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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Introduction to Pathogenic Parasites: Pathogenesis and Chemotherapy of Parasitic Diseases

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Parasites: Diagnosis

Although parasitic diseases are not as common in the United States as elsewhere, they do occur and may, at times, be life threatening. In addition, the continuous arrival of travel-ers and immigrants from endemic areas necessitates consideration of these diseases in differential diagnoses.

DIAGNOSIS

Although parasitic diseases are not as common in the United States as elsewhere, they do occur and may, at times, be life threatening. In addition, the continuous arrival of travel-ers and immigrants from endemic areas necessitates consideration of these diseases in differential diagnoses. Unfortunately, the clinical manifestations of parasitic infections are seldom sufficiently characteristic to raise this possibility in the clinician’s mind. Moreover, routine laboratory tests are seldom of aid. Although eosinophilia has been rec-ognized as an important clue to the diagnosis of parasitic disease, this phenomenon is characteristic only of helminthic infection, and even in these cases it is frequently absent. Eosinophilia, which presumably reflects an immunologic response to the complex foreign proteins possessed by worms, is most marked during tissue migration. Once migration ceases, the eosinophilia may decrease or disappear entirely. Thus, the clinician must usu-ally rely on a detailed travel, food intake, transfusion, and socioeconomic history to raise the possibility of parasitic disease.

Once considered, diagnosis is usually straightforward. Typically, it rests on the demonstration and morphologic identification of the parasite or its progeny in the stool, urine, sputum, blood, or tissues of the human host.

In intestinal infections, a simple wet mount or stained smear, or both, of the stool is often adequate. Some parasites, however, are passed in the feces intermittently or in fluc-tuating numbers, and repeated specimens are needed. Ova of worms and cysts of protozoa may be concentrated by sedimentation or flotation techniques to increase their numbers for diagnosis. Occasionally, specimens other than stool must be examined. In the case of small bowel infections such as giardiasis and strongyloidiasis, aspirates of the duodenum or a small bowel biopsy may be required to establish the diagnosis. Similarly, the recov-ery of large bowel parasites such as E. histolytica and Schistosoma mansoni may require proctoscopy or sigmoidoscopy, with aspiration or biopsy of suspect lesions. Eggs of pin-worms (Enterobius) and tapeworms (Taenia) may be found on the perineal skin when they are absent from the stool.

Parasites dwelling within the tissue and blood of the host are more difficult to iden-tify. Direct examination of the blood is useful for the detection of malarial parasites, leishmania, trypanosomes, and filarial progeny (microfilariae). The concentration of or-ganisms in the bloodstream often fluctuates, however, requiring the collection of multiple specimens over several days. Both wet mount and stained preparations of thin and thick blood smears  are used. Lung flukes and occasionally other helminths discharge their offspring in the sputum and may be found there with appropriate concen-tration techniques. In others, larvae can be recovered with skin (onchocerciasis) or muscle (trichinosis) biopsy.

In some infections, parasite recovery is uncommon. Immunodiagnostic and nucleic acid hybridization techniques provide diagnostic alternatives for these situations. Al-though tests for circulating antibodies have long been available for a number of parasitic diseases, they have often lacked sensitivity and specificity. The replacement of crude, antigenically complex parasitic extracts with purified homologous antigens, together with the adaptation of highly reactive test systems, has significantly increased the sensitivity and specificity of such tests. Currently, reliable serologic procedures are available for amebiasis, cysticercosis, echinococciasis, paragonimiasis, schistosomiasis, strongyloidia-sis, toxocariasis, toxoplasmosis, and trichinosis. More will undoubtedly follow in the near future.

Techniques for the detection of parasitic antigens in blood, body fluids, tissues, and excreta also have been developed. Commercial immunofluorescent and immunosorbent kits for Pneumocystis carinii (pulmonary secretions), T. vaginalis (genitourinary fluids), and E. histolytica, Giardia, and Cryptosporidium (feces) are now commonly found in clinical laboratories. Less generally available are systems for the detection of malaria antigens in blood and T. gondii in tissue.

DNA probes are available for the detection of P. falciparum, T. cruzi, T. brucei, On-chocerca species, and the etiologic agents of lymphatic filariasis. The probes for P. fal-ciparum and lymphatic filariae have demonstrated sensitivities that match or exceed thoseof traditional techniques. The major limitations of DNA probes as diagnostic tools, relate to the technical aspects of the hybridization procedure which should soon be overcome.


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