What is parasitology?

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Medical parasitology is the study of parasites and the human diseases caused by these organisms. By definition, parasites are dependent upon their hosts for survival. Numerous species of organisms, ranging from unicellular protozoa to large, physiologically complex helminths, parasitize human hosts. Additionally, several species of arthropods act as parasitic disease vectors. Parasites remain a significant cause of global morbidity and mortality.
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Definition

Medical parasitology is the study of parasites and the human diseases caused by these organisms. By definition, parasites are dependent upon their hosts for survival. Numerous species of organisms, ranging from unicellular protozoa to large, physiologically complex helminths, parasitize human hosts. Additionally, several species of arthropods act as parasitic disease vectors. Parasites remain a significant cause of global morbidity and mortality.

Intestinal Protozoa

Intestinal protozoa are single-celled parasites that commonly infect humans through a person’s ingestion of fecal-contaminated water or food. Entamoeba histolytica causes amebiasis, also known as amebic dysentery, characterized by abdominal pain and tenderness, bloody diarrhea, and fever. Giardia intestinalis (also known as G. lamblia) is the most commonly diagnosed intestinal parasite in the United States. Symptoms of giardiasis include abdominal pain, diarrhea, increased flatulence, and steatorrhea. Cryptosporidiumparvum, C. hominis, and other species of this sporozoan are common intestinal parasites found throughout the world. Balantidium coli, a ciliated protozoan, is found predominantly in tropical and subtropical climates.

Blood and Visceral Protozoa

Diseases caused by blood and visceral protozoa are among the most debilitating and potentially life-threatening of all parasitic infections, including malaria, trypanosomiasis, and leishmaniasis. Malaria, caused by Plasmodium falciparum, P. vivax, P. ovale, P. malariae, and P. knowlesi, is transmitted by the female Anopheles mosquito. The parasite initially infects hepatocytes before spreading to the erythrocytes. Classic symptoms include fever, chills, sweats, head and body aches, nausea, vomiting, and malaise. Malaria is the fifth leading cause of infectious-disease-related deaths worldwide.

Trypanosomes are flagellated protozoans carried by arthropod vectors. African trypanosomiasis (African sleeping sickness), caused by Trypanosoma brucei gambiense and T. b. rhodesiense, is carried by the tsetse fly. The parasite migrates from the circulatory system to the central nervous system. American trypanosomiasis (Chagas’ disease) is caused by T. cruzi and is carried by triatomine bugs. Chagas’ disease occurs commonly in Central America, South America, and Mexico. Acute signs and symptoms include fever, lymphadenopathy, and hepatosplenomegaly. Potentially fatal complications include meningoencephalitis and myocarditis.

Leishmaniasis describes a group of diseases transmitted by sandflies and caused by Leishmania species, including L. aethiopica, L. amazonensis, L. donovani, L. infantum, L. major, L. mexicana, L. tropica, and L. venezuelensis. With cutaneous leishmaniasis, the parasites infect tissue macrophages, typically leading to localized, painless ulcers. Visceral leishmaniasis, also known as kala-azar, involves diffuse parasitic invasion of the macrophages of the liver, spleen, and other organs. Symptoms and signs include hepatosplenomegaly, malaise, anorexia, and weight loss. Leishmaniasis is most prevalent in India, the Middle East, and parts of South America.

Intestinal Helminths

Intestinal helminths are complex organisms that include numerous species of nematodes (roundworms), trematodes (flukes), and cestodes (tapeworms). The World Health Organization estimates that approximately two billion people worldwide are infected with one or more soil-transmitted nematodes, including common roundworms (Ascaris lumbricoides), whipworms (Trichuris trichiura), and hookworms (Necator americanus and Ancylostoma duodenale). Other prevalent intestinal nematodes include Enterobius vermicularis and Strongyloides stercoralis. Fasciolopsis buski and Heterophyes heterophyes are flukes that parasitize the small bowel.

Intestinal tapeworms are typically contracted from the inadvertent ingestion of larvae in raw or undercooked animal meats. The larvae mature into adult worms in the host’s gastrointestinal tract. Human intestinal cestodes include Diphyllobothrium latum (fish tapeworm), Taenia saginata (beef tapeworm), T. solium (pork tapeworm), and Hymenolepis nana (dwarf tapeworm).

