Page 114 - AIDSBK23C

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GASTROINTESTINAL PROTOZOAL INFECTIONS

These infections occur from such organisms as Entamoeba histolytica, Entamoeba coli,
Giardia lamblia, Cryptosporidium sp, Microsporidium sp, and Isospora belli. Only
Cryptosporidium and Isospora are part of definitional criteria for AIDS, though one or more of
these agents may be identified in the GI tract by stool examination at some point in the course of
AIDS.[392] Cryptosporidium is far more frequently identified than Microsporidium or Isospora,
at least in developed nations, while the others are more sporadic in occurrence. In the U.S. less
than 5% of HIV infected persons develop cryptosporidiosis, with an increased risk for infection
when the CD4 lymphocyte count is less than 100/µL.[485] Appearance of these protozoa may
explain clinically significant diarrheas, though patients with such organisms diagnosed may be
asymptomatic. Cryptosporidium and Isospora are more common in patients in developing
nations than in the U.S.[401,486,487] Acid fast staining is useful for identification of
Cryptosporidium and Microsporidium in stool specimens.[488]
Cryptosporidiosis in immunocompromised hosts can be the cause for diarrhea that is
refractory to therapy. The species that most often infects humans has been designated
Cryptosporidium parvum. There are no specific gross pathologic features and it is usually
diagnosed from stool specimen examination. Cryptosporidial infection is usually
unaccompanied by inflammation, hemorrhage, or ulceration. After ingesting infective oocysts,
there is asexual multiplication of the organisms in host intestinal epithelial cells within a vacuole
so that the organisms are intracellular but extra cytoplasmic located on the brush border.
Gametogeny follows next, leading to production of oocysts that are either thin-walled and auto
infective or thick-walled and passed in feces to become infective to others. More thin-walled
oocysts are present in immunocompromised hosts, leading to the persistence with greater
severity of the disease. The incubation period is 2 to 14 days.[401,486]
The cryptosporidia appear histologically as quite small 2 micron uniform rounded shapes;
they develop outside of human cells but within a vacuole derived from the host cell. Thus, in
tissue sections stained with hematoxylin-eosin, these organisms are small pale blue dots found
lined along the mucosal brush border of the intestine. They can be highlighted with acid fast
staining. Unfortunately, they may also resemble tissue fragments or karyorrhectic nuclei in
tissue biopsies, so care must be taken in diagnosis. They are recognizable as 4-6 micron oocysts
that are most distinctive from background in stool specimens with an acid-fast stain.
Cryptosporidia are more easily recovered from diarrheal stools than from formed stools.[401]
Cryptosporidia may rarely be found outside of the GI tract in the biliary tree or respiratory
tract.[417] An immunohistochemical stain may aid in detecting them.[489]
Since the major route for infection with Cryptosporidium is through fecal-oral
contamination and through contaminated water, HIV-infected persons should avoid drinking
untreated water, avoid contact with either human or animal feces, and wash hands after contact
with pets, soils, and fecal material.[208] Death from cryptosporidiosis may occur rarely in AIDS
patients because of intractable diarrhea with fluid loss and electrolyte imbalance. The cachectic
state and concomitant infection with other opportunistic agents in many AIDS patients
potentiates the effects of the severe diarrhea. Therapy with spiramycin or eflornithine has shown
very limited success.[396]
Isospora belli infections occur less frequently than cryptosporidial infections in AIDS,
but produce an indistinguishable clinical appearance. Immunocompetent persons have mild