Monday, September 20, 2010

Answer to Case 131

Answer: Entamoeba histolytica/dispar
Congratulations to all of the viewers who wrote in with the answer - you recognized that the morphologic features and size were consistent with these two closely related protozoa. E. histolytica is a recognized pathogen, although it only causes disease in approximately 10% of the people it infects. E. dispar, on the other hand, is generally considered a non-pathogen. Unfortunately, the two are morphologically indistinguishable, and require isoenzyme, antigen, or molecular methods to distinguish them. The only exception to this is when E. histolytica trophozoites are seen invading the bowel wall on histologic section, or contain ingested RBCs on ova and parasite exam. No ingested RBCs are seen in this case, so it is not possible to differentiate between the two organisms.

The answer to the second part of the question is that an asymptomatic host could still be infected with either E. histolytica
or E. dispar (remember that most E. histolytica infections are asymptomatic).

Finally, just to make life difficult for clinical parasitologists, there is now a THIRD species of Entamoeba which is morphologically indistinguishable from E. histolytica and E. dispar. Entamoeba moshkovskii is now recognized to be a wide spread environmental organism and occasional human parasite. Therefore, I suppose that the most correct answer to this case is "Entamoeba histolytica/dispar/moshkovskii!!


neuro_nurse said...

I don't doubt that there are asymptomatic E. histolytica infections, but I wonder about the prevalence of asymptomatic E. histolytica versus the prevalence of E. dispar.

"Approximately 90% of the asymptomatic intestinal infections previously attributed to infection with E. histolytica were actually due to E. dispar."
Long: Principles and Practice of Pediatric Infectious Diseases, 3rd ed.

"All E. moshkovskii and E. dispar infections and as many as 80% of E. histolytica infections are asymptomatic."
Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases, 7th ed.

Those two statements do not necessarily contradict each other.

Parasite Gal said...

Neuro_nurse raises an excellent point. Both of the statements listed above are true.
"Approximately 90% of the asymptomatic intestinal infections previously attributed to E. histolytica were actually due to E. dispar" (or E. moshkovskii). In addition, we also know that of PCR-proven E. histolytica infections, approximately 90% are asymptomatic. So that means that E. histolytica infection is much less common than previously thought (especially in countries such as the US where almost all infections were actually due to E. dispar), AND that invasive infection by PCR-proven E. histolytica is actually a rare occurrence. I hope this clarifies any confusion. Thanks for the question!

Salbrent said...

Ferric Fang, MD said earlier
the once saw a patient with RBC-containing E. dispar in his stool who turned out to have inflammatory bowel disease on biopsy; the gastrointestinal bleeding caused by the inflammation provided RBCs for ingestion by the non-pathogenic amoebae. E. histolytica was excluded by a negative serology, negative ELISA for the GAL/GALNAc-specific lectin of E. histolytica, and colonoscopy
with biopsy which showed no evidence of amebic colitis. In their review of the laboratory diagnosis of amebiasis (Clin Microbiol Rev
16:713-29, 2003), Tanyuksel & Petri observe that "A poor man's way to distinguish E. dispar from E. histolytic microscopically is erythrophagocytosis. . . However in some cases E. dispar is also observed to contain RBCs." ("Poor man's" is not meant pejoratively here, just to designate settings in which more definitive methods are unavailable).

Parasite Gal said...

Thanks Salbrent. That's a very good point. There are also some studies out there that show that E. dispar will rarely ingest RBCs, (albiet much less readily than E. histolytica). Here's one such article:

Boettner et al. Entamoeba histolytica and Entamoeba dispar Utilize Externalized Phosphatidylserine for Recognition and Phagocytosis. Infection and Immunity 2005.

of Erythrocytes