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Essay title Diagnosis: Parasitic diseases
Category: Diagnosis
Also known as: N/A
Definition
For the purpose of this essay, parasites consist of helminths and ectoparasites. Helminths of medical importance consist of roundworms, tapeworms, and flukes. Ectoparasites are arthropods that infest the skin of humans from which they derive sustenance. The most important ectoparasites are mites, lice, and fleas. Protozoa, which also can be parasitic, are discussed in a separate essay.
Helminths
Diagnosis of parasitic infections begins with a clinical evaluation of symptoms presented by the patient together with epidemiological considerations (geographical region in which the patient lives or has traveled in, and his or her exposure to contaminated food and water). Most helminths infect the intestinal tract, so detection and differentiation is usually accomplished by preparing smears of fecal samples, and examining under a microscope.
Microscopic examinations are unsatisfactory under conditions of low parasitic infection, if the patients do not exhibit symptoms, or if the parasite cannot be identified. In these instances, serology tests are more sensitive and specific. These tests include enzyme-linked immunosorbent assay (ELISA), hemagglutination test, and immunoblot. Newer molecular methods use nucleic-acid based technologies to diagnose parasitic infections. The polymerase chain reaction (PCR) is the primary technology used. The primary advantages of molecular methods are speed and sensitivity. Although much promising research has been done, the methods have not yet progressed to the stage of routine clinical use.
Roundworm (Nematode) infections can be confined to the intestines, or can invade other tissues, depending on the species. Pinworms (Enterobius vermicularis) commonly infect children in the U.S. Diagnosis is accomplished by patting a sticky tape in the anal folds of the child and examining for ova. Strongyloidiasis (Strongyloides stercoralis) (threadworm) infection occurs when feces contaminated with larva come in contact with and penetrate the skin. Examination of feces of infected persons for larva is a confirmed diagnosis. Larva migrans are diseases in which larva of various nematode parasites normally infecting dogs or cats migrate in human tissues as an unnatural host. Cutaneous larva migrans is caused by a hookworm that produces a winding, threadlike trail of inflammation in the epidermis. Discovery of larva in a skin biopsy confirms diagnosis. Visceral larva migrans is caused by ingestion of soil or food contaminated with Toxocara roundworm ova. The disease can be confirmed by liver biopsy or serologic tests. Larva migrans is usually self-limiting (goes away on its own).
. Trichinosis (Trichinella spiralis) is caused by eating inadequately cooked pork. Infection can cause a painful burrowing of larvae in muscle tissue. Larvae or cysts found in a muscle biopsy is a confirmed diagnosis. Large roundworm (Ascaris lumbricoides) infection may result after handling infected pets or soil with improper hand washing. Eggs can be found in the feces.
Tapeworms (Cestodes) can infect the intestines or other parts of the body. Humans can be infected by consuming raw or undercooked meats from animals infected with beef (Taenia saginata), pork (Taenia solium), or fish (Diphyllobothrium latum) tapeworms. Tapeworms attach to the intestinal wall, and contain a chain of segments (proglottids) that grow and mature. The segments break off and become part of the stool. Examining segments obtained from the stool allows differentiation between the three tapeworm species. Infection with Echinococcus granulosis tapeworm is largely limited shepherds and their families, with dogs or other canines acting as intermediates. The infection can be diagnosed by the presence of cysts in the liver detected by ultrasound scans or computerized tomography.
Flukes (Trematodes) can be important parasites in many world regions, but are not common in the U.S. Schistosomiasis is a major debilitating disease caused primarily by three species of Schistosoma. The parasite enters the skin of people who drink or bathe in polluted water and migrates to the liver and other internal organs. Eggs are found in the stool or urine. Clonorchiasis is a liver fluke particularly important in Asia. eggs are found in the feces or duodenal contents.
Ectoparasites
Scabies is caused by the itch mite Sarcoptes scabiei. The mite burrows in the external layer of skin (stratum corneum) to deposit their eggs. In order to diagnose scabies, scrapings of burrows should be taken and examined microscopically for the presence of mites or their eggs. Burrows appear as dark wavy lines in the epidermis, but can be difficult to find since they may be obscured by secondary lesions.
Lice infestations (pediculosis) feed on human blood, and can be caused by three species. Pediculus humanus capitis affects the head, P. humanis corporis affects the body, and Phthirus pubis affects the genital area. Diagnosis of head lice is confirmed by examining the scalp with a magnifying glass. Ova (nits) are found fixed to the hair shafts. Body lice are most readily found in clothing worn next to the skin. Pubic lice are difficult to find, but may be present as brown spots on the undergarments. Tunga penetrans (sand flea) is an important parasite of tropical regions.
Insects that cause bites and stings are not considered parasites in this essay as they do not invade the body. Examples include bed bugs, mosquitoes, spiders, bees, ticks, and ants. Insects can act as vectors transmitting protozoan parasites
Impact
Parasitic infections are generally much less of a problem in the developed world than in tropical or subtropical areas. However, travelers to the underdeveloped world must be fully aware of the possibility of contracting parasitic diseases, and take the necessary precautions. Importation of foods from regions endemic to parasites represents potential sources of infection, particularly for produce eaten raw. Many organizations are involved in programs to reduce the incidence of parasitic infections in underdeveloped countries through diagnostic research and its applications.
David A. Olle, M.S.
Further Reading
Fritsche, Thomas and Rangaraj Selvarangan. “Medical Parasitology” In Henry’s Clinical Diagnosis and Management by Laboratory Methods, edited by Richard McPherson and Matthew Pincus. 21st ed. Philadelphia: W.B. Saunders Co., 2007. A detailed discussion of parasitic infections, including illustrations of parasites’ life cycles and characteristics.
Merck Manual Home Health Handbook, edited by Robert Porter, Justin Kaplan and Barbara Homeier. 3rd ed. Warehouse Station, NJ: Merck Research Laboratories, 2009. This is the layman’s version of the professional Merck Manual. Provides a concise discussion of the essential features of protozoan diseases.
Harrison’s Principles of Internal Medicine, edited by Eugene Braunwald, Anthony Fauci, Dennis Kasper, et al. 15th ed. New York: McGraw-Hill, 2001. Provides a summary table and extensive discussions of all significant protozoan diseases.
Web Sites of Interest
Centers for Disease Control and Prevention. “Parasites” http://www.cdc.gov/parasites/ accessed February 2011