Common Parasitic Infections of the Eye
Parasitic infections of the eye, although less common than bacterial, viral, or fungal infections, can cause devastating visual impairment and blindness if not diagnosed and managed promptly. These conditions are particularly significant in tropical and developing countries, where environmental, socioeconomic, and healthcare factors contribute to their prevalence. Parasites affecting the eye include protozoa, helminths, and ectoparasites. The most important ocular parasitic infections are Acanthamoeba keratitis, Onchocerciasis, Toxoplasmosis, Toxocariasis, and Malaria-related eye disease. This unit explores each of these conditions in detail, covering etiology, pathogenesis, clinical features, diagnosis, treatment, and preventive strategies.
1. Acanthamoeba Keratitis
Acanthamoeba keratitis (AK) is a rare but serious corneal infection caused by Acanthamoeba, a free-living protozoan found in soil, water, and air. It is most commonly associated with contact lens wear, especially when lenses are improperly cleaned or stored in contaminated water.
Etiology and Risk Factors
- Use of contact lenses, particularly soft lenses
- Exposure to contaminated water (swimming, hot tubs, tap water rinsing of lenses)
- Corneal trauma followed by exposure to contaminated material
Pathogenesis
The trophozoite stage of Acanthamoeba invades the corneal epithelium and stroma, leading to intense inflammation. The organism can transform into a cyst form, which is highly resistant to treatment and environmental stress, making eradication difficult.
Clinical Features
- Severe ocular pain disproportionate to clinical signs
- Redness, tearing, photophobia
- Ring-shaped stromal infiltrate (a hallmark feature in advanced cases)
- Recurrent epithelial breakdown and persistent keratitis
Diagnosis
- Corneal scraping and culture on non-nutrient agar with E. coli overlay
- Confocal microscopy – visualization of cysts and trophozoites
- PCR-based methods for rapid confirmation
Treatment
Combination therapy with topical biguanides (polyhexamethylene biguanide, chlorhexidine) and diamidines (propamidine isethionate). Oral antifungals or azoles may be used in resistant cases. Severe infections may require keratoplasty.
Prevention
- Proper contact lens hygiene – no tap water rinsing, use sterile solutions
- Avoid swimming or bathing with lenses
- Early reporting of symptoms in contact lens wearers
2. Onchocerciasis (River Blindness)
Onchocerciasis is a parasitic infection caused by the filarial worm Onchocerca volvulus, transmitted by blackfly (Simulium) bites. It is a major cause of preventable blindness in endemic areas of Africa, Latin America, and Yemen.
Life Cycle and Pathogenesis
The blackfly deposits larvae into human skin. These mature into adult worms, which produce microfilariae that migrate through the skin and eyes. The death of microfilariae induces intense inflammatory reactions, leading to ocular damage.
Ocular Manifestations
- Punctate keratitis (“snowflake opacities”)
- Sclerosing keratitis – progressive corneal opacification
- Chorioretinitis, optic atrophy
- Visual field loss and blindness (“river blindness”)
Diagnosis
- Skin snip test for microfilariae
- Slit lamp examination for motile microfilariae
- Serological and molecular tests
Treatment
Ivermectin (given annually or biannually) is the drug of choice. It kills microfilariae and reduces transmission. Doxycycline targeting Wolbachia endosymbionts may also be used.
Prevention and Control
- Mass drug administration (MDA) programs with ivermectin
- Vector control (blackfly breeding site reduction)
- Health education in endemic regions
3. Ocular Toxoplasmosis
Caused by the protozoan Toxoplasma gondii, ocular toxoplasmosis is the most common cause of infectious posterior uveitis worldwide. Infection may occur congenitally (transplacental) or postnatally (ingestion of oocysts in contaminated food or water).
Pathogenesis
Tachyzoites disseminate and encyst within retinal tissue. Reactivation of latent cysts leads to recurrent necrotizing retinochoroiditis, a hallmark of the disease.
Clinical Features
- Focal necrotizing retinochoroiditis (“headlight in the fog” appearance – bright lesion amidst vitritis)
- Recurrent episodes of posterior uveitis
- Visual impairment from macular involvement or optic nerve damage
Diagnosis
- Clinical presentation is diagnostic in most cases
- Serology: IgG and IgM antibodies
- PCR of aqueous or vitreous samples in atypical cases
Treatment
Triple therapy: Pyrimethamine + Sulfadiazine + Folinic acid, often combined with corticosteroids. Alternatives include clindamycin or atovaquone. Intravitreal clindamycin may be used in refractory cases.
Prevention
- Avoid undercooked meat
- Practice hand hygiene after handling raw meat or soil
- Pregnant women should avoid cat litter exposure
4. Ocular Toxocariasis
Ocular toxocariasis results from human infection with the larvae of Toxocara canis or Toxocara cati (dog and cat roundworms). Children are most commonly affected due to soil contamination with animal feces.
Clinical Features
- Unilateral vision loss
- Granulomatous inflammation of the retina or optic disc
- Endophthalmitis-like picture
- Leukocoria (white pupillary reflex), mimicking retinoblastoma
Diagnosis
- Ophthalmoscopic visualization of granulomas
- ELISA for Toxocara antibodies
- Ultrasound B-scan in opaque media
Treatment
Corticosteroids to reduce inflammation. Antihelminthics like albendazole or mebendazole may be used in active disease. Vitrectomy may be indicated in severe vitreous opacities or retinal detachment.
Prevention
- Deworming of pet dogs and cats
- Public health measures to reduce soil contamination
- Educating children on handwashing after playing outdoors
5. Malaria-related Eye Disease
Malaria, caused by Plasmodium species (particularly P. falciparum), may cause a range of ocular manifestations, especially in cerebral malaria.
Ocular Manifestations
- Retinal whitening, vessel changes, and hemorrhages
- Papilledema in cerebral malaria
- Retinopathy strongly correlates with disease severity and outcome
- Occasional involvement of the anterior segment (uveitis, keratitis)
Diagnosis
- Blood smear for malarial parasites
- Rapid antigen detection tests
- Ophthalmic examination for malarial retinopathy
Treatment
Antimalarial drugs (artemisinin-based combination therapies for P. falciparum). Supportive therapy for cerebral malaria. Ocular complications are managed symptomatically.
Prevention
- Vector control (mosquito nets, insecticides)
- Antimalarial prophylaxis in endemic regions
- Public health campaigns for awareness
Conclusion
Parasitic infections of the eye, though less common than bacterial, viral, or fungal causes, remain a significant cause of visual morbidity worldwide. Acanthamoeba keratitis primarily threatens contact lens wearers, Onchocerciasis continues to blind millions in endemic regions, Toxoplasmosis is a major cause of infectious posterior uveitis, Toxocariasis often affects children, and Malaria can lead to blinding retinopathy in severe cases. Timely diagnosis, appropriate therapy, preventive measures, and global public health initiatives are essential to reduce the burden of these diseases.
