Protozoan & Helminthic Parasites
Single-celled protozoa and multicellular worms — life cycles, transmission, and disease patterns.
Interactive Diagram
Tap a teal label to learn about each structure.
Protozoa vs Helminths
Protozoa are single-celled eukaryotes with life stages (trophozoite = active; cyst = infectious). Helminths are multicellular worms visible to the eye, divided into nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes). Parasites are often neglected in temperate practice but cause enormous global burden and appear in travelers, immigrants, and the immunocompromised. Diagnosis usually rests on seeing the organism or eggs in stool, blood, or tissue.
Intestinal Protozoa
Giardia lamblia causes foul-smelling, fatty, non-bloody diarrhea via contaminated water; trophozoites with the 'falling leaf' motion and 'owl-eye' cysts are diagnostic; treat with metronidazole or tinidazole. Entamoeba histolytica causes bloody dysentery and liver abscesses; cysts have four nuclei; treat with metronidazole plus a luminal agent (paromomycin). Cryptosporidium, Cyclospora, and Isospora cause watery diarrhea, especially in AIDS. These are acquired from water and resist chlorination (filtration is key).
Blood and Tissue Protozoa
Plasmodium species cause malaria, transmitted by female Anopheles mosquitoes: P. falciparum (severe, cerebral malaria, no relapse), P. vivax and P. ovale (hypnozoite liver stage, relapse — need primaquine), P. malariae (quartan), P. knowlesi. Diagnosis is by blood smear (ring forms, gametocytes). Toxoplasma gondii from cats/undercooked meat causes CNS toxoplasmosis in AIDS and congenital infection (ring-enhancing lesions, choroidoretinitis). Trypanosoma causes Chagas disease (T. cruzi, reduviid bug) and sleeping sickness (T. brucei, tsetse). Leishmania is transmitted by sandflies and causes visceral (kala-azar) or cutaneous disease.
Nematodes (Roundworms)
Enterobius vermicularis (pinworm) causes perianal pruritus; tape test is diagnostic; treat with mebendazole. Ascaris lumbricoides is the largest intestinal roundworm and may cause obstruction. Hookworm (Necator, Ancylostoma) and Strongyloides larvae penetrate the skin (barefoot) and cause anemia; Strongyloides can autoinfect and disseminate with steroids (hyperinfection — fatal). Trichinella from undercooked pork causes myalgia and eosinophilia (larvae encyst in muscle). Wuchereria bancrofti causes lymphatic filariasis (elephantiasis), transmitted by mosquitoes. Onchocerca volvulus (blackfly) causes river blindness.
Cestodes and Trematodes
Taenia solium from pork can cause cysticercosis (brain cysts, seizures) and taeniasis; Echinococcus granulosus (dog tapeworm) forms hydatid cysts in liver/lung. Diphyllobothrium latum from fish causes B12 deficiency. Trematodes need a water snail: Schistosoma species (mansoni, haematobium, japonicum) cause portal hypertension (S. mansoni/japonicum) and bladder cancer (S. haematobium); cercariae penetrate skin during freshwater exposure ('swimmer's itch' and Katayama fever). Clonorchis and Opisthorchis (liver flukes from fish) cause biliary cancer. Praziquantel treats most cestodes and trematodes; albendazole treats many nematodes and cysticercosis (with steroids).
