Amoebiasis
1. Identification
– an intestinal protozoan parasitic infection in two forms
1) hardy infective cyst
2) fragile, potentially pathogenic trophozoite that can invade tissues
- severe infection – amoebic dysentery – acute fever, chills and bloody or mucoid diarrhea
- mild infection – mild abdominal discomfort with bloody or mucoid diarrhea alternating with periods of constipation or remission
- can get amoebic granuloma in the wall of the large intestine which can be mistaken for carcinoma
- can spread through the bloodstream to cause abscess of the liver or less commonly lung or brain
- can mimic ulcerative colitis
- steroids make it worse
2. Infectious agent
- Entamoeba histolytica, a protozoan parasite
- nine pathogenic strains
- morphologically (microscopically) identical to nonpathogenic strain called Entamoeba dispar, 13 strains
-most asymptomatic cyst passers have strains of E. dispar, no treatment necessary
3. Occurrence
- ubiquitous, mostly Mexico, India, Central and South America, tropical Asia and West and southern Africa
- 10% of world population has Entamoeba, mostly E. dispar
- 3rd commonest cause of death from parasites after schistosomiasis and malaria
- amoebic colitis is rare in travelers and is frequently overdiagnosed in
developing countries, the presence of the harmless E. dispar can confuse the diagnosis
- 90% of infections are asymptomatic
- 10% of infections range from dysentery to abscess of the liver, lung or brain
- invasive amoebiasis is mostly a disease of young adults
- liver abscesses occur predominantly in males
- rare below age 5 and especially below age 2 years, when dysentery is due primarily to shigellae
- rates are higher in areas with poor sanitation eg parts of the tropics
- low proportion of cyst passers develop clinical disease
4. Reservoir
- humans, usually a chronically ill or asymptomatic cyst passer
5. Transmission
- by fecally contaminated food or water
- flies can serve as carriers of the amoebic cysts
- cysts are chlorine resistant
- sexually by oral-anal contact
- even amoebic dysentery poses little risk to others because of the absence of cysts in dysenteric stools and the fragility of triphozoites
- trophozoites are released from cysts and in 90% are harmless commensals
- in some invade bowel mucosa (colitis) or the bloodstream to form abscess
6. Incubation
- variable, from a few days to months or years, commonly 2 to 4 weeks after ingestion of cysts
7. 7. Period of communicability
8. - during the period E. histolytica cysts are passed which may last for years
9. 8. Susceptibility and resistance
10. - susceptibility to infection is general
11. - those harboring E. dispar do not develop disease
- reinfection is rare
9. Prevention
- hand washing, avoid uncleaned, unpeeled or uncooked fruits or vegetables
- filter or boil water
- use bottled water or iodine in local water
- treat known carriers
- use of chemoprophylactic agents not advised
- educate high risk group, safe sexual practices
- personal hygiene education, especially food handlers
10. Diagnosis
- can mimic appendicitis or present with weight loss and malaise
- stools essentially blood and mucous
- only 40% are febrile
- liver abcess usually presents within 5 months of travel
- only 1/3 have diarrhea
- can get a fistula to the lung
- rare brain abscess, less than 0.1%
- painful genital ulcers
- definitive diagnosis by seeing trophozoites that have ingested erythrocytes
- an amoebic antibody test can differentiate between the two identically appearing strains, E. histolytica and E. dispar
- differential diagnosis: campylobacter, shigella, salmonella and vibrio species
- Crohn’s disease and ulcerative colitis can mimic amoebiasis and must be considered in the younger patient
- diverticulitis or malignancy should be suspected in older persons
11. Treatment
- amoebic colitis patients (or abscess) should receive IV or PO Metronidazole 750mg tid for 5 to 10 days, followed by iodoquinol, 650 mg tid for 20 days
- this regimen cures 100% of those with amoebic liver abscess and 93% of those with colitis
- asymptomatic cyst passers and those without documented invasive disease require iodoquinol alone
- if can prove E. dispar cysts by antigen/antibody tests no treatment is necessary
- metronidazole does not kill cysts
- luminal agent ( iodoquinol) eradicates cysts
12. Bibliography
- Control of Communicable Diseases Manual, 2000
- Harrison’s Principles of Internal Medicine, 2001 - - - International Travel Health Guide, Rose, 2001
jmcmurray@shaw.ca