16.1 Blood, Tissue, and Brain Protozoa

16.1 Blood, Tissue, and Brain Protozoa

Quick Answer: Speciate malaria on a STAT Giemsa thin film: P. falciparum has multiple rings in normal-sized RBCs, Maurer dots, and a banana gametocyte; P. vivax enlarges the RBC and shows Schüffner dots; P. ovale is oval and fimbriated with stippling; P. malariae produces band forms. Thick films screen and quantitate; thin films speciate; travel history is required. T. cruzi trypomastigotes are C-shaped with a large kinetoplast; T. brucei is slender with a small kinetoplast and reaches CSF in late disease. Brain: Toxoplasma cysts in AIDS or congenital infection, Naegleria motile CSF amebae after warm freshwater (PAM), and Acanthamoeba keratitis or GAE.

Domain III (Mycobacteriology, Virology, Parasitology, Mycology) is 20–30% of the M examination. Official outline III.C Parasitology (content guideline revised 2025-09-25) tests specimen sources, major pathogens and disease states, microscopic identification, and antigen/molecular detection. This section is the blood, tissue, and brain map. Intestinal and urogenital protozoa plus helminths are 16.2. O&P methods, antigen/NAAT, and arthropods are 16.3. Parasite culture (for example Trichomonas spp. and Strongyloides spp.) is SM-only and is not M-tested content.

Specimen sources: blood, tissue, CSF

Blood-and-tissue parasitology is a matched-source problem. The wrong tube, a delayed film, or a refrigerated CSF can erase the organism the outline is asking you to recognize.

A malaria smear is a critical STAT. Untreated P. falciparum can progress to cerebral malaria in hours. Report species and percent parasitemia from the thin film; do not hold the specimen for a later daytime draw if the patient is febrile now. Travel, immune status, freshwater swimming, and contact-lens hygiene belong on the requisition because they choose the method: films versus CSF wet mount versus tissue stains.

Plasmodium species morphology

Four human species dominate M items: P. falciparum, P. vivax, P. ovale, and P. malariae. Always collect thick and thin films from the same stick or draw.

The thick film is a concentrated drop that is laked (RBCs lysed; not methanol-fixed first). It examines about 20 times more blood than a thin film and is the screening and quantitation preparation. Species morphology is poorer after lysis, so do not speciate from the thick film alone. The thin film is a methanol-fixed monolayer used to speciate: parasite size versus the host RBC, stippling, number of rings per cell, gametocyte shape, and whether mature schizonts circulate. Giemsa buffer pH about 6.8–7.2 is required to show Schüffner dots; an overly acidic stain washes stippling away and can make vivax look like falciparum.

P. falciparum is the medical emergency. It infects RBCs of all ages, so parasitemia can exceed 2–5% and may be much higher. Rings are small, often with double chromatin dots, multiple rings per RBC, and appliqué (accolé) forms plastered on the RBC margin. Maurer dots (clefts) may appear in older trophozoites. Infected RBCs are not enlarged. Mature trophozoites and schizonts usually sequester in deep vasculature and are absent from peripheral blood except in overwhelming infection — a smear that shows only rings plus crescents is itself a falciparum clue. The banana (crescent) gametocyte is pathognomonic and is the morphology the outline expects you to name on sight. There are no hypnozoites; return of symptoms is recrudescence of blood stages, not a liver relapse. Cerebral malaria, sequestration, and high-output hemolysis distinguish it clinically.

P. vivax infects reticulocytes, so RBCs are enlarged (often about 1.5×) and parasitemia is usually <2%. Trophozoites are ameboid. Schüffner dots stipple the RBC. Schizonts with about 12–24 merozoites can circulate. Liver hypnozoites cause relapses months later, so a delayed fever after travel to a vivax-endemic region is still malaria. Duffy-negative RBCs, common in West Africa, resist vivax invasion, which is why falciparum dominates that geography.

P. ovale is the other relapsing tertian malaria. RBCs are modestly enlarged, often oval with fimbriated (ragged) edges, and show Schüffner (James) stippling. Schizonts have fewer merozoites (about 8–12) than vivax. Treat ovale as vivax-like morphology plus oval or fimbriated cells, not as falciparum.

