Practical Guide to Diagnostic Parasitology, Second Edition

Although common names are often used to describe parasites and parasitic infections, these names may refer to different parasites in different parts of the world. To eliminate these problems, a binomial system of nomenclature is used in which the scientific name consists of the genus and species. Based on life cycles and organism morphology, classification systems have been developed to indicate the relationship among the various parasite species. Closely related species are placed in the same genus, related genera are placed in the same family, related families are placed in the same order, related orders are placed in the same class, and related classes are placed in the same phylum, one of the major categories in the animal kingdom. Parasites of humans are classified into six major divisions. These include the Protozoa (amebae, flagellates, ciliates, sporozoans, coccidia, and microsporidia), the Nematoda or roundworms, the Platyhelminthes or flatworms (cestodes, trematodes), the Pentastomids or tongue worms, the Acanthocephala or thorny-headed worms, and the Arthropoda (insects, spiders, mites, ticks). This section talks about parasite classification and relevant body sites such as intestine, blood, tissue, liver and lungs.

This section focuses on ova and parasite examination of stool specimens using routine diagnostic methods and body sites. Diagnostic parasitology includes laboratory procedures that are designed to detect organisms within clinical specimens by using morphologic criteria and visual identification, rather than culture, biochemical tests, and/ or physical growth characteristics. The most common specimen submitted to the diagnostic laboratory is the stool specimen. The most commonly performed procedure in parasitology is the ova and parasite examination (O&P exam), which consists of three separate protocols: the direct wet mount, the concentration, and the permanent stained smear. Several other diagnostic techniques are available for the recovery and identification of parasitic organisms from the intestinal tract. A roundworm parasite that has worldwide distribution and is commonly found in children is Enterobius vermicularis, known as pinworm or seatworm. Diagnosis of pinworm infection is usually based on the recovery of typical eggs, which are described as thick-shelled, football-shaped eggs with one slightly flattened side. Most routine clinical laboratories do not have the animal care facilities necessary to provide animal inoculation capabilities for the diagnosis of parasitic infections. In certain parasitic infections, the standard diagnostic laboratory procedures may not be sufficient to confirm infection or specimen collection may not be practical or cost-effective. In these circumstances, alternative methods may be helpful; these include antibody, antigen, and nucleic acid detection. In the absence of reliable serologic diagnostic tests, skin tests have been used to provide indirect evidence of infection.

This section focuses on diagnostic parasitology. Specimen collection and processing procedures constitute the ova and parasite examination (O&P exam). The direct wet smear, concentration, and permanent stained smear constitute the routine O&P exam on fresh stool specimens. The section provides some of the immunoassay options available for stool protozoa. Currently, immunoassays are available for Giardia lamblia, Cryptosporidium spp., the Entamoeba histolytica/E. dispar group, and E. histolytica. Most effective techniques for the identification of the intestinal protozoa, the wet preparation smear and the permanent stained smear are recommended as the most relevant and accurate procedures for identification. Some helminth eggs are quite heavy and will not float, even when zinc sulfate with a specific gravity of 1.20 is used. The scientific (genus and species) names should be used on the final report that goes to the physician and on the patient’s chart. It is also recommended that the stage of the organism be included (trophozoite, cyst, oocyst, spore, egg, larvae, adult worm); various stages of the malaria parasites affect the outcome of therapy.

This section talks about the diagnostic characteristics for organisms. Characteristics (including size, motility, nucleus number and visibility) of intestinal protozoa such as amebae, flagellates, ciliates, coccidia characteristics of trophozoites, cysts of each species are given separately. Comparisons of various diagnostic techniques such as light microscopy, routine histology, electron microscopy, serologic antibody detection for Microsporidia identification are provided. Comparison of Naegleria fowleri, Acanthamoeba spp., Balamuthia mandrillaris, and Sappinia diploidea have also been provided in the section. Normal life spans and characteristics of the most common intestinal nematodes, tissue cestodes, liver and lung trematodes, human paragonimiasis, blood trematodes, are listed in the tables. Potential problems using EDTA anticoagulant for the preparation of thin and thick blood films are listed in detail. Features of human leishmanial infections and characteristics of American trypanosomiasis are also provided in a tabular form. Key characteristics of helminthes, blood parasites, rapid diagnostic procedures are listed separately in tables.