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Chapter 19 : Parasitic Infections in the Compromised Host
This chapter discusses some of the representative opportunistic organisms that can cause disease in immunocompromised patients. Any parasitic infection in the immunocompromised host may cause more severe symptoms; however, the organisms presented in the chapter have been identified as causing severe disease in this population. Parasitic organisms causing infection include Entamoeba histolytica, Giardia lamblia and Toxoplasma gondii. Macrophages or monocytes can look like the trophozoite form of E. histolytica, and polymorphonuclear leukocytes, when they have been in the stool for a while, can mimic the four-nucleus E. histolytica mature cyst. When organisms invade the mucosal lining and are carried via the bloodstream to the liver, a somewhat different approach to diagnosis is necessary. Invasive amebiasis appears to be an emerging parasitic disease in patients infected with HIV in areas where amebic infection is endemic. Infections caused by small, free-living amebae are becoming recognized clinically as important parasitic pathogens, particularly in immunocompromised patients. Amebic meningoencephalitis caused by Naegleria fowleri is an acute, suppurative infection of the brain and meninges. The cerebrospinal fluid (CSF) may have the predominantly polymorphonuclear leukocytosis, increased protein concentration, and decreased glucose concentration that are seen with bacterial meningitis. In contrast to E. histolytica, G. lamblia inhabits the duodenal area of the intestine, tends to adhere very tightly to the mucosa, and can be very difficult to recover, even after a series of five or six stool examinations. For this reason, other techniques, such as the Entero-Test string capsule, duodenal aspirate, or biopsy, may have to be used.
(Upper left) Entamoeba histolytica trophozoite; (upper right) Entamoeba histolytica/E. dispar cyst; (lower) gross specimen, amebic liver abscess.
(Top) Naegleria fowleri trophozoite (note the large karyosome within the nucleus); (middle) Naegleria fowleri trophozoites within brain tissue; (bottom) Balamuthia mandrillaris in brain tissue. (Bottom, Armed Forces Institute of Pathology photograph.)
(Left) Acanthamoeba trophozoites (note the sharp, spiky pseudopodia); (right) Acanthamoeba cysts (note the hexagonal double wall).
(Upper left) Giardia lamblia trophozoite (note two nuclei, curved median bodies, and linear axonemes); (upper right) Giardia lamblia cyst; (lower) Giardia lamblia cyst (large) and Cryptosporidium oocysts (small) demonstrating fluorescence in the fecal FA immunoassay (note that the background demonstrates use of the counterstain).
Toxoplasma gondii in bone marrow.
Cryptosporidium oocysts stained using the modified acid-fast stain (note the spherical shape; oocysts measure 4 to 6 μm).
Cyclospora cayetanensis oocysts stained using the modified acid-fast stain (note the spherical shape; oocysts measure 8 to 10 μm; some oocysts do not stain, thus the organisms are said to be “modified acid-fast variable”).
Isospora (Cystoisospora) belli. (Upper) Immature oocyst (contains single sporocyst) stained using modified acid-fast stain; (lower) more mature oocyst (contains two sporocysts) in a saline wet mount of stool concentration sediment.
Sarcocystis sp. in muscle tissue (note the bradyzoites contained within the sarcocyst).
Microsporidia. (Top) Microsporidial spores seen in fecal specimen (concentration sediment) stained with Ryan modified trichrome stain (note the horizontal line through some of the spores, representing the presence of the polar tubule); (middle) spores stained with specific FA reagents for the detection of Encephalitozoon spp.; (bottom) spores within a urine sediment after staining with Ryan modified trichrome stain (taken at a lower magnification).
Leishmania donovani in bone marrow (note the individual amastigotes, each one containing the bar and nucleus); specimen stained using Giemsa stain.
Strongyloides stercoralis. (Upper) Rhabditiform larvae seen in bronchoalveolar lavage fluid specimen (larvae can also be seen in sputum in heavy infections or in the hyperinfection syndrome); (lower) rhabditiform larva from fecal concentration sediment (note the short mouth opening/buccal capsule and the packet of genital primordial cells at the bottom left).
Sarcoptes scabiei “itch mite.” (Left) Mite from skin scraping preparation (note the four pairs of legs); (right) hand of an individual with severe scabies (Norwegian scabies). (Right, Armed Forces Institute of Pathology photograph.)
Host defense mechanisms
Selected procedures for determination of host defense defects a
Diagnosing parasitic infection in the compromised host
Parasitic infections: clinical findings in normal and compromised hosts
Acanthamoeba infections in immunocompromised patients
Toxoplasma gondii infections in immunocompromised patients
Cryptosporidium infections in immunocompromised patients
Prevention of cryptosporidiosis in immunocompromised patients a
Cyclospora cayetanensis: updated information a
Isospora (Cystoisospora) belli: parasite development and disease
Encysted pathogenic protozoan parasites seen in human feces
Sarcocystis: parasite development and disease a
Microsporidia: updated information (AIDS patients)
Leishmania infections in immunocompromised patients
Strongyloides stercoralis infections in immunocompromised patients