Ocular toxoplasmosis is an eye infection caused by the microscopic parasite Toxoplasma gondii. It often leads to inflammation of the retina and choroid, the vascular layer beneath the retina. This condition is a common cause of posterior uveitis worldwide and can lead to various vision problems. Globally, approximately 25% to 30% of the population carries the Toxoplasma parasite, making it a widespread foodborne parasitic infection.
How Ocular Toxoplasmosis Develops
Toxoplasma gondii enters the human body through two main pathways: congenital transmission or acquired infection. Congenital transmission occurs when an infected mother passes the parasite to her unborn child during pregnancy. This can lead to ocular involvement in the developing fetus, with symptoms appearing years after birth.
Acquired infection happens through the ingestion of parasite oocysts, commonly found in contaminated soil, water, or cat feces. Consuming undercooked or raw meat, particularly pork, lamb, or venison, is another source, as it may contain tissue cysts of the parasite.
Once ingested, the parasite travels through the bloodstream and can localize in various tissues, including the eye. Within the eye, the parasite proliferates inside host cells, leading to their rupture and invasion of neighboring cells. This initiates an inflammatory response, primarily affecting the retina (retinitis) and secondarily involving the choroid (choroiditis).
The host’s immune response can convert the rapidly multiplying tachyzoites into inactive bradyzoites, which form cysts that can remain dormant. These cysts can rupture later, releasing organisms that reactivate the infection and cause new lesions or inflammation near old scars.
Recognizing Ocular Toxoplasmosis
Individuals with ocular toxoplasmosis report various symptoms. Blurred vision is common, along with floaters, which are dark specks that drift across the field of vision. Eye pain, redness, and increased sensitivity to light (photophobia) are also experienced.
A sudden decrease in vision may occur, especially if the infection impacts the macula, the central part of the retina responsible for sharp, detailed central vision. The inflammation in the eye, known as uveitis, can lead to discomfort, including headaches and, less commonly, fever. These symptoms can manifest years after the initial infection, particularly in cases of congenital toxoplasmosis or when dormant cysts reactivate.
Medical Approaches to Ocular Toxoplasmosis
Diagnosing ocular toxoplasmosis involves a comprehensive eye examination. The eye care provider looks for characteristic signs of infection, such as inflammation, retinal scarring, or active lesions. Fundoscopy, which examines the back of the eye, is a common procedure to visualize these lesions and assess retinal and choroidal involvement.
Additional imaging tests, such as optical coherence tomography (OCT) or fundus photography, may be used to provide clearer images of affected retinal areas. While diagnosis is often based on the distinctive appearance of eye lesions, blood tests can detect antibodies to Toxoplasma gondii, indicating past or present exposure. PCR analysis of aqueous or vitreous samples from the eye can confirm parasite DNA.
Treatment for ocular toxoplasmosis involves medications to control the infection and reduce inflammation. Antiparasitic medications like pyrimethamine and sulfadiazine are prescribed to target the Toxoplasma gondii parasite. Corticosteroids may also be used to manage inflammation and minimize damage to the retina and optic disc caused by the immune response. The goal of treatment is to arrest the parasite’s activity during the acute phase of retinochoroiditis and preserve vision. While treatment can manage the active infection, it may not completely eradicate the parasite, meaning dormant cysts can remain.
Preventing and Living with Ocular Toxoplasmosis
Preventing Toxoplasma gondii infection involves adopting hygiene and food safety practices. Thoroughly cooking meat, especially pork, lamb, and venison, to safe temperatures destroys potential tissue cysts. Washing hands with soap and water after handling raw meat, gardening, or touching soil is also recommended. For cat owners, regularly cleaning litter boxes and washing hands after handling cat feces helps reduce exposure to oocysts.
For those who have experienced ocular toxoplasmosis, long-term management focuses on monitoring for potential recurrences and addressing lingering vision impairment. Regular eye check-ups with an eye care professional are advised to detect signs of reactivation or new lesions promptly. Even after an active infection resolves, scarring in the retina can occur, which may affect vision depending on its location and size. If vision impairment persists, strategies such as low-vision aids or rehabilitation may be explored to help individuals manage their daily activities.