• Multicellular organisms with organ systems
  • Aka helminths
  • Divided into nematodes and platyhelminths


  • Toxocara canis is a ubiquitous canine parasite transmitted by dogs and cats
  • Helminth 
  • Eggs are secreted in the faeces
  • Larvae migrate through the GI tract and disseminate via the bloodstream (eg to the eye). Systemic disease is rarely fatal.

    • The larvae themselves are not inflammatory but when they die, the inflammatory response is triggered

  • Infection triggers an IgE-mediated immune response with eosinophilia
  • Appears as a large inferior white intraocular vitreoretinal mass (leucocoria)
  • Can manifest as a low-grade fibrous reaction or can be rapid retinal inflammation with vitritis and pars planitis

  • Tractional retinal detachment may occur
  • Associated with ‘dragging’ of the optic disc
    • But no intraocular calcification as seen in retinoblastoma 
  • Puppies and lactating bitches are the main source of infective ova (in adult dogs, most larvae become encysted in a dormant state and do not mature therefore are not transmitted)

  • Treatment: tiabendazole or albendazole. Steroids to reduce inflammation


  • Onchocerca volvulus of the filariasis subdivision of nematodes
  • River blindness
  • Simulium blackfly acts as a vector and breeds in fast flowing rivers
  • Blackfly bite transfers infective larvae to the human host
  • Adult worms are only found in humans
  • They sequester in collagenous nodules and can survive in the skin for 30 months
  • Ocular infection occurs via subcutaneous migration of worms
  • Anterior and posterior segment inflammation with sclerosing keratitis, chorioretinitis and optic neuritis, depigmentation, pruritis and “hanging groin”

    • Secondary glaucoma and cataract
  • Stimulates a low-grade immunosuppressive response which facilitates the longevity of the adult and larval worms to perpetuate the life cycle

  • The filariae also harbour Rickettsia-like bacteria known as Wolbachia and are thought to mediate the immunopathology associated with ocular onchocerciasis

    • Susceptible to doxycycline
  • Mazzotti skin test detects infection
  • Ivermectin can be used for treatment

Loa loa

  • Chrysops flies are vectors
  • Can live for 17 years in humans and migrate through subcutaneous tissues
  • High eosinophilia is strongly suggestive
  • Calabar swellings are pathognomonic
  • Worms may be seen traversing the conjunctiva
  • Diagnosed by finding microfilariae in the blood

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