Toxoplasmosis is a widespread parasitic infection caused by the microscopic organism Toxoplasma gondii. This parasite can infect various warm-blooded animals, including humans, affecting up to a third of the world’s population. While many individuals remain without symptoms, the parasite can sometimes cause health issues, particularly when it affects specific organs. One such manifestation is ocular toxoplasmosis, an infection targeting the eye.
Understanding Ocular Toxoplasmosis
Ocular toxoplasmosis is an infection of the eye. It is a leading cause of posterior uveitis, inflammation affecting the back part of the eye. The infection primarily impacts the retina, the light-sensitive tissue at the back of the eye, causing retinitis. The choroid, a layer of blood vessels supplying the retina, can also become inflamed (choroiditis).
The hallmark of ocular toxoplasmosis is necrotizing retinochoroiditis, where the parasite causes tissue death in the retina. This can result in inflammation, scarring, and vision impairment. The disease can be acquired after birth or be present from birth, known as congenital toxoplasmosis. In congenital cases, the disease can be bilateral, affecting both eyes.
How People Contract It
People can contract Toxoplasma gondii through several routes. A common way is by consuming undercooked meat containing parasite tissue cysts. This includes meats like lamb, ground beef, and game. The parasite’s cysts can survive and infect if meat is not cooked thoroughly enough to kill them.
Another route involves accidental ingestion of oocysts, the environmentally resistant form of the parasite. These oocysts are shed in the feces of infected cats, the definitive host for T. gondii. Humans can ingest these oocysts through contact with contaminated soil (e.g., gardening) or by consuming unwashed fruits and vegetables contaminated by soil. Contaminated water can also be a source of infection.
Congenital transmission occurs when the parasite passes from an infected mother to her fetus during pregnancy. If a woman acquires the infection during pregnancy, there is a risk of transmitting it to the developing baby, which can lead to severe complications, including retinal infection. While less common, transmission can also occur through blood transfusions or organ transplants.
Signs and Symptoms to Watch For
The symptoms of ocular toxoplasmosis involve the visual system. Blurred vision is common, making clear sight difficult. Individuals may also notice floaters, small specks or cobweb-like shapes drifting across their vision. These are caused by inflammatory cells in the vitreous humor, the jelly-like substance filling the eye.
Light sensitivity (photophobia) can develop, causing discomfort in bright environments. Some experience eye pain or redness, indicating inflammation within the eye. In more severe cases, particularly if the macula (the central part of the retina responsible for sharp vision) is affected, vision loss can occur. Symptoms may recur over time, often at the margins of old scars.
Diagnosis and Management
Diagnosing ocular toxoplasmosis involves a comprehensive eye examination. An eye care professional will perform a fundoscopy, using an ophthalmoscope to visualize the retina and choroid at the back of the eye. The presence of a white, focal lesion on the retina, often described as a “headlight in the fog” due to overlying vitreous inflammation, is a classic sign of active infection. Older, pigmented retinochoroidal scars adjacent to new lesions are also indicative of previous or recurrent infection.
Blood tests may detect antibodies to Toxoplasma gondii, indicating past exposure. However, these serological tests alone are not definitive for diagnosing active ocular disease due to the high prevalence of past exposure. The clinical appearance of the eye during examination remains the primary diagnostic tool for active ocular toxoplasmosis.
Management strategies for ocular toxoplasmosis involve a combination of medications. Antiparasitic drugs target the Toxoplasma gondii parasite directly. Common medications include pyrimethamine, sulfadiazide, and clindamycin, sometimes combined with folinic acid to mitigate side effects. Trimethoprim-sulfamethoxazole has also shown effectiveness.
Corticosteroids, such as prednisone, may be prescribed to reduce inflammation caused by the infection. These are started a few days after antiparasitic therapy begins to prevent the parasite from multiplying unchecked while inflammation is suppressed. Treatment plans are individualized based on lesion size, location, and severity. Regular monitoring is necessary to assess treatment response and detect recurrences. Long-term prophylactic treatment with certain medications may be considered to reduce the risk of future recurrences.
Preventing Ocular Toxoplasmosis
Preventing ocular toxoplasmosis involves measures to reduce exposure to the Toxoplasma gondii parasite. Thoroughly cooking meat is a primary prevention strategy. Meat should be cooked to safe internal temperatures to kill any tissue cysts. Using a food thermometer can ensure proper cooking.
Good hygiene practices are important, particularly after handling raw meat or gardening. Washing hands thoroughly with soap and water after these activities removes any parasite oocysts or cysts. Similarly, washing fruits and vegetables before consumption removes any potential contamination from soil.
For individuals with cats, managing litter boxes carefully can reduce exposure. Changing cat litter daily helps prevent oocysts from becoming infectious, as they need 1 to 5 days to sporulate. Pregnant individuals or those with weakened immune systems should avoid changing cat litter altogether, or wear gloves and wash hands immediately afterward. Avoiding contact with stray cats can also limit exposure, as these animals may be shedding the parasite.