Blood and Visceral Helminths

Parasitic blood and tissue nematodes and cestodes cause a variety of human infections, which range in severity from mild to potentially life-threatening. Rare or undercooked meats infested with Trichinella larvae cause trichinellosis. Worms mature in the intestine and give rise to larvae, which invade systemically. Larvae-contaminated water is the source of guinea worm (Dracunculus medinensis) infections. Adult female worms migrate to the subcutaneous tissues and burrow through the skin surface to discharge larvae. Onchocerca volvulus, Wuchereria bancrofti, Brugia malayi, B. timori, Loa loa, Mansonella ozzardi, and M. perstans are parasitic blood and tissue nematodes transmitted to humans by arthropod vectors common in tropical and subtropical regions.

Snails are the primary intermediate hosts of several blood and tissue flukes that parasitize humans. The most common human blood flukes include Schistosoma mansoni, S. japonicum, and S. haematobium. Adult worms reside in the venules of the mesenteric, vesical, and pelvic plexuses. Clonorchis sinensis, Opisthorchis viverrini, O. felineus, Fasciola hepatica, and F. gigantic are parasitic liver flukes, or flatworms. Potential complications of liver fluke infestation include bile duct obstruction, biliary cirrhosis, cholangitis, cholangiohepatitis, and cholangiocarcinoma.

Lung flukes, including Paragonimus westermani, P. kellicotti, P. mexicanus, and P. africanus, are contracted from ingesting raw or undercooked fresh-water crustaceans contaminated with metacercariae. The parasites migrate from the intestine to the lungs and mature, potentially causing chronic bronchitis and bronchiectasis. Adult worms may migrate to other body tissues, including the central nervous system.

Invasive larvae from certain tapeworm species cause parasitic cysts in human tissues. Ingestion of T. solium eggs causes cysticercosis, characterized by larval cyst formation in various body tissues. Neurocysticercosis may prove life-threatening. Echinococcosis, caused by Echinococcus granulosus, E. vogeli, and E. multilocularis,most commonly affects the liver or lungs. Less commonly, the brain, bones or heart may be involved.

Impact

Although far from eradicated, parasitic diseases cause significantly less morbidity and mortality in wealthy, industrialized nations compared with the developing world. On a global scale, however, parasites continue to exact a significant toll on humankind, adversely affecting quality of life, productivity, and longevity.

Bibliography

Bogitsh, Burton J., Clint Earl Carter, and Thomas N. Oeltmann. Human Parasitology. 3d ed. Boston: Academic Press/Elsevier, 2005. A medical parasitology text with detailed information on the biology, epidemiology, and pathophysiology of human parasites, including protozoans, helminths, and arthropod vectors.

Despommier, Dickson D., et al. Parasitic Diseases. 5th ed. New York: Apple Tree, 2005. Provides a list of parasitic diseases of special concern to public health professionals. Also describes the assessment of and treatment options for a variety of these diseases.

Garcia, Lynne Shore. Diagnostic Medical Parasitology. 5th ed. Washington, D.C.: ASM Press, 2007. A good reference source on the diagnostic aspects of parasitology.

Gillespie, Stephen H., and Richard D. Pearson, eds. Principles and Practice of Clinical Parasitology. New York: John Wiley & Sons, 2001. Reference text that discusses the epidemiology, pathophysiology, and management of the most prevalent human parasitic infections.

Sheorey, Harsha, John Walker, and Beverley-Ann Biggs. Clinical Parasitology. Carlton South, Vic.: Melbourne University Press, 2000. Reviews global parasitic diseases and includes information regarding classification and geographical distribution of parasites, details of diagnostic tests, availability and treatment regimens of drugs, and means of obtaining uncommon drugs.

World Health Organization. Expert Committee on the Control of Schistosomiasis. Prevention and Control of Schistosomiasis and Soil-Transmitted Helminthiasis. Geneva: Author, 2001. Technical report from the World Health Organization reviews the epidemiology, burden, and management of soil-transmitted parasitic diseases.

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