Protozoa Classification, Reproduction And Diseases Caused

Microscopic life forms are everywhere on Earth. These small forms of life are usually single-celled organisms: protozoa and bacteria. Though both protozoa and bacteria are single-celled organisms, bacteria do not have some of the internal structures (organelles) found in protozoa, such as nuclei and vacuoles, that mimic the organs of multicellular organisms.

With about 35,000 species, protozoa are remarkably diverse in form and activity. There are nearly as many kinds of protozoa as there are plants and animals in the visible world. Three main groupings of protozoa are based largely on how they move. Ciliates move by beating action of very small hair-like projections on the surface of their body. Flagellates have one or more long whip-like projections, called flagella. The beating motion of the flagella moves protozoa through water, much like the action of a fish's tail. Amoebae (Sarcodina) are flattened protozoa that move by bulging outward along their edges.1 Many protozoa are permanently attached to surfaces. Sporozoa are parasitic protozoa which generally obtain nutrients by absorbing organic molecules from the host organism. They often have very complicated life cycles.

Unlike other single-celled organisms such as algae and fungi, protozoa do not have rigid cell walls. The shapes of protozoa are maintained, instead, by an interior protein skeleton called the cytoskeleton. The cytoskeleton is a network of protein fibers that provide structural rigidity but also flexibility. The flexible cell surface allows protozoa to engulf and digest bacteria and smaller eukaryotic microorganisms. Like algae, protozoa reproduce by transverse (ciliates) or longitudinal (flagellates) division, although in some protozoa sexual reproduction also occurs. During this division process, called binary fission, the organism divides into two equal-sized daughter cells.6

Protozoa graze on other life forms by ingesting and killing bacteria or smaller eukaryotes. Others, such as the protozoa that cause malaria, live by parasitizing larger animals.


Parasitic Protozoa

Most protozoa are free-living (they have no hosts). Parasitic protozoa live in or upon animal or human in cell (cytozoic) or in tissue (histozoic) and may cause diseases. Examples of infections caused by protozoa are malaria and sleeping thickness. Pathogenic protozoa have several common features. For example, many protozoa have both a dormant (immotile) cyst stage that permits survival when enviromental conditions are hostile, and a motile, actively feeding and reproducing, vegetative (trophozoite) stage.4

Some protozoa cause disease only under certain conditions and in this case they are called opportunistic pathogens, as is the case with Herpetomonas, parasites of insects. A case of infection with this parasite was reported in a homosexual immunodeficiency virus (HIV)-positive man.13

Protozoa in the environment have been implicated as both potential hosts harboring pathogens and as agents enhancing pathogen survival and pathogenicity. Presence of the shiga toxin-encoding prophage in Escherichia coli is reported to enhance their survival in the food vacuoles of grazing Tetrahymena pyriformis. The passage of Salmonella enterica through and excretion from a soilborne Tetrahymena species is reported to convey increased survival of the organism.15

Following is the list of important parasitic protozoa:

