Amebiasis Paul R. Earl Facultad de Ciencias Biol

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AmebiasisPaul R. EarlFacultad de Ciencias
BiológicasUniversidad Autónoma de Nuevo LeónSan
Nicolas, NL, Mexico pearl_at_dsi.uanl.mx
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Amebiasis or amebic dysentary is caused by the
protozoan Entamoeba histolytica. Improved
sanitation and clean water supply decrease the
incidence of amebiasis. The amount of chlorine
normally used to control pathogens is inadequate
in killing the cysts. Drinking water can be
rendered safe by boiling or iodination with
tetraglycine hydroperiodide. Nevertheless,
drinking water is usually not much of the
problem.
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Generalities.
Amebiasis is an intestinal infection
in which cysts are passed in the feces. Symptoms
can include fever, chills and diarrhea, sometimes
bloody or with mucus and often with cramps. Some
people may have only mild abdominal discomfort or
no symptoms at all. Symptoms can start 2 or more
weeks after infection. Rarely, trophozoites (the
mobile amebas) may invade the liver, lung or
brain, or perforate the colon causing septicemia.
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E. dispar is a nonpathogenic protozoon
morphologically identical to E histolytica.
Previously reported asymptomatic infections due
to the so-called nonpathogenic strains of E
histolytica now are recognized to be due to E.
dispar. These 2 species of Entameba can be
distinguished by monoclonal antibodies. Other
morphologically distinct organisms, such as
Entamoeba coli ( E. coli ! ! ) and Entamoeba
hartmanni are also nonpathogenic.
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Amebiasis is the third leading parasitic cause of
death worldwide, surpassed only by malaria and
schistosomiasis. On a global basis, amebiasis
affects approximately 50 million persons each
year, resulting in nearly 100,000 deaths.
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Laboratory diagnosis.
An iodine-stained cyst of
the pathogen Entamoeba hystolytica with 4 nuclei
is illustrated. The harmless commensal Entamoeba
coli has larger cyts with 8 nuclei. Furthermore,
recall that E. histolytica has a lookalike E.
dispars that is harmless.
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The cyst of E histolytica averages 12 ?m, ranging
from 5-20 ?m. It has 1-4 nuclei that are
morphologically similar to the nuclei of the
trophozoite. The cyst may have iodine-stainable
glycogen clumps and chromatoid bodies with smooth
rounded edges. The ending oid means LIKE so
chromatoid bodies are like chromatin in that they
stain with hematoxylin.
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TrophozoitesEntamoeba
coli Entamoeba histolytica15 mm - 40 mm in
size 10 mm - 35 mm sizeNondirectional motility
Unidirectional motilityMultiple pseudopodia
Single pseudopodiaNo ingested erythrocytes
Ingested erythrocytesCytoplasm rough
looking Finely granular cytoplasmLarge,
eccentric karyosome Small, central
karyosomeClumped nuclear chromatin Finely
beaded chromatin
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CystsEntamoeba coli Entamoeba histolytica10
mm - 35 mm in size 10 mm - 20 mm in sizeMay
have 8 nuclei Never more than 4
nucleiKaryosomes eccentric Karyosomes small,
centralNuclear chromatin clumped Chromatin
finely beadedSplintered chromatoidal bars
Rounded chromatoidal bars
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Leukocytosis and mild anemia can occur.
Erythrocyte sedimentation rate generally is
elevated. Liver function tests reveal elevated
alkaline phosphatase in 80 of patients, elevated
transaminases and reduced albumin. Urinalysis may
reveal proteinuria.Rectosigmoidoscopy and
colonoscopy may show small mucosal ulcers covered
with yellowish exudates. The intervening mucosa
appears normal. Biopsy results and scrapings of
ulcer edge may locate trophozoites.
