Medical Parasitology: Protozoa, Helminths, and Vectors

Posted on May 16, 2026 in Naval Architecture and Marine Engineering

Protozoa

Protozoa are unicellular eukaryotic parasites classified mainly into amoebae, flagellates, apicomplexans, and ciliates.

  • Motility: Pseudopodia (amoebae), flagella (flagellates), cilia (ciliates), and gliding (Apicomplexa).
  • General stages: Trophozoite (active/feeding) and cyst/oocyst (resistant infective stage).
  • Reproduction: Asexual via binary fission or schizogony; Apicomplexa also undergo sexual reproduction.

Amoebae

  • Entamoeba histolytica: Intestinal amoeba; infective mature cysts are ingested fecal-orally. Trophozoites colonize the large intestine, causing flask-shaped ulcers, dysentery, and bloody stool. Key ID feature: ingested RBCs. Extraintestinal spread leads to amoebic liver abscess (“anchovy paste”).
  • Naegleria fowleri: Free-living freshwater amoeba; enters the nose during swimming and migrates via olfactory nerves. Causes primary amebic meningoencephalitis (PAM), characterized by rapid headache, fever, and death. Not acquired by drinking water.

Flagellates

  • Giardia duodenalis/lamblia: Pear-shaped trophozoite with a ventral sucking disc and two nuclei (“face”). Infective cysts are ingested via contaminated water. Causes malabsorption, greasy diarrhea, and bloating.
  • Trichomonas vaginalis: STI flagellate; exists only as a trophozoite. Causes vaginitis, frothy green discharge, and strawberry cervix.
  • Trypanosoma brucei: Causes African sleeping sickness via tsetse fly bites. Antigenic variation causes recurrent fever; CNS involvement leads to neuro-decline and coma.
  • T. cruzi: Causes Chagas disease via triatomine (“kissing bug”) feces. Acute signs include Romaña sign; chronic complications include cardiomyopathy and megacolon.
  • Leishmania spp.: Transmitted by sand flies. Amastigotes survive in macrophages, causing cutaneous, mucocutaneous, or visceral (kala-azar) disease.

Apicomplexa

Defined by an apical complex for host-cell invasion. No visible locomotor organelles.

  • Plasmodium spp.: Malaria transmitted by female Anopheles mosquitoes. Cyclic RBC rupture causes periodic fever and anemia. P. falciparum is the most deadly; P. vivax features dormant liver hypnozoites.
  • Babesia spp.: Tick-transmitted RBC parasite causing hemolytic anemia; shows “Maltese-cross” tetrads.
  • Toxoplasma gondii: Cats are definitive hosts. Humans infected via oocysts or tissue cysts. Congenital triad: chorioretinitis, hydrocephalus, and intracranial calcifications.
  • Cryptosporidium spp.: Chlorine-resistant, waterborne, acid-fast oocysts causing diarrhea in immunocompromised patients.

Ciliates

  • Balantidium coli: The only major human ciliate; pigs are the reservoir. Causes dysentery-like colitis.

Helminths

Multicellular eukaryotic worms categorized into cestodes (tapeworms), trematodes (flukes), and nematodes (roundworms).

Cestodes (Tapeworms)

Flat, segmented, and lack a digestive tract. Taenia solium (pork tapeworm) causes cysticercosis if eggs are ingested. Diphyllobothrium latum (fish tapeworm) may cause B12 deficiency. Echinococcus granulosus causes hydatid cysts in the liver or lungs.

Trematodes (Flukes)

Flattened, unsegmented worms (except schistosomes). Most require snails as intermediate hosts.

  • Blood flukes: Schistosoma species penetrate skin directly. Pathology is driven by eggs/granulomas.
  • Liver/Lung flukes: Fasciola hepatica (liver), Clonorchis sinensis (bile duct), and Paragonimus westermani (lung).

Nematodes (Roundworms)

  • Ascaris lumbricoides: Giant intestinal nematode; larvae migrate through lungs (Loeffler syndrome).
  • Enterobius vermicularis: Pinworm; causes nocturnal perianal itching.
  • Strongyloides stercoralis: Unique for autoinfection; can cause fatal hyperinfection in immunosuppressed patients.
  • Hookworms: Ancylostoma and Necator penetrate skin and cause iron-deficiency anemia.

Somatic/Filarial Nematodes

  • Trichinella spiralis: Acquired from undercooked meat; larvae form nurse-cell cysts in muscle.
  • Wuchereria bancrofti: Causes lymphatic filariasis (elephantiasis).
  • Onchocerca volvulus: Causes river blindness via black fly bites.
  • Dracunculus medinensis: Guinea worm; transmitted via copepod-contaminated water.

Ectoparasitic Arthropods and Vectors

Arthropods act as disease vectors or ectoparasites.

  • Fleas: Xenopsylla cheopis (plague vector).
  • Bugs: Triatoma (Chagas vector); Cimex lectularius (bed bug).
  • Mosquitoes: Anopheles (malaria), Aedes (arboviruses).
  • Ticks: Ixodes scapularis (Lyme disease, babesiosis).
  • Lice: Pediculus humanus (typhus).
  • Mites: Sarcoptes scabiei (scabies).