Toxocariasis refers to an infection caused by roundworm parasites, primarily Toxocara canis from dogs and Toxocara cati from cats. This condition is classified as a zoonotic disease, meaning it can transmit naturally from animals to humans. Humans become infected when they accidentally ingest eggs shed in the feces of infected animals. While the larvae can migrate and affect various organs throughout the body, the eyes represent a particularly significant site of infection, especially in children. Ocular involvement can lead to notable vision impairment.
Specific Eye Manifestations
Ocular Toxocariasis (OT), also known as Ocular Larva Migrans (OLM), occurs when Toxocara larvae invade the human eye. This often results in unilateral vision loss or decreased vision, a symptom reported by over 80% of patients and affecting one eye in approximately 90% of cases. Another common sign is strabismus, or misaligned eyes, observed in more than a quarter of affected individuals. A distinct white pupil reflex, known as leukocoria, can also be present in around 15% of patients.
The presence of the migrating larva triggers an immune response, leading to inflammation within the eye. This inflammation can manifest as uveitis, particularly posterior uveitis, or more severe endophthalmitis, which occurs in roughly 25% of cases. Retinal detachment, where the light-sensitive tissue at the back of the eye pulls away, is also a serious complication, occurring in over a quarter of cases. Granulomas, which are yellowish or whitish inflammatory masses, frequently form in the retina or vitreous humor. Less commonly, patients might experience eye pain or redness.
Diagnosis of Ocular Toxocariasis
Diagnosing Ocular Larva Migrans presents challenges because its symptoms can resemble other eye conditions, and the parasite itself is rarely observed directly within the eye. A thorough eye examination by an ophthalmologist is a primary step, utilizing specialized tools like an ophthalmoscope to identify characteristic lesions such as retinal granulomas or signs of inflammation, including vitritis, which is seen in over 90% of patients. Imaging studies, including optical coherence tomography (OCT), ultrasound biomicroscopy (UBM), scanning laser ophthalmoscopy, and fundus fluorescein angiography, can also provide detailed views of internal eye structures to aid diagnosis.
Blood tests, specifically serologic tests like Enzyme-Linked Immunosorbent Assay (ELISA), are used to detect anti-Toxocara antibodies. A positive ELISA indicates exposure to the parasite, but it does not definitively confirm active ocular disease, as many individuals may have been exposed without developing eye symptoms. Clinical suspicion remains an important factor, particularly in patients with a history of exposure to puppies or kittens, or in children with geophagia, the ingestion of soil.
Treatment of Ocular Toxocariasis
Treatment for Ocular Larva Migrans primarily focuses on managing inflammation and preventing further eye damage, as directly eliminating the encysted parasite within the eye is often complex. Corticosteroids are the main form of anti-inflammatory medication used. They are administered orally, as topical eye drops, or through injections around or into the eye, to reduce inflammation and minimize vision loss. These medications help to control the immune response triggered by the parasite.
Anthelmintic drugs, such as albendazole or mebendazole, are sometimes prescribed. However, their effectiveness in treating OLM is considered limited, especially once the larva has become encysted within the eye tissues, as they primarily target migrating larvae in other bodily tissues. Nevertheless, combining anthelmintic therapy with corticosteroids has shown favorable outcomes in some studies, contributing to reduced recurrence of inflammation. In cases involving severe complications like retinal detachment or dense vitreous opacities, surgical interventions such as vitrectomy may be necessary. Laser photocoagulation or cryosurgery are also options.
Preventing Toxocariasis Infection
Preventing toxocariasis infection involves several practical measures to reduce exposure to the parasite, thereby lowering the risk of ocular involvement. Regular deworming of pets, particularly puppies and kittens, is a primary preventative strategy. Veterinarians often recommend a consistent deworming schedule for young animals.
Maintaining good hygiene practices is also important. This includes thorough handwashing with soap and water, especially after handling pets, playing outdoors in soil, or before preparing and consuming food. Prompt and proper disposal of pet feces helps to minimize environmental contamination with Toxocara eggs. Avoiding contact with and ingestion of contaminated soil, a behavior sometimes seen in young children known as geophagia, is another preventative action; covering sandboxes when not in use can also reduce contamination risk. Children are generally at a higher risk of infection due to their play habits and increased likelihood of contact with contaminated environments.