Protozoan Diseases: Major Human Infections, Transmission, and Treatment

Protozoa: Single-Celled Eukaryotic Parasites

Protozoans cause significant global morbidity. Key human pathogens:

1. Malaria — Plasmodium spp.

• Species: P. falciparum (most severe), P. vivax, P. ovale, P. malariae, P. knowlesi.
• Vector: Anopheles mosquito.
• Symptoms: Cyclic fever, chills, rigors, anemia, cerebral malaria (falciparum).
• Treatment: ACT (artemisinin combination therapy) — e.g., artemether-lumefantrine; severe: IV artesunate. P. vivax/ovale need primaquine/tafenoquine for hypnozoites (G6PD test first).
• Prevention: ITNs, IRS, chemoprophylaxis (doxycycline, atovaquone-proguanil, mefloquine), RTS,S/AS01 vaccine (children in Africa).

2. African Trypanosomiasis (Sleeping Sickness) — Trypanosoma brucei

• T. b. gambiense (West/Central Africa, chronic); T. b. rhodesiense (East Africa, acute).
• Vector: Tsetse fly (Glossina).
• Stages: Hemolymphatic (fever, lymphadenopathy) → meningoencephalitic (sleep disruption, neuro deficits, coma).
• Treatment: Stage-dependent: fexinidazole (oral, both stages, gambiense), pentamidine/suramin (early), melarsoprol/nifurtimox-eflornithine (late).

3. Chagas Disease — Trypanosoma cruzi

• Vector: Triatomine "kissing bugs" (fecal contamination of bite).
• Also: Congenital, transfusion, transplant, oral (contaminated food).
• Phases: Acute (mild/febrile) → indeterminate → chronic (cardiomyopathy 30%, megaviscera 10%).
• Treatment: Benznidazole or nifurtimox — best in acute/early chronic; limited efficacy late.

4. Leishmaniasis — Leishmania spp.

• Visceral (kala-azar): L. donovani, L. infantum — fever, splenomegaly, pancytopenia, fatal if untreated.
• Cutaneous: L. major, L. tropica, L. mexicana — ulcers, scars.
• Mucocutaneous: L. braziliensis — destructive nasopharyngeal lesions.
• Vector: Sandfly (Phlebotomus, Lutzomyia).
• Treatment: Liposomal amphotericin B (visceral); miltefosine, pentavalent antimonials, paromomycin (cutaneous).

5. Giardiasis — Giardia duodenalis

• Waterborne/foodborne/fecal-oral; diarrhea, steatorrhea, bloating.
• Treatment: Metronidazole, tinidazole, nitazoxanide.

6. Amebiasis — Entamoeba histolytica

• Dysentery, liver abscess.
• Treatment: Metronidazole/tinidazole → then paromomycin/diloxanide furoate (luminal agent).

7. Toxoplasmosis — Toxoplasma gondii

• Cats definitive host; humans: undercooked meat, oocysts (soil, litter), congenital.
• Immunocompetent: asymptomatic or mono-like; immunocompromised: encephalitis; congenital: chorioretinitis, hydrocephalus.
• Treatment: Pyrimethamine + sulfadiazine + leucovorin; spiramycin in pregnancy.

8. Babesiosis — Babesia microti etc.

• Tick-borne (Ixodes scapularis); malaria-like; severe in asplenic/elderly.
• Treatment: Atovaquone + azithromycin; clindamycin + quinine (severe).

9. Cryptosporidiosis — Cryptosporidium

• Waterborne; watery diarrhea; severe in HIV/CD4<100.
• Treatment: Nitazoxanide; supportive; ART for HIV.

10. Trichomoniasis — Trichomonas vaginalis

• STI; vaginitis, urethritis.
• Treatment: Metronidazole/tinidazole (single dose); treat partners.

Diagnostic Tools

• Microscopy (Giemsa, trichrome, Kinyoun), antigen detection (malaria RDT, Crypto/Giardia EIA), PCR (multiplex panels), serology (Chagas, Toxo, Leishmania).

Travel medicine, vector control, sanitation, and early diagnosis are pillars of protozoan disease control.