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170 Tropical and Parasitic Infections in the ICU
transaminases and bilirubin levels are abnormal in a much smaller
proportion (20-50%) (17).
Detection of Amoebae. The mainstay of laboratory diagnosis of invasive
amoebic colitis is still microscopic detection of haematophagous amoebae
in stool; examination must be within 30 minutes of stool collection, and
several specimens may be required. Microscopy of trichrome stained
stool smears is ideal but its availability is limited. Amoebic culture,
although more sensitive than microscopy is not available outside some
specialised laboratories. Immunological tests for detection of E.
histolytica-specific antigens or antibodies in stool, serum or saliva (23, 24)
will ultimately displace microscopy in the routine laboratory; some such
antigen tests are already commercially available (25). Colonoscopy with
ulcer scrapings or biopsies may be indicated if the stool antigen test is
negative or unavailable. Aspiration, ideally under ultrasound guidance,
and culture of liver abscess material will help to distinguish bacterial from
amoebic aetiology. Amoebic trophozoites are located in the abscess wall,
rather than its centre, and are often very difficult to visualise
microscopically.
Serological Tests. Outside endemic areas, in which up to a third or more
of residents have persistent antibodies, positive amoebic serological tests
(at presentation, or seroconversion) are very useful corroborative evidence
of invasive amoebiasis, especially extraintestinal infection. Sensitivities
of 90%-96% and 70%-85% for amoebic liver abscess and colitis,
respectively, are mentioned (16, 17).
Treatment
Luminal carriage and invasive amoebiasis are treated differently (Table
3). Eradication of luminal amoebae after treatment of invasive amoebiasis
is recommended to prevent relapses, as tissue amoebicides are not
efficient at eliminating carriage. Luminal agents are variably available
(e.g. in South Africa, none are sold). In this case, metronidazole (or
tinidazole) is the only option for elimination of intestinal carriage, ideally
with confirmation of cyst (or antigen) clearance. Because of high risk of
reinfection and the lack of facilities for distinguishing E. histolytica from
E. dispar, treatment of asymptomatic intestinal carriage in endemic areas
is of questionable value.
Agents regarded as second-line or obsolete for treatment of invasive
amoebiasis are dehydroemetine and emetine, chloroquine, and
tetracycline.
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