Definition

• Parasitic worm

Lifecycles

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

Nematodes (roundworms)

GI symptoms

• Pinworm (enterobius vermicularis): perianal itch

• Whipworm, Hookworm, Ascaris

• Strongyloides

• Anisakis

• 
  

Mosquito

Filaria

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

https://www.cdc.gov/parasites/sth/index.html

Trematodes (flukes)

• Lifecycles that include eggs, miracidium, cercariae, adults/metacercaria/mesocercaria

• Examples include: schistosoma (blood), paragominas (lung fluke), fasciolia (liver fluke), Opisthorchis / clonorchis (oriental liver flukes)

• Treatment is with praziquantel or triclabendazole (fascioliasis, paragonimiasis)

Cestodes (tapeworms)

• 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

Vegetation

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

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

Meat

• 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

Fish

• 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

Water

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

Transcutaneous

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.

2. 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

Albendazole/mendazole

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

Triclabendazole

Ivermectin

• Strongyloides

Praziquantel

• 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