Diagnosis of parasitic diseases

Definition

Parasites, including helminths and ectoparasites, are organisms that depend on a host organism for their food source and survival. This relationship does not benefit the host and often leads to infection. Helminths of medical importance comprise roundworms, tapeworms, and flukes. Ectoparasites are arthropods that infest the skin of humans, from which they derive sustenance. The most significant ectoparasites are mites, lice, and fleas.

Helminths

Diagnosis of parasitic infections begins with a clinical evaluation of symptoms presented by the affected person; epidemiology (such as the geographical region in which the person lives or has traveled and his or her exposure to contaminated food and water) is considered too. Most helminths infect the intestinal tract, so detection and differentiation is usually accomplished by preparing smears of fecal samples and examining them under a microscope.

Microscopic examinations are unsatisfactory under conditions of low parasitic infection, if the person does 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 immunoabsorbent 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 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 United States. Diagnosis is accomplished by patting a sticky tape in the anal folds of the child and examining for ova.

Strongyloidiasis (Strongyloides stercoralis), or 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 (it 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 and then not washing one’s hands properly (or at all). Eggs can be found in 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 (T. solium), or fish (Diphyllobothrium latum) tapeworms. Tapeworms, which attach to the intestinal wall, 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 to 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, which are detected by ultrasound scans or computerized tomography.

Flukes (trematodes) can be significant parasites in many world regions, but they are not common in the United States. Schistosomiasis is a major debilitating disease caused primarily by three species of Schistosoma. The parasite enters the skin of persons 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 significant in Asia. Eggs are found in the feces or the 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 its eggs. To diagnose scabies, a clinician takes scrapings of burrows and then examines them microscopically for the presence of mites or their eggs. Burrows appear as dark wavy lines in the epidermis, but they can be difficult to find because 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. h. 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, such as bed bugs, mosquitoes, spiders, bees, ticks, and ants, do not invade the body and are, therefore, not considered here. Insects can, however, act as vectors that transmit protozoan parasites.

Impact

Parasitic infections are generally much less of a problem in the developed world than in tropical and subtropical areas. However, travelers to developing nations should be fully aware of the possibility of contracting a parasitic disease and should take the necessary precautions. The importation of foods from endemic regions represents a potential source of infection, particularly for produce that is eaten raw.

Many organizations around the world are involved in programs to reduce the incidence of parasitic infections in developing countries. These organizations are working in diagnostic research and applications.

Bibliography

Fritsche, Thomas, and Rangaraj Selvarangan. “Medical Parasitology.” In Henry’s Clinical Diagnosis and Management by Laboratory Methods, edited by Richard A. McPherson, Matthew R. Pincus, and John B. Henry. 22d ed. Philadelphia: Saunders/Elsevier, 2011.

Dabrowska, Julia, et al. "Effective Laboratory Diagnosis of Parasitic Infections of the Gastrointestinal Tract: Where, When, How, and What Should We Look For?" Diagnosis, vol. 14, no. 19, 27 Sept. 2024, p. 2148, doi: 10.3390/diagnostics14192148. Accessed 2 Feb. 2025.

Garcia, Lynne Shore. Diagnostic Medical Parasitology. 5th ed. Washington, D.C.: ASM Press, 2007.

Weller, P. F., and T. B. Nutman. “Intestinal Nematodes.” In Harrison’s Principles of Internal Medicine, edited by Anthony Fauci et al. 17th ed. New York: McGraw-Hill, 2008.