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Symptoms and pathology. Primary intestinal
flask-shaped (button hole) necrotic ulcers occur
in the submucosa of the large intestine, most
commonly the cecal and sigmoidorectal
regions.Ulcers contain necrotic debris, actively
feeding trophozoites with ingested erythrocytes,
cytolyzed cells and mucous polymorphonuclear
leukocytes and round inflammatory cells.
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Extraintestinal features. Hematogenous spread may
result in abscesses of the liver, spleen, lung or
brain. Hepatic amebiasis (abscess, hepatitis) is
the most common and grave complication Enlarged,
tender liver and upper abdominal pain that may
radiate to the right shoulder. Mild jaundice may
be evident, transaminases and alkaline
phosphatase elevations may be seen.
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Drugs for treatment. Five pharmaceuticals
are briefly noted. Asymptomatic intestinal
infection may be treated with iodoquinol,
paromomycin or diloxanide furoate. Recommended
drugs for treatment of symptomatic intestinal
disease and for hepatic abscess are metronidazole
and tinidazole. Since these drugs may not
eliminate the cysts of the intestine, immediately
follow metronidazole and tinidazole with
iodoquinol, paromomycin or diloxanide furoate.
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1/ Metronidazole (Flagyl, Protostat). Kills
trophozoites of E. histolytica in intestine and
tissue. Does not eradicate cysts from intestines.
Adult oral dose 500-750 mg 3 times per day for
10 day. Elimination is accelerated by
simultaneous use of phenytoin and phenobarbital
clearance is decreased by cimetidine.
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2/Tinidazole (Fasigyn). 5-nitroimidazole
derivative with selective antimicrobial activity
against anaerobic bacteria and protozoa. Not
available in United States. Adult oral dose 600
mg bid or 800 mg 2 times a day for 5 days.
Pediatric dose 50-60 mg/kg for 5 days, not to
exceed 2 g/day.
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3/Paromomycin (Humatin). Amebicidal
aminoglycoside antibiotic that is poorly
absorbed. Active only against intestinal form of
amebiasis. Used to eradicate cysts of E.
histolytica following treatment with
metronidazole or tinidazole for an invasive
disease. Adult oral dose 25-35 mg/kg/day divided
3 times for 7 days. Pediatric dose Administer as
in adults.
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4/Diloxanide furoate (Furamid, Entamizole,
Furamide). Luminal amebicide acts primarily in
bowel lumen since it is poorly absorbed. Used to
eradicate cysts of E. histolytica after treatment
of invasive disease. Available through US CDC
Drug Service (404-639-3670). Adult oral dose 500
mg 2 times a day for 10 days. Pediatric dose 20
mg/kg/ divided twice a day for 10 days, not to
exceed 1500 mg/day.
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5/Iodoquinol (Yodoxin). Halogenated
hydroxyquinoline. Luminal amebicide acts
primarily in bowel lumen since it is poorly
absorbed. Best tolerated when given with meals.
Since active only against intraluminal form of
amebiasis, used to eradicate cysts of E.
histolytica after treatment of invasive disease.
Adult oral dose 650 mg 2 times a day for 20 days.
Pediatric dose 30-40 mg/kg/day divided 2 times
for 20 days not to exceed 2 g/day.
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The control of infection. The epidemiology of
amebiasis is folkloric, beginning with infected
foodhandlers. While elements of truth are
scattered through this neglected syndrome, much
more can be done. Frankly, the ecology is
unknown. Are there reservoir animals?
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What civic water treatments kill and which ones
tolerate E. histolytica? What part does chronic
malnutrition play in susceptibility to infection?
What part does exposure then the rise of natural
antibodies play in defense? What are the
water-borne pathogens intimately associated with
E. histolytica? Has radioactivity been used to
trace ecological dispersion of an element like
phosphorus in a parasite? If ever, how is E.
histolytica considered in civic water management?
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The main point of this lecture is to sketch E.
histolytica as a distinct pathogen differing from
E. coli and E. dispar. More, decades pass without
fruitful reseach results. Perhaps you find this a
challenge.