and 1716, he described, in addition to free-living protozoa, several parasitic varieties from animals, and Giardia lamblia from his own stools. Practically all humans have protozoa living in or on their body at some time, and also many persons are infected with one or much more species throughout your life. Some species are thought about commensals, i.e., typically not harmful, vice versa, others are pathogens and usually develop disease. Protozoan diseases range from an extremely mild come life-threatening. Individuals whose defenses space able come control yet not eliminate a parasitic infection end up being carriers and constitute a resource of epidemic for others. In geographic locations of high prevalence, well-tolerated infections are often not cure to eradicate the parasite due to the fact that eradication would lower the individual"s immune to the parasite and result in a high likelihood that reinfection.

Many protozoan infections that space inapparent or soft in normal individuals deserve to be life-threatening in immunosuppressed patients, specifically patients with got immune deficiency syndrome (AIDS). Evidence suggests that many healthy persons harbor low numbers that Pneumocystis carinii in your lungs. However, this parasite produce a generally fatal pneumonia in immunosuppressed patient such as those with AIDS. Toxoplasma gondii, a very common protozoan parasite, usually reasons a rather mild initial illness followed by a long-lasting implicitly infection. AIDS patients, however, can build fatal toxoplasmic encephalitis. Cryptosporidium was defined in the 19th century, yet widespread person infection has only freshly been recognized. Cryptosporidium is another protozoan the can create serious symptom in patients with AIDS. Microsporidiosis in human beings was report in just a few instances prior to the appearance of AIDS. It has actually now end up being a an ext common infection in AIDS patients. As much more thorough researches of patients with AIDS are made, the is most likely that other rare or unusual protozoan infections will be diagnosed.

Acanthamoeba types are free-living amebas the inhabit soil and water. Cyst stages have the right to be airborne. Significant eye-threatening corneal ulcers due to Acanthamoeba species are gift reported in individuals who use contact lenses. The parasites presumably space transmitted in contaminated lens-cleaning solution. Amebas of the genus Naegleria, i beg your pardon inhabit body of new water, are responsible for nearly all cases of the commonly fatal disease primary amebic meningoencephalitis. The amebas are thought to get in the body from water that is splashed top top the upper nasal tract throughout swimming or diving. Human infections of this kind were predicted before they to be recognized and reported, based on laboratory research studies of Acanthamoeba epidemic in cabinet cultures and in animals.

The lack of reliable vaccines, the paucity of reliable drugs, and also other problems, including challenges of vector control, motivated the world Health company to target six illness for boosted research and training. Three of these to be protozoan infections—malaria, trypanosomiasis, and leishmaniasis. Although new information top top these diseases has to be gained, most of the difficulties with control persist.


Most helminth protozoa in people are much less than 50 μm in size. The smallest (mainly intracellular forms) room 1 come 10 μm long, yet Balantidium coli may measure 150 μm. Protozoa space unicellular eukaryotes. As in all eukaryotes, the nucleus is attached in a membrane. In protozoa various other than ciliates, the cell nucleus is vesicular, v scattered chromatin offering a diffusive appearance to the nucleus, every nuclei in the separation, personal, instance organism show up alike. One form of vesicular nucleus has a much more or less central body, referred to as an endosome or karyosome. The endosome lacks DNA in the parasitic amebas and also trypanosomes. In the phylum Apicomplexa, top top the other hand, the vesicular nucleus has one or much more nucleoli the contain DNA. The ciliates have both a micronucleus and macronucleus, which show up quite homogeneous in composition.

The oribelles of protozoa have actually functions comparable to the organs of higher animals. The plasma membrane enclosing the cytoplasm additionally covers the projecting locomotory frameworks such together pseudopodia, cilia, and also flagella. The external surface great of part protozoa, termed a pellicle, is sufficiently rigid to keep a distinctive shape, as in the trypanosomes and Giardia. However, this organisms can readily twist and bend when moving through your environment. In most protozoa the cytoplasm is differentiated into ectoplasm (the outer, transparent layer) and endoplasm (the inner class containing organelles); the framework of the cytoplasm is most easily seen in varieties with projecting pseudopodia, such as the amebas. Part protozoa have a cytosome or cabinet “mouth” because that ingesting fluids or hard particles. Contractile vacuoles for osmoregulation happen in some, such as Naegleria and Balantidium. Countless protozoa have actually subpellicular microtubules; in the Apicomplexa, which have actually no exterior organelles for locomotion, these administer a method for sluggish movement. The trichomonads and trypanosomes have actually a distinctive undulating membrane between the body wall surface and a flagellum. Plenty of other structures occur in parasitic protozoa, consisting of the Golgi apparatus, mitochondria, lysosomes, food vacuoles, conoids in the Apicomplexa, and also other committed structures. Electron microscopy is crucial to visualize the details the protozoal structure. Indigenous the point of check out of functional and also physiologic complexity, a protozoan is an ext like an pet than like a single cell. Figure 77-1 reflects the framework of the bloodstream kind of a trypanosome, as identified by electron microscopy.


