Protozoan diseases
Protozoan diseases are infections caused by protozoa, which are unicellular organisms that can act as parasites in humans and other animals. These diseases can affect various systems, particularly the gastrointestinal system, and their symptoms can range from mild gastrointestinal disturbances to severe conditions like anemia and skin lesions. Protozoa are incredibly diverse, with approximately ten thousand known species capable of parasitism, and they can replicate rapidly within their hosts, leading to significant health impacts.
Many protozoan diseases are prevalent globally, with millions infected by parasites such as Entamoeba (causing amebic dysentery) and Plasmodium (responsible for malaria, which claims millions of lives annually). Other notable protozoan diseases include giardiasis, Chagas disease, and toxoplasmosis, each caused by different protozoan species and transmitted through various routes, including contaminated water and insect bites.
Treatment for protozoan diseases typically involves specific drug therapies tailored to the type of infection. Common treatments include metronidazole for amebiasis and giardiasis, and pentavalent antimonials for leishmaniasis. Despite advancements in treatment, protozoan infections continue to pose significant health challenges, particularly in regions with limited access to healthcare. Understanding protozoan diseases is crucial for prevention and effective management, especially in vulnerable populations.
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Subject Terms
Protozoan diseases
ANATOMY OR SYSTEM AFFECTED: Gastrointestinal system
DEFINITION: Disease caused by protozoa, a diverse group of free-living, unicellular animals that function as parasites
CAUSES: Parasitic infection
SYMPTOMS: Wide ranging; may include chills, fever, sweating, anemia, spleen enlargement, gastrointestinal disorders, skin lesions
DURATION: Acute to chronic
TREATMENTS: Drug therapy, supportive therapy
Causes and Symptoms
Protozoa exist in almost every ecological niche. As parasites, they infect all species of vertebrates and many invertebrates, adapting to nearly all available sites within their hosts’ bodies. There are nearly sixty-six thousand known species of protozoa; half of these are represented in the fossil record, and about ten thousand of the living species are known to be parasitic. Unlike many other parasites, protozoa replicate within their hosts to produce hundreds of thousands of their kind within several days of infection. Because of their variety, adaptability, and reproductive rates, parasitic protozoa exert a major influence on human existence.
The phylum Protozoa is subdivided into four subphyla, but only the subphylum Sarcomastigophora is of real concern to humans. Within this subphylum are the twelve genera of parasitic protozoa that cause diseases in humans and their domestic animals, the most important of which are Trypanosoma, Leishmania, Trichomonas, Entamoeba, Eimeria, Toxoplasma, Babesia, Theileria, Giardia, and Plasmodium.
The most startling characteristic of parasitic protozoan diseases in humans is the sheer number of cases that exist. For example, a conservative estimate suggests at least two hundred million persons harbor the protozoa Entamoeba, which causes amebic dysentery. Of this number, some fifteen to twenty million suffer severe cases of amebic dysentery. One to two billion humans are estimated to be infected with Toxoplasma, the protozoa that cause toxoplasmosis. Trypanosoma, the agents of and Chagas disease, infect fifteen to twenty million people worldwide; Giardia, two hundred million; and Leishmania, which are responsible for the disease kala-azar, one to two million. The most important of all protozoan diseases, said to be the greatest killer in history, is malaria, which is caused by the Plasmodium and results in the death of two to three million people annually.
Protozoa that infect humans are commonly found in the intestinal tract, various tissues and organs, and the bloodstream. Of the many protozoa that reside in the human gut, only invasive Entamoeba histolytica causes serious disease. E. histolytica parasites are transmitted by the ingestion of water contaminated with human feces containing E. histolyticacysts and can result in the disease or, in its severe form, amebic dysentery. If the disease spreads by way of intestinal to the liver, the result can be amebiasis. Though E. histolytica can prove highly invasive, mostly it remains in the gut as nonpathogenic infections. Another waterborne intestinal protozoan familiar to campers and vacationers is the flagellate Giardia lamblia, which causes giardiasis. Giardiasis results in a mild-to-serious, long-lasting diarrhea. It is usually acquired from the ingestion of water fouled by animal waste. It can also be transmitted by human sources and, if the outbreak occurs in a closed population, frequently results in a rapidly spreading infection.
