What is parasitology?

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.