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Entamoeba histolytica
Entamoeba histolytica is the only amoebic species capable of invading tissues & causing disease.
Epidemiology:
1) endemic is 80% of the world
- often seen in travelers from Central America, South America, Africa & Asia [5]
2) 2nd leading cause of death from parasitic infections [4]
3) AIDS patients not more susceptible to invasive infections
4) adult male/female ratio 7-10/1, but equal gender distribution for children & elderly [2,3]
5) infection results from ingestion of cystic protozoa in fecally contaminated soil, food or water [4]
6) maturation of cysts into trophozoites occurs in the terminaal ileum, cecum, or colon [4]
7) trophozoites penetrate the colonic mucosa & may produce tissue secretory bloody diarrhea & colitis
Pathology:
1) causes dysentery
2) hemagenous spread may lead to abscess from deposition of trophozoites in liver & rarely pulmonary, cardiac or brain tissue [3]
3) liver abscesses most common extraintestinal manifestation
a) chocolate-colored fluid or paste
b) acellular debris, necrotic hepatocytes
c) trophozoites seen on microspopy < 20% of cases [3]
- generally in periphery of abscess
d) eosinophils, with surrounding neutrophils [2]
4) galactose binding receptor on amoeba binds to mucin-containing cells [2]
5) trophozoites
a) measure 10-60 um
b) commensal forms are generally 15-20 um
c) invasive forms are generally > 20 um
d) progressively motile on wet mounts with hyaline finger-like pseudopods
e) unstained nucleus is not visible
f) with invasive disease, some trophozoites may contain ingested erythrocytes (pathognomonic)
g) both pathogenic & commensal forms may contain ingested bacteria in the cytoplasm
h) stained specimens, peripheral nuclear chromatin is evenly distributed along nuclear membrane
6) cysts
a) spherical in shape
b) measure 10-20 um in diameter (generally 12-15 um)
c) precyst has single nucleus without refractile cyst wall
d) mature cyst has 4 nuclei, each 1/6 the diameter of the cyst
e) glycogen is generally diffuse
f) chromatoid bodies: elongated bars
Clinical manifestations:
1) incubation period 7-21 days
2) colitis
3) manifestations of liver abscess
a) fever, right upper quadrant pain
b) < 1/3 with bloody diarrhea or history of diarrhea
c) hepatomgaly, anemia & weight loss with chronic infection
4) 90% of infections are asymptomatic, including those with liver absecess
5) in different parts of the world, amoeba have predilection for different organs, South Africa liver, Egypt intestine
Laboratory:
1) Entamoeba histolytica serology
- IgA, IgM, IgG response; no IgE response
2) liver function tests
a) elevated serum ALP in 80%
b) serum ALT may also be elevated
3) complete blood count (CBC) may show leukocytosis
- no eosinophilia
4) urinalysis may show proteinuria
5) stool for ova & parasites
- Entamoeba histolytica identified in stool
- direct microscopy identifies trichrome-stained cysts < 18% of cases [3]
6) fine needle aspiration of abscess
a) failure of response to therapy (3-5 days)
b) risks include amoebic peritonitis, inadvertant puncture of echinococcal cyst
c) PCR of aspirate may be useful
7) Entamoeba histolytica antigen
8) Entamoeba histolytica DNA
- Entamoeba histolytica+E dispar+E ecuadoriensis+E nuttalli DNA
9) Entamoeba histolytica 18S rRNA
* image of cyst [4]
Special laboratory:
- flexible sigmoidoscopy
- diffuse colitis
- intestinal biopsy
- flask-shaped intestinal ulcers [5]
Radiology:
1) chest X-ray may be abnormal
2) imaging of liver is primary diagnostic tool [3]
a) computed tomography
b) ultrasound
c) magnetic resonance imaging
d) single abscess in right lobe of liver (70-80%) [3]
3) radiologic resolution of abscess may take 2 years
4) gallium scan may distinguish amoebic (cold)* vs pyogenicb(hot) abscess
* rim of amoebic abscess may be bright (surrounding neutrophils)
Differential diagnosis:
- Cyclospora cayetanensis
- nausea/vomiting, explosive watery diarrhea
- recovery of oocysts in feces
- Giardia lamblia
- explosive watery diarrhea, steatorrhea, foul smelling stool
- associated with contaminated fresh water, campinng ...
- Cryptosporidium
- self-limited watery diarrhea, except HIV1 & other immunocompromised hosts
- public water supply, recreational water facilities
- Isospora
- watery diarrhea
- self-limited except HIV1 & other immunocompromised hosts
- Microsporidia
- immunocompromised hosts
Management:
1) asymptomatic carriers (lumenal agents)
a) iodoquinol 650 mg PO TID for 20 days
b) diloxanide furoate 500 mg PO TID for 10 days
c) paromomycin 500 mg PO TID
2) acute colitis &/or liver abscess
a) metronidazole 750 mg PO or IV TID for 5-10 days, plus ceftriaxone
b) metronidazole plus lumenal agent
c) lumenal agents (see above)
3) other agents
a) tinidazole 2 g PO
b) nitazoxanide (Alinia) 500 PO BID with food for 3 days
c) ornidazole 2 g PO ( not available in USA)
4) aspiration & percutaneous drainage of liver abscesses may be indicated
5) corticosteroids are contraindicated
- may result in toxic megacolon & death
Related
amebiasis
trophozoite
General
Entamoeba
Properties
KINGDOM: animal
PHYLUM: protozoa
References
- Clinical Diagnosis & Management by Laboratory Methods,
19th edition, J.B. Henry (ed), W.B. Saunders Co., Philadelphia,
PA. 1996, pg 1271-74, 1276
- Ravidin J, UC Davis Grand Rounds, Oct 2005
- UpToDate 13.2
- Grimm L
What's Eating You: 12 Common Intestinal Parasites.
Medscape. November 25, 2019
https://reference.medscape.com/slideshow/intestinal-parasites-6010996
- NEJM Knowledge+ Gastroenterology