Diagnosis of protozoan diseases

Definition

Protozoa are members of an informal grouping of simple, usually unicellular, heterotrophic phyla that share similar characteristics. Some protozoa are pathogenic.

Diagnostic Tools

Diagnosis of protozoan 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 their exposure to contaminated food and water) is considered too. Definitive diagnosis has traditionally been accomplished by microscopically detecting the protozoa or their eggs in stool, blood, urine, bone marrow, cerebrospinal fluid, or tissue samples.

Immunodiagnostic (serologic) tests can be valuable in those situations in which insufficient protozoa or eggs are available for detection. Similar to other infecting organisms, protozoa have unique proteins known as antigens on or in the organism. These antigens stimulate the body to produce specialized antibodies to react against the parasite. Serologic tests have been developed to detect either antigens or antibodies. These tests include the enzyme-linked immunoabsorbent assay (ELISA), hemagglutination, immunofluorescence, immunoblot, and rapid diagnostic tests.

Molecular methods use nucleic-acid-based technologies to diagnose protozoan 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.

Intestinal Protozoans

In the United States, the most common protozoan parasites that infect the intestinal tract include Giardia lamblia (giardiasis) and Cryptosporidium parvum (cryptosporidiosis). Cyclospora cayetanensis, which causes cyclosporiasis, is related to Cryptosporidium. Significant infectious intestinal protozoa worldwide include Isospora belli (isosporiasis), Cyclospora species, Enterocytozoon bieneusi (microsporidosis), and Entamoeba histolytica (amebiasis).

Diagnosis of intestinal protozoans is definitive with the microscopic detection of the organism in fecal contents and its differentiation from other species. Wet mounts from watery or loose stools that are prepared to detect the motile stage of the parasite (known as trophozoites) are particularly important in detecting amebiasis. Permanent mounts prepared with various stains are used to detect the cyst, oocyst, or spore stages of protozoa species.

Protozoa Infecting Blood and Tissue

Malaria is the most significant parasitic disease in humans. It is caused principally by species of the genus PlasmodiumP. falciparum, P. vivax, P. malariae, P. ovale, and P. knowlesi. The disease is endemic to most tropical regions. Malaria is transmitted by the bite of infected female Anopheles mosquitoes. The asexual part of the malaria life cycle affects humans and is known as schizogony. The parasite first enters the liver and then the red blood cells.

For diagnosis, a clinician draws blood from the affected person; slides are then prepared for microscopic examination. Diagnosis is confirmed by the presence of parasitized erythrocytes. Newer tests known as rapid antigen detection tests have been applied in many laboratories to avoid misdiagnosis of malaria. This test involves applying a blood sample to a nitrocellulose strip containing antibodies. If positive, the resultant antigen-antibody can be visualized.

Babesiosis caused by Babesia species also infects red blood cells. Babesia resembles malarial parasites morphologically, so a differential diagnosis is necessary upon examining blood samples. This increasingly important parasite is found primarily in the northeastern United States and is spread by ticks. The initial symptoms of babesiosis are quite similar to bacterial Lyme disease, which also is contracted from ticks in the same geographical region. Usually, blood or cerebrospinal fluid tests are performed to diagnose Lyme disease, using an FDA-cleared serologic two-test process.

Trichomoniasis, caused by Trichomonas vaginalis, is considered to be the most common parasitic infection in the United States. It is a sexually transmitted disease that affects the genitourinary tract. The usual diagnosis is to examine slides of vaginal secretion (women) or urethral secretion (men) under a dark field, phase-contrast, or ordinary light microscope. The parasite, if present, will exhibit rapid motility and flagella.

Toxoplasmosis, caused by Toxoplasma gondii, is also an important parasitic infection in the United States. Transmission usually takes place by oral ingestion of food or soil contaminated by the feces of infected cats. From the intestinal tract, the parasite invades a variety of tissues. This invasion stimulates a strong immune response from the host person and serves as a diagnostic tool. Diagnosis usually involves a blood test to detect IgM, IgG, IgA, or IgE antibodies. PCR tests or imaging scans may also be used.

Trypanosomiasis consists of two separate diseases depending on the region where they occur. In South America, Chagas disease is caused by Trypanosoma cruzi, while in Africa, African sleeping sickness is caused by T. brucei. Diagnosis depends upon identifying the organism. Blood samples are easy to obtain, but if the protozoan is not found under observation, the parasite can be concentrated by centrifugation before microscopic examination. The parasite is more concentrated in lymph node fluid, and T. brucei is found in cerebrospinal fluid during the latter stages of the disease. A lumbar puncture may also be performed to diagnose or stage the protozoan disease using cerebrospinal fluid.

Leishmaniasis is a collective term for the many diseases caused by species of the genus Leishmania. Visceral leishmaniasis (kala-azar) is caused by L. donovani and occurs over a wide geographical area. The parasite invades the blood and becomes established in the spleen, liver, bone marrow, and lymph nodes. Diagnosis is established by finding the parasite in biopsies of those tissues or in cultures from those tissues or from blood. Cutaneous leishmaniasis (oriental sore) is caused by many species and results in skin lesions. The parasite can be found in aspirates, smears, or dermal scrapings of the ulcer. Mucosal leishmaniasis is caused by species of the Viannia subgenus. The parasite causes lesions in the nasal and pharyngeal mucosal lining. Diagnosis is made by examining biopsy material or culture of aspirated material from the lesion.

Impact

Protozoan infections are generally much less of a problem in the developed world than in tropical or subtropical areas. However, travelers to developing countries should be fully aware of the possibility of contracting parasitic diseases and should take the necessary precautions.

Bibliography

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Fauci, Anthony, et al., editors. Harrison’s Principles of Internal Medicine. 21st ed., McGraw-Hill, 2022.

Fitri, Loeki Enggar, et al. “Diagnostic Methods of Common Intestinal Protozoa: Current and Future Immunological and Molecular Methods.” Tropical Medicine and Infectious Disease, vol. 7, no. 10, 2022, p. 253, doi.org/10.3390/tropicalmed7100253. Accessed 2 Nov. 2024.

Fritsche, Thomas, and Rangaraj Selvarangan. “Medical Parasitology.” Henry’s Clinical Diagnosis and Management by Laboratory Methods, edited by Richard A. McPherson, et al., 24th ed., Saunders/Elsevier, 2022.

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Parker, Steve. Protozoans, Algae, and other Protists. Compass Point Books, 2009.