Spot The Difference
A small intestinal amoeba measures about 4–12 µm and resembles E. histolytica morphologically. What organism should be considered?
Entamoeba hartmanni
For most intestinal amoebae, what is the general job of the cyst compared with the trophozoite?
Cyst = environmentally resistant/transmissible stage
Trophozoite = active feeding/motile stage
What information can a fresh wet preparation provide that may be lost on a permanent stained smear?
Motility
What type of intestinal disease pattern should make you think about malabsorption rather than an invasive dysentery?
Greasy/foul-smelling, non-bloody diarrhea associated with organisms such as Giardia.
Why does stool consistency matter when looking for intestinal protozoa?
Different stages are more likely to be recovered depending on how rapidly material passes through the intestine.
Name two nuclear or cytoplasmic characteristics that help separate E. coli from E. histolytica.
Coarse/uneven peripheral chromatin, eccentric karyosome, "dirty"/vacuolated cytoplasm; mature cyst can have 8 nuclei.
A sand fly introduces one form of Leishmania, but another form is found multiplying inside human macrophages. Name both forms.
Promastigote → amastigote
Microscopy shows an Entamoeba cyst compatible with the E. histolytica/E. dispar/E. moshkovskii complex, but there are no ingested RBCs.
Why should the laboratorian avoid automatically calling it E. histolytica?
The species can be morphologically indistinguishable; additional antigen/immunoassay testing may be necessary.
Why are nonpathogenic intestinal amoebae still clinically important to the MLS?
They must be distinguished from pathogens to avoid misidentification and inappropriate treatment.
Why can immune status dramatically change the significance of an opportunistic intestinal Apicomplexan?
Immunocompromised patients may develop prolonged, severe, or disseminated disease.
What morphologic feature best distinguishes T. vaginalis from T. hominis?
Undulating membrane approximately half the body length vs. the entire length.
Why could T. cruzi appear as different forms in a peripheral blood specimen and a cardiac tissue biopsy?
Trypomastigotes circulate in blood; amastigotes occur in tissues.
Why are tissue and bone-marrow specimens useful when visceral leishmaniasis is suspected?
Diagnostic amastigotes may be present within macrophages.
Compare the typical timeline/speed of primary amebic meningoencephalitis vs granulomatous amebic encephalitis.
PAM is rapidly progressive/fulminant
GAE is more subacute or chronic
Why can Cryptosporidium maintain a persistent infection within the same patient?
Thin-walled oocysts permit autoinfection.
How would you approach differentiating Cryptosporidium, Cyclospora, and Cystoisospora if all three are being considered?
Compare oocyst size first, then staining behavior/shape and internal structures.
Contrast the roles of Toxoplasma tachyzoites and bradyzoites.
Tachyzoites = rapidly multiplying/disseminating
Bradyzoites = dormant forms in tissue cysts
Match the general diagnostic approach to Toxoplasma: routine patient diagnosis, direct tissue demonstration, and investigation of congenital infection.
Serology; tissue demonstrating tachyzoites/bradyzoites; PCR of amniotic fluid.
Explain how the clinical picture of Chagas disease can change from the initial infection to chronic disease.
Early localized/systemic findings can progress years later to cardiac and/or GI disease.
Why doesn't Babesia typically produce the synchronized paroxysm pattern associated with malaria?
Its erythrocytic replication is not synchronized in the same manner as Plasmodium.
Give three findings that would favor Babesia over Plasmodium in an intraerythrocytic parasite differential.
Tick exposure, no malarial pigment/stippling, possible Maltese-cross tetrads, no RBC enlargement, no cyclic malarial paroxysm.
Where do the sexual and asexual portions of the Plasmodium life cycle occur?
Sexual/sporogony in the mosquito
Asexual schizogony in humans
A clinician strongly suspects malaria. Explain why the laboratory may use both thick and thin blood films and why the investigation may continue after an initially negative set.
Thick films improve detection/sensitivity; thin films preserve morphology for identification/speciation; parasitemia can vary, so repeat specimens may be necessary.
Put the three major forms of leishmaniasis in order according to the major tissue involved: skin only, mucous membranes, reticuloendothelial organs.
What are the organisms involved in each major form?
Cutaneous → mucocutaneous → visceral
L. mexicana and L. tropicalis - Cutaneous
L. braziliensis - Mucocutaneous
L. donovani - Visceral
Explain the cycle of Plasmodium spp. from infection to diagnosis.
Female Anopheles mosquito injects sporozoites → sporozoites travel to the liver and undergo schizogony → merozoites are released and invade RBCs → trophozoites and schizonts develop in RBCs → parasites are detected on thick and thin blood smears for diagnosis.