P. malariae infects older RBCs, so cells are normal to small. The diagnostic trophozoite is the band form stretching across the cell. Schizonts often show 6–12 merozoites in a rosette. There are no Schüffner dots and no hypnozoites, but low-level infection can persist for decades as recrudescence. Quartan (72-hour) fever is classic. Parasitemia is typically <1%.

Travel history is part of identification. An Anopheles-exposed traveler from sub-Saharan Africa with banana gametocytes is falciparum until the thin film says otherwise. Enlarged stippled RBCs after travel to temperate or highland foci point to vivax or ovale. Babesia is the domestic malaria look-alike: extra-erythrocytic parasites, Maltese-cross tetrads, no hemozoin pigment, no tropical travel, and Ixodes exposure. Do not call every ring falciparum if the epidemiology is Cape Cod, not Cameroon.

Trypanosoma: location on the smear

Both T. cruzi and T. brucei are hemoflagellates with a nucleus, kinetoplast, undulating membrane, and flagellum. The exam split is geography plus smear shape and kinetoplast size, then which compartment is positive.

T. cruzi trypomastigotes circulate mainly in acute Chagas disease, congenital infection, and reactivation after transplant. The large kinetoplast often looks as if it will burst the posterior end — that is the smear clue. Chronic disease is a tissue-amastigote problem, diagnosed serologically more often than by films. T. brucei trypomastigotes remain extracellular. Once they are in CSF, the patient has crossed into CNS-stage disease; a negative blood film does not exclude late African trypanosomiasis. Do not look for a banana gametocyte or a bradyzoite cyst.

A kinetoplast also marks Leishmania amastigotes inside macrophages (bone marrow, spleen, or skin). Use it to refuse Histoplasma, which has no kinetoplast. Leishmania is not the named outline example; Trypanosoma is.

Brain protozoa

Toxoplasma gondii is an apicomplexan. Cats shed oocysts; humans acquire tissue cysts in undercooked meat, oocysts from cat feces, or transplacental infection. Bradyzoite cysts persist in brain and muscle. In immunocompetent hosts primary infection is often mild. In AIDS (typically CD4 counts below about 100 cells/µL) cysts reactivate as toxoplasma encephalitis: multiple ring-enhancing brain lesions. Congenital infection (TORCH) produces chorioretinitis, hydrocephalus, and intracranial calcifications. Tachyzoites mark active disease. Laboratory first-line tests are serology in immunocompetent and pregnant patients; CSF or tissue PCR and cysts on histopathology support CNS disease in the immunocompromised. A brain cyst in an AIDS patient is Toxoplasma, not motile Naegleria.

Naegleria fowleri is a free-living thermophilic ameba of warm freshwater (lakes, poorly chlorinated splash pads, neti pots filled with tap water). Trophozoites travel the olfactory nerve through the cribriform plate and produce primary amebic meningoencephalitis (PAM): explosive headache, fever, meningismus, and rapid death in previously healthy children and young adults. Examine fresh CSF as a wet mount: motile amebae with explosive pseudopodia are the classic finding. Bacterial Gram stain and culture are negative. PAM is not a cyst-in-AIDS disease and not a contact-lens disease.

Acanthamoeba spp. cause two syndromes. Keratitis follows corneal trauma or contact-lens hygiene failures (homemade saline, tap-water rinsing, swimming in lenses). Cysts and spiny trophozoites (acanthopodia) are seen in corneal scrapings. Granulomatous amebic encephalitis (GAE) is a subacute-to-chronic infection of immunocompromised hosts. Unlike PAM, GAE is slower, CSF rarely shows motile amebae, and diagnosis is often biopsy cysts plus trophozoites. Do not interchange Naegleria PAM with Acanthamoeba GAE.

Malaria films and CSF wet mounts for suspected PAM are critical results: telephone the clinician, document read-back, and do not refrigerate a rule-out-ameba CSF overnight.