Figure 77-1

Fine framework of a protozoan parasite, Typanosoma evansi, as revealed by transmission electron microcopy of thin sections. (Adapted from Vickerman K: Protozoology. Vol. 3 London college of Hygiene and also Tropical Medicine, London, 1977, through permission.) (more...)


In 1985 the culture of Protozoologists released a taxonomic scheme that dispersed the Protozoa right into six phyla. Two of this phyla—the Sarcomastigophora and also the Apicomplexa--contain the most important varieties causing person disease. This system is based upon morphology as revealed by light, electron, and scanning microscopy. Dientamoeba fragilis, for example, had been thought to be an ameba and placed in the family Entamoebidae. However, internal structures watched by electron microscopy proved that it is properly put in the bespeak Trichomonadida of flagellate protozoa. In part instances, biology that appear identical under the microscope have been assigned different types names top top the communication of such criteria as geographical distribution and also clinical manifestations; a great example is the genus Leishmania, because that which subspecies surname are frequently used. Biochemical methods have been employed on strains and species to determine isoenzyme trends or to recognize relevant nucleotide order in RNA, DNA, or both. Extensive studies have actually been do on the kinetoplast, a distinctive mitochondrion uncovered in the hemoflagellates and other members that the stimulate Kinetoplastida. The DNA linked with this organelle is of great interest. Cloning is widely provided in taxonomic studies, for instance to study distinctions in virulence or an illness manifestations in isolates of a single species derived from various hosts or geographical regions. Antitoxin (particularly monoclonal antibodies) come known varieties or to details antigens indigenous a types are gift employed to recognize unknown isolates. Eventually, molecular taxonomy might prove to it is in a an ext reliable basis 보다 morphology because that protozoan taxonomy, however the microscope is quiet the most practical tool because that identifying a protozoan parasite. Table 77-1 list the medically important protozoa.


Life bicycle Stages

During that life cycle, a protozoan generally passes through numerous stages the differ in structure and activity. Trophozoite (Greek for “animal the feeds”) is a general term for the active, feeding, multiplying phase of most protozoa. In parasitic species this is the stage usually linked with pathogenesis. In the hemoflagellates the terms amastigote, promastigote, epimastigote, and also trypomastigote point out trophozoite stages the differ in the absence or presence of a flagellum and also in the place of the kinetoplast associated with the flagellum. A range of terms room employed for stages in the Apicomplexa, such as tachyzoite and bradyzoite because that Toxoplasma gondii. Other stages in the facility asexual and sexual life cycles viewed in this phylum space the merozoite (the form resulting from fission the a multinucleate schizont) and sexual stages such together gametocytes and gametes. Some protozoa kind cysts that contain one or more infective forms. Multiplication wake up in the cysts the some types so the excystation releases much more than one organism. For example, once the trophozoite of Entamoeba histolytica an initial forms a cyst, it has actually a solitary nucleus. As the cyst matures nuclear division produces four nuclei and also during excystation four uninucleate metacystic amebas appear. Similarly, a fresh encysted Giardia lamblia has the same number of internal frameworks (organelles) together the trophozoite. However, together the cyst matures the organelles twin and 2 trophozoites space formed. Cysts happen in stools have a safety wall, allowing the parasite to endure in the outside environment for a period ranging from days come a year, relying on the varieties and eco-friendly conditions. Cysts developed in tissues do not usually have a heavy protective wall and count upon carnivorism because that transmission. Oocysts are stages resulting from sexual reproduction in the Apicomplexa. Part apicomplexan oocysts room passed in the feces the the host, but the oocysts that Plasmodium, the certified dealer of malaria, build in the human body cavity the the mosquito vector.


