• Parasitic worm


Host: a larger organism that hosts a smaller organism

  • Definitive host: adult stage (reproduction)

  • Intermediate host: immature stage

    • [ păr′ə-tĕn′ĭk ] n. An intermediate host whose presence may be required for the completion of a parasite's life cycle but in which no development of the parasite occurs.

  • Dead-end, incidental, or accidental host - an organism that generally does not allow transmission to the definitive host, thereby preventing the parasite from completing its development.

  • Reservoir:

  • Mechanical transmitted:

  • Amplifying host:

Eggs, cysts, larva, worms

All have larval stage

GI symptoms

  • Pinworm (enterobius vermicularis): perianal itch

  • Whipworm, Hookworm, Ascaris

  • Strongyloides

  • Anisakis



Guinea worm

Soil-transmitted helminths, top 3:

Soil-transmitted helminths refer to the intestinal worms infecting humans that are transmitted through contaminated soil:

  • Ascaris (RW)

  • Trichuris trichura (WW)

  • Hookworm (HW); Anclostoma duodenale and Necator americanus


  • Fish (diphyllobothriasis nihonkaiense)

  • Beef: taeniasis saginata

  • Pork: taenia solium (cysticercosis)

  • Dog: (echinococcosis), Hydatid cyst

Treatment is with praziquantel

Helminths by transmission route

Oral ingestion

  1. Faecal oral

  • Ingestion of embryonated egg (can be autoinfection): Enterobius (Pinworm)

  • Ingestion of embryonated egg in soil: Trichuris (Whipworm)

  • Ingestion of embryonated egg in soil: Ascaris lumbricoides

  • Eggs in dog/cat faeces: toxocara canis/cati

  • Embryonated egg in dog faeces: Echinococcus granulosus


  • Larvae (gastropod) in snails or contaminated produce: angiostrongylus cantonensis

  • Metacercariae on vegetation (e.g. water cress): fasciola spp.


  • Cysts in meat (pigs, bears): Trichinella spiralis

  • Cysticerci in raw or undercooked meat (Pork): intestinal Taeniasis, ingestion of embryonated eggs in faeces and passed into environment: cysticercosis

  • Cysticerci in raw or undercooked beef: Taenia saginata


  • Larva in fish: Anisakis simplex

  • Larvae in raw food (fish/frogs): gnathostoma spinigerum

  • Metacercariea in flesh or skin of freshwater fish: chlonorchis sinensis / opisthorchis

  • Metacercariae in crustaceans: paragonimus

  • Plerocercoids in infected fish: Diphylobothryium

  • Echinococcus multilocularis


  • Drinking water containing copepods with L3 larvae: dracunculus medinensis (guinea worm)


  1. Parasite burrows through the skin

  • Filariform larva in soil penetrates skin: Hookworm

  • Filariform larva in soil penetrates skin: Strongyloides stercoralis

  • Cercariae in fresh water penetrate skin: Schistosoma spp.

  1. Insect vector

  • Mosquito, larvae enter skin: Wuchereria bancrofti, brugia malayi/timori (lymphatic filariasis)

  • Midges or black fly: mansonella

  • Black fly (genus Simulium), larvae enter bite wound: Onchocerca volvulus

  • Chrysops fly, larvae enter bite wound: loa loa

By treatment


  • GI nematodes (pinworm, whipworm, hookworm, ascaris), not strongyloides



  • Strongyloides


  • Trematodes (schistosomiasis, clonorchis, ), except

Top 3: STHs (hookworm, ascaris, trichuris).... why hookworm has thin wall? ascaris needs to survive outside for >6 (egg is infective) weeks to become infective whereas hookworm will hatch larvae in around 24hrs, as the larvae is infective

Schistosoma, taenia