A number of flagellate parasites infect the human skin, bloodstream, and viscera. The flagellate Trypanosoma cruzi is the agent of Chagas disease, a major cause of debilitation and chronic heart disease among poorly housed populations of Central and South America. T. cruzi is transmitted when the liquid feces of an insect (genus Triatoma) are scratched into the skin or rubbed in the eye. In Africa, the tsetse fly carries the parasitic agents of the disease trypanosomiasis, also known as African sleeping sickness. The disease is generated by the flagellate parasites Trypanosoma brucei gambiense and T. brucei rhodesiense, which infect the bloodstream and can be fatal if they cross the blood-brain barrier. Parasitic flagellates of the Leishmania are transmitted by bloodsucking midges and sandflies and result in infection. Cutaneous is characterized by infected macrophages in the skin, resulting in long-lasting skin lesions. A similar condition, mucocutaneous leishmaniasis, common to the Amazon Basin, begins as skin ulcers and often progresses to the destruction of nasal mucosa, cartilage, and soft facial and pharyngeal tissues. Visceral leishmaniasis, or kala-azar, infects the spleen, liver, bone marrow, and lymph nodes.
Only one species of ciliate protozoa is parasitic in humans: Balantidium coli, a free-living, large protozoan covered with rows of cilia. It is found in the large intestine, where it can cause the ulcerative disease balantidiasis.
The group of protozoa known as sporozoans are all parasitic and include many deadly parasites of humans: Isopora, Sarcocystis,Cryptosporidium, and Toxoplasma. Toxoplasma gondii, the agent of toxoplasmosis, is of great medical concern. It is estimated to infect between 20 and 50 percent of the world’s population, and it can penetrate the and subsequently infect the of pregnant women lacking the proper antibodies. However, the most important sporozoans are the agents of malaria.
Malaria is a disease caused by a group of sporozoans in the genus Plasmodium that infect the human liver and red blood cells. Malaria is characterized by periodic chills, fever, and sweats, leading to anemia, enlargement of the spleen, and complications that result in death, especially among infants. The parasites are transmitted to humans by the bite of any of sixty species of infected Anopheles mosquitoes. The disease is found in all tropical and temperate regions, but control efforts have eliminated it from North America, Europe, and the northern Asian continent. Despite control efforts and effective medical treatments, is historically the greatest killer of all human infectious diseases, and it remains the single most dangerous threat to humankind from an infectious agent. More than one million children die of malaria in Africa yearly.
Treatment and Therapy
Treatment is generally by antiprotozoal agents, antibiotics, or both, with the specific agent depending on the type of protozoa. Amebiasis, giardiasis, and trichomoniasis can be treated with metronidazole, which functions as both an antibiotic and an antiprotozoal. Leishmaniasis is usually treated with pentavalent antimonials, which are compounds containing the metalloid antimony. Various treatments for malaria exist, including chloroquine and quinidine; the best course of treatment depends on the type of malaria and where it was contracted.
Bibliography
Behnke, Jerzy M., ed. Parasites: Immunity and Pathology; The Consequences of Parasitic Infection in Mammals. New York: Taylor & Francis, 1990.
Despommier, Dickson D., et al. Parasitic Diseases. 5th ed. New York: Apple Tree, 2006.
Dugdale, David C., III, Jatin M. Vyas, and David Zieve. “Amebiasis.” MedlinePlus, 10 Sept. 2022, medlineplus.gov/ency/article/000298.htm. Accessed 8 Apr. 2024.
Dugdale, David C., III, Jatin M. Vyas, and David Zieve. “Leishmaniasis.” MedlinePlus, 26 Aug. 2023, medlineplus.gov/ency/article/001386.htm. Accessed 8 Apr. 2024.
Engleberg, N. Cary, Victor DiRita, and Terence S. Dermody, eds. Schaechter’s Mechanisms of Microbial Disease. 5th ed. Philadelphia: Lippincott Williams & Wilkins, 2013.
Frank, Steven A. Immunology and Evolution of Infectious Disease. Princeton, N.J.: Princeton University Press, 2002.
Gutteridge, W. E., and G. H. Coombs. Biochemistry of Parasitic Protozoa. Baltimore: University Park Press, 1977.
Koo, Ingrid. “Protozoa and the Illnesses They Cause.” VeryWell Health, 14 Oct. 2022, www.verywellhealth.com/definition-of-protozoa-1958837. Accessed 7 Aug. 2023.
“Malaria.” MedlinePlus, 28 July 2016, medlineplus.gov/malaria.html. Accessed 7 Aug. 2023.
Reyes-Lopez, Magda, et al. "Hemoglobin Uptake and Utilization by Human Protozoan Parasites: A Review." Frontiers in Cellular and Infection Microbiology, vol. 13, 2023. DOI: 10.3389/fcimb.2023.1150054. Accessed 8 Apr. 2024.
Roberts, Larry S., John Janovy Jr., and Steve Nadler. Gerald D. Schmidt and Larry S. Roberts’ Foundations of Parasitology. 9th ed. New York: McGraw-Hill, 2013.
Vorvick, Linda J., Jatin M. Vyas, and David Zieve. “Giardia Infection.” MedlinePlus, 10 Mar. 2022, medlineplus.gov/ency/article/000288.htm. Accessed 8 Apr. 2024.