Reproduction in the Protozoa might be asexual, as in the amebas and also flagellates that infect humans, or both asexual and sexual, as in the Apicomplexa of clinical importance. The most common type of asexual multiplication is binary fission, in i m sorry the organelles room duplicated and the protozoan climate divides right into two finish organisms. Department is longitudinal in the flagellates and also transverse in the ciliates; amebas have no apparent anterior-posterior axis. Endodyogeny is a kind of asexual department seen in Toxoplasma and also some related organisms. Two daughter cells form within the parent cell, which climate ruptures, releasing the smaller sized progeny which grow to complete size before repeating the process. In schizogony, a common kind of asexual division in the Apicomplexa, the cell nucleus divides a number of times, and then the cytoplasm divides into smaller uninucleate merozoites. In Plasmodium, Toxoplasma, and other apicomplexans, the sex-related cycle requires the manufacturing of gametes (gamogony), fertilization to type the zygote, encystation that the zygote to form an oocyst, and also the formation of infective sporozoites (sporogony) in ~ the oocyst.

Some protozoa have complicated life cycles requiring two various host species; rather require just a solitary host to finish the life cycle. A single infective protozoan entering a at risk host has the potential to create an enormous population. However, reproduction is restricted by events such as fatality of the host or by the host"s defense mechanisms, which may either remove the parasite or balance parasite reproduction to productivity a chronic infection. Because that example, malaria can result when only a couple of sporozoites that Plasmodium falciparum—perhaps ten or fewer in rare instances—are presented by a feeding Anopheles mosquito into a human being with no immunity. Repeated cycles of schizogony in the bloodstream can an outcome in the infection of 10 percent or an ext of the erythrocytes—about 400 million helminth per milliliter that blood.


The nutrition of all protozoa is holozoic; that is, they need organic materials, which might be particulate or in solution. Amebas engulf particulate food or droplets through a type of momentary mouth, perform digestion and absorption in a food vacuole, and also eject the rubbish substances. Countless protozoa have a permanent mouth, the cytosome or micropore, v which ingested food overcome to come to be enclosed in food vacuoles. Pinocytosis is a technique of eating nutrient materials through which fluid is attracted through small, momentary openings in the body wall. The ingested material becomes enclosed within a membrane to kind a food vacuole.

Protozoa have actually metabolic pathways similar to those of higher animals and also require the same species of organic and inorganic compounds. In current years, far-ranging advances have been do in devising chemically defined media for the in vitro growing of parasitic protozoa. The result organisms are complimentary of various substances the are present in biology grown in complex media or isolated from a host and also which can interfere v immunologic or biochemical studies. Research on the line of helminth is of prompt interest due to the fact that pathways the are important for the parasite yet not the host are potential targets because that antiprotozoal compounds that would block that pathway yet be safe for humans. Plenty of antiprotozoal medicine were used empirically long before their device of action was known. The sulfa drugs, i m sorry block folate synthetic in malaria parasites, are one example.

The quick multiplication price of plenty of parasites boosts the chances for mutation; hence, changes in virulence, drug susceptibility, and other characteristics may take place. Chloroquine resistance in Plasmodium falciparum and also arsenic resistance in Trypanosoma rhodesiense are two examples.

Competition for nutrient is not usually critical factor in pathogenesis because the quantities utilized by helminth protozoa are relatively small. Some parasites the inhabit the small intestine can substantially interfere v digestion and also absorption and influence the nutritional condition of the host; Giardia and Cryptosporidium space examples. The destruction of the host"s cells and tissues as a result of the parasites" metabolic tasks increases the host"s nutritional needs. This may be a significant factor in the outcome of an infection in a malnourished individual. Finally, extracellular or intracellular helminth that ruin cells while feeding have the right to lead to body organ dysfunction and serious or life-threatening consequences.

Englund PT, Sher A (eds): The biological of Parasitism. A Molecular and Immunological Approach. Alan R. Liss, new York, 1988.
Goldsmith R, Heyneman D (eds): tropic Medicine and Parasitology. Appleton and also Lange, eastern Norwalk, CT, 1989.
Lee JJ, Hutner SH, Bovee EC (eds): An shown Guide to the Protozoa. Culture of Protozoologists, Lawrence, KS, 1985.

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Neva FA, Brown H: simple Clinical Parasitology, sixth edition, Appleton & Lange, Norwalk, CT, 1994.