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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