  1. Intestinal flagellates
    1. Histomonas species are the most important parasitic protozoan diseases in poultry; turkeys, grouse and partridge develop severe disease and have mortality rate that can exceed 75%.
    2. Trichomonas species cause necrotic ulcers in pigeons, turkeys and chickens; they also occur as oral parasites on various hosts and tend to multiply in presence of pus-forming inflammation.11 Trichomonas vaginalis is the most important sexually transmitted protist, causes vaginitis in women and urethritis in men. It causes vaginitis when the protist adheres to the host epithelium and changes from a flagellated to an ameboid form. Nonpathogenic species occur in cecum of various domestic and wild animals without causing disease.
    3. Giardia infections cause chronic diarrheal disease in a wide number of hosts that does not respond to treatment with antibiotics. In dogs, diarrhea may begin 5 days after exposure to infection. The feces of infected dogs and cats are more than usually malodorous. Giardia infection in humans may be mild or may cause severe inflammatory disease (enteritis).11
    4. Opalina species inhabit intestines of frogs and toads. They are symbionts and do not harm the host. They are pretty large (1 mm) and can be seen with the naked eye.
  2. Hemoflagellates
    1. Leishmania are heteroxenous, meaning that they are able to colonize two hosts. They live in the phagocytes of the reticulo-endothelial system of mammals and in the intestinal tract of sandflies as well as some tick species. Canine leishmaniasis is a serious problem, and it is estimated that 2.5 million dogs are infected in the Mediterranean basin only.7
    2. Trypanosoma species are major parasites of animals in Africa and Americas; T. cruzi causes Chaga's disease in humans.
  3. Sporozoa
    1. Monocystis is a parasite of earthworms.
    2. Sarcocystis calchasi - induces central nervous signs, causative agent of pigeon protozoal encephalitis.
    3. Sarcocystis neurona -causes equine protozoal myeloencephalitis (EPM) and EPM-like illness in several other mammals, including domestic dogs and cats.
    4. Eimeria are intracellular parasites of host's alimentary canal; cause heavy losses to the poultry industry.
    5. Isospora are widespread parasites similar to Eimeria. They infect intestines (dogs), lungs, liver and spleen (birds).8
    6. Plasmodium species infect erythrocites of humans causing malaria. Among four species of Plasmodium that infect humans, P. falciparum is the most virulent species of malaria transmitted by female mosquitoes.
    7. Nosema species are highly destructive parasites of insects, including honey bees and bumble bees, causing bee sickness disease that destroys entires colonies.
    8. Babesia species are primarily of veterinary importance but human cases are also reported from North America and Europe. Signs of babesiosis, a tick-borne disease, vary in severity from silent infection to acute circulatory shock with anemia.
    9. Toxoplasma species are obligate intracellular parasites that are considered of the world's most successful pathogens. The parasite employs efficient propagation within both its primary (cats) and intermediate hosts, extensive mechanisms to evade and disarm host immunity, and an ability to form chronic lifelong infections (toxoplasmosis).
    10. Cryptosporidium species are parasites of medical and veterinary importance that cause gastroenteritis in a variety of vertebrate hosts. Cryptosporidiosis characteristically results in watery diarrhea that may sometimes be profuse and prolonged. Diarrhea and abdominal pain are generally the symptoms which cause patients to seek medical attention, leading to a laboratory diagnosis of cryptosporidiosis. Other clinical features include nausea, vomiting, and low-grade fever.9
  4. Rhizopoda
    1. Entamoeba species occur as commensal organisms in animal tissues and organs, while some are pathogenc. E. histolytica, E. invadence, E. rananrum and E. anatis cause lethal infection in human, reptiles, amphibians and birds respectively. Four species cause non-lethal mild dysentery.
  5. Ciliata are the most complex single-celled eukaryotes, with some genomes containing more than 20,000 genes. Despite this diversity, ciliates have one common and unique feature: they possess two types of nuclei, each with its own specific function.
    1. Balantidium coli inhabits the large intestine of pigs, monkeys and humans. The organism has also been reported in chimpanzees, new world monkeys, domestic and wild hawks and wild rats. Within the tissues, the B. coli propagates, produces ulcers and forms abscesses that may extend to the muscular layer. Invasion of the colonic tissue leads to necrosis of the epithelium. In the acute form of the disease patients frequently have bloody stools, associated with stomach pain, weight loss and dehydration.10
    2. Nyctotherus symbiont species have been isolated from amphibians, fishes, invertebrates, cockroaches, and reptiles (particularly tortoises and herbivorous lizards).
  6. Suctoria are complex protozoans having two phases in their life cycle: stalked and free-swimming. Instead of mouths, they have tentacles (hollow tubes to suck their nutrients iniside the cell). They are common symbionts in copepods. A small number of Suctoria have been found in insects. Most of them are harmless, but some may cause disease, especially when they colonize respiratory passages.12
  7. Amoebozoa are protozoa that use that use pseudopodia as organelles of motility, are unicell, rarely bicell or multicell. Within this group, there are several groups that can cause diseases among vertebrates:
    1. Entamoeba species. Humans are primary hosts of E. histiolytica, the third most common cause of parasite-induced deaths among humans.
    2. Entamoeba invadens is an important parasite of reptiles, causing death in snakes.
    3. Free-living amebae such as Balamythia and Acanthamoeba, free-living amebae that are found everywhere in the environment such as soil and freshwaters but are also found in a wide range of environments including dust particles in the air, bottled water, chlorinated pools, water taps and sink drains, flowerpots, aquariums, sewage, and brackish and marine waters; they cause skin, respiratory system, eye and nervous system infections. Acanthamoeba may enter the body via a break in the skin or inhalation of wind-blown cysts.


  1. Explore the world of Protozoa. O. Roger Anderson & Marvin Druger, editors.
  2. Invertebrates: protozoa to echinodermata. Ashok Verma
  3. Microbiology: Diversity, Disease, and the Environment. Abigail A. Salyers & Dixie D. Whitt.
  4. Lippincott's Illustrated Reviews: Microbiology. Richard A. Harvey, Pamela C. Champe, Bruce D. Fisher
  5. Killer germs: microbes and diseases that threaten humanity. Barry E. Zimmerman, David J. Zimmerman
  6. Handbook of water and wastewater microbiology. N. J. Horan
  7. A Historical Overview of the Classification, Evolution, and Dispersion of Leishmania Parasites and Sandflies. Mohammad Akhoundi,1,* Katrin Kuhls,2 Arnaud Cannet,3 Jan Votýpka,4,5 Pierre Marty,1,3 Pascal Delaunay,1,3 and Denis Sereno6,7 PLoS Negl Trop Dis. 2016 Mar; 10(3): e0004349.
  8. Nutrition and Feeding Strategies in Protozoa. Brenda Nisbet
  9. Cryptosporidium Pathogenicity and Virulence. Maha Bouzid,a,* Paul R. Hunter,corresponding authora,* Rachel M. Chalmers,b,* and Kevin M. Tylera Clin Microbiol Rev. 2013 Jan; 26(1): 115–134.
  10. Necrotizing lung infection caused by the protozoan Balantidium coli. Sat Sharma, MD FCCP1 and Godfrey Harding, MD FRCPC2 Can J Infect Disv.14(3); May-Jun 2003
  11. Georgis' Parasitology for Veterinarians. Dwight D. Bowman
  12. Insect Pathology. Yoshinori Tanada, Harry K. Kaya
  13. Isolation of a Protozoan Parasite Genetically Related to the Insect Trypanosomatid Herpetomonas samuelpessoai from a Human Immunodeficiency Virus-Positive Patient. J Clin Microbiol. 2008 Nov; 46(11): 3845–3847.
  14. Foundations of Wildlife Diseases. Richard G. Botzler, Richard N. Brown
  15. Bradley J. Hernlem, Subbarao V. Ravva, Chester Z. Sarreal Front Cell Infect Microbiol. 2014; 4: 57.



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