STEPTRACKERMICROBIOLOGY + HEMATOLOGY + PHARMACOLOGY | BLOOD PROTOZOA | HIGH-YIELD REVIEW

Blood Protozoa & Trichomonas

Plasmodium and Babesia live inside red cells — cyclic fevers, hemolysis and thick-and-thin blood smears. Trichomonas is the lone flagellate STI.

FOCUS ON
Malaria (female Anopheles): P falciparum — most severe, irregular fevers, banana-shaped gametocytes, many ring forms, cytoadherence → cerebral malaria, renal failure, ARDS; P vivax/ovale — 48-h fevers, hypnozoites in liver → relapse → add primaquine (check G6PD first), Schüffner dots; P malariae — 72-h fevers, nephrotic syndrome. Protection: sickle trait, G6PD deficiency, Duffy-negative RBCs (vivax). Babesia (Ixodes tick; Northeast US): ring forms and Maltese cross, severe in asplenia → atovaquone + azithromycin. Trichomonas: motile trophozoites, frothy discharge → metronidazole for patient and partner.
PARASITE
BIOLOGY
CLINICAL PICTURE
DIAGNOSIS · TREATMENT
Plasmodium falciparum
Infects RBCs of all ages (high parasitemia); PfEMP1 makes RBCs stick to endothelium
Irregular high fevers; cerebral malaria (coma, seizures), severe anemia, hypoglycemia, AKI ("blackwater"), ARDS
Multiple rings per cell, crescent gametocytes · artemisinins (IV artesunate if severe)
Plasmodium vivax · ovale
Infect reticulocytes (vivax needs Duffy antigen); dormant liver hypnozoites
Fever every 48 h (tertian); relapses months later
Enlarged RBCs with Schüffner stippling · chloroquine (where sensitive) + primaquine or tafenoquine
Plasmodium malariae
Infects old RBCs; low parasitemia; can persist decades
Fever every 72 h (quartan); immune-complex nephrotic syndrome
Band forms · chloroquine
Plasmodium knowlesi
Macaque malaria in Southeast Asia; 24-h cycle
Can be severe
Looks like malariae on smear
Malaria diagnosis
Thick smear (detect) + thin smear (species, % parasitemia); rapid antigen test
Fever in any returning traveler from endemic areas
Repeat smears every 12–24 h × 3 if negative
Malaria prophylaxis
By region and resistance
Atovaquone-proguanil, doxycycline, mefloquine (neuropsychiatric effects); chloroquine where sensitive
Plus bed nets, repellent
Antimalarial toxicities
—
Chloroquine: retinopathy · quinine/quinidine: cinchonism, QT prolongation, hypoglycemia · primaquine: hemolysis in G6PD deficiency
Test G6PD before primaquine/tafenoquine
Natural protection
Altered RBC environment
Sickle trait (HbAS), thalassemia trait, G6PD deficiency, Duffy-negative (vivax)
Balanced polymorphism
Babesiosis
Babesia microti via Ixodes tick (same as Lyme, Anaplasma); transfusion
Fever, hemolytic anemia, hemoglobinuria; severe in asplenia, elderly
Rings, tetrad "Maltese cross", no pigment · atovaquone + azithromycin; exchange transfusion if severe
Trichomoniasis
Trichomonas vaginalis, flagellated, sexual transmission; no cyst form
Women: frothy yellow-green malodorous discharge, itching, strawberry cervix; men often asymptomatic urethritis
pH > 4.5, motile trophozoites on wet mount, NAAT · metronidazole (treat partners)

Picture it — the malaria life cycle

Anopheles biteinjects sporozoitesLIVERschizonthypnozoitevivax, ovalerelapse →primaquineringtrophozoiteschizontRBC bursts→ FEVERRBC cycle48–72 hmerozoitesGametocytessome RBC formstaken upby mosquitoblood-stage drugs: artemisinins, chloroquine
Plasmodium in humansSporozoites from the mosquito infect hepatocytes, which release merozoites into the blood. Each RBC cycle ends with synchronous rupture and fever. Vivax and ovale leave dormant hypnozoites in the liver, so primaquine is added to prevent relapse. Some parasites become gametocytes that the next mosquito picks up.

Schematic diagrams, not to scale.

Mechanism — why fevers are periodic

Merozoites invade RBCs
→
Synchronous growthRing → trophozoite → schizont
→
RBCs burst togetherPyrogens released
→
Fever spike every 48–72 h

P falciparum is less synchronous, so fevers are irregular.

Plasmodium species compared

SPECIES
CYCLE
LIVER DORMANCY
CLUE
falciparum
Irregular
No
Crescent gametocytes, cerebral
vivax
48 h
Yes (hypnozoites)
Big RBCs, Schüffner dots
ovale
48 h
Yes
Oval RBCs
malariae
72 h
No
Nephrotic syndrome

How it's tested

COMMON CONFUSIONBabesia (Maltese cross, Northeast US tick, no travel, no pigment) vs P falciparum (multiple rings, travel to Africa, hemozoin pigment).
THE TRAPTreating vivax or ovale with blood-stage drugs alone leaves hypnozoites — relapse months later unless primaquine is added.
YOUR APPROACHFever + travel or tick → smear → species → severity → blood-stage drug ± primaquine (after G6PD test).

If you see… think…

Returning traveler from Africa with fever and coma → P falciparum (cerebral malaria)
Banana-shaped gametocytes → P falciparum
Fever relapse months after treated malaria from India → P vivax hypnozoites
Test needed before primaquine → G6PD
Fever every 72 hours + nephrotic syndrome → P malariae
Why Duffy-negative people resist vivax → no RBC receptor
Asplenic Massachusetts gardener with hemolysis and Maltese cross → Babesia
Treatment of babesiosis → atovaquone + azithromycin
Frothy green discharge with strawberry cervix → Trichomonas
Antimalarial causing tinnitus, headache, QT prolongation → quinine/quinidine