Protozoa

Parasites

• Divided into subkingdoms
• Metazoa (see below)
• Protozoa: unicellular organisms

Acanthamoeba

• Free-living, ubiquitous protozoan

• Single celled eukaryotic microorganism and “amoeba”: motile eukaryotes that alter their shape and crawl using pseudopods

• A. castellani and A. polyphaga are the most common subtypes

• Associated with contact lens wear, ocular trauma and exposure to high levels of protozoa in water supplies (soil, air, swimming pools, hot tubs, tap water)

• Biphasic life cycle

• Active trophozoite: during this phase, ingests bacteria and fungi. These can replicate within the acanthamoeba and be released within the cornea as “trojan horses” (eg. Legionella, Pseudomonas, mycobacteria, chlamydia).

• Inert dormant cyst form. Extremely resistant to environmental agents and the immune system. 

• Survives chlorinated water.
• The organism can convert between these states eg. convert to a cyst when under duress
• Virulence factors
• Mannose-binding protein
• Mannose-induced protein which has collagenase and cytopathic activity
• Attaches to epithelial surface 
• Collagenases cause stromal changes
• Clinical features
• Presents with classic symptoms of infectious keratitis
• Protracted clinical course and failed therapy 

• Initial findings may be non-specific: punctate epithelial erosions, pseudodendrites, mild conjunctival injection

• Ring infiltrates
• Perineural infiltrates (radial keratoneuritis)
• Progressive corneal thinning and perforation
• Corneal neovascularisation is rare despite the chronicity of the infection
• Diagnosis
• Visualisation with in-vivo confocal microscopy: spindle shaped trophozoites

• Charcoal yeast agar
• PCR: sensitivity and specificity of 90%

• Corneal scrapes stained with: acridine Orange, calcofluor white, PAS, wright-giemsa, Wheatley-trichrome

• Corneal biopsy

• Management: 

• Topical chlorhexidine (a biguanide which disrupt DNA) and a diamidine which disrupt cell membranes (propamidine isetionate eg. Brolene or hexamidine)

• Polyhexamethylene biguanide can also be considered (PHMB)
• Econazole 
• Additional cover with broad spectrum antibiotics

Toxoplasmosis (Toxoplasma gondii)

• Obligate intracellular protozoan
• Most common cause of infectious retinochoroiditis worldwide

• Mammals eg. farm animals and pets: cats are the definitive hosts as they support the sexual stage of the parasite in their intestinal epithelium

• Oocysts are excreted in faeces and spore in soil
• Eggs become infective after ingestion by intermediate host eg. humans
• Transmission occurs via
• Ingesting food contaminated with cat faeces
• Undercooked meat

• Maternal transmission especially if infected during the third trimester (however disease is more severe if infection occurs in first trimester)

• NB: only antibody-negative mothers are at risk of vertical transmission

• Toxo proteins activate toll-like receptors on monocytes and dendritic cells to produce IL-2 and an indirect T cell response

• The conoid facilitates cell penetration
• In the cell, the microbe resides in a vacuole resistant to lysosomal destruction

• The parasite survives best in cerebral neurons hence the brain and retina are the predominantly affected tissues

• Uveal and retinal inflammation
• Uveitis: usually self-limiting in immunocompetent patients

• Retinochoroiditis (can replicate in Muller cells and RPE): the retina is affected in preference to the choroid due to its lower immunoglobulin levels