Nematodes

Metazoa

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

Toxocariasis

• 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

Onchocerciasis

• 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