The microscopic parasite Toxoplasma gondii causes an infection known as toxoplasmosis. While it is one of the most common human infections and is often asymptomatic or causes mild flu-like symptoms, it can lead to significant health problems. A more serious form of the disease occurs when the parasite invades the eye, a condition that can cause potential vision complications.
How Toxoplasmosis Affects the Eye
When Toxoplasma gondii infects the eye, it causes a condition called ocular toxoplasmosis, a form of retinochoroiditis. The parasite travels to the retina, the light-sensitive tissue lining the back of the eye, and forms dormant tissue cysts. These cysts can remain inactive for extended periods, sometimes for the host’s entire life.
Damage to the eye occurs when a cyst ruptures and releases active parasites called tachyzoites. These parasites invade and destroy neighboring retinal cells, triggering a strong immune response. This reaction causes intense inflammation in the retina and the underlying choroid, a layer of blood vessels that nourishes it. The resulting inflammation is the direct cause of vision problems and the lesions seen during an eye exam.
Symptoms of an Eye Infection
Symptoms of ocular toxoplasmosis are caused by inflammation and damage to the retina and vitreous, the gel-like substance filling the eye. Common symptoms include:
- Floaters, which are dark spots drifting through the field of vision caused by inflammatory cells.
- Blurred vision, resulting from inflammation of the macula or general haziness in the eye.
- Eye pain, which can be a dull ache or a sharper pain that worsens with eye movement.
- Photophobia, or sensitivity to light, making bright environments uncomfortable.
- Redness in the affected eye.
These symptoms usually affect only one eye. The severity varies depending on the location and size of the inflammatory lesion. If the inflammation is near the center of the retina or the optic nerve, the impact on vision can be significant. Mild inflammation in the periphery of the retina may cause only minor symptoms that go unnoticed.
Causes and Diagnosis
A person can become infected with Toxoplasma gondii through several routes. The most common is consuming undercooked or raw meat, such as pork or lamb, containing the parasite’s tissue cysts. Another major route is accidentally ingesting oocysts, a form of the parasite shed in cat feces. This can happen through direct contact with a litter box or indirect contact with contaminated soil or water. The infection can also be passed from a mother to her child during pregnancy.
An ophthalmologist diagnoses ocular toxoplasmosis through a detailed eye examination. After dilating the pupils, the doctor views the retina and looks for the classic sign of an active infection: a white, focal area of retinal inflammation often described as a “headlight in the fog.” The presence of an old, pigmented scar next to a new area of inflammation is also a strong indicator of the disease.
Blood tests can detect antibodies, confirming exposure to T. gondii. A positive test shows a person has been infected at some point but does not prove the eye symptoms are from toxoplasmosis. However, a negative antibody test can effectively rule out the diagnosis in individuals with a healthy immune system. In difficult cases, a doctor may test fluid from the eye for the parasite’s DNA using a polymerase chain reaction (PCR) test.
Medical Management and Treatment
The goal of treatment is to stop the parasite from multiplying and reduce inflammation, minimizing harm to the retina and optic nerve. Treatment is not always necessary for small lesions in the periphery of the retina that do not threaten central vision. In these instances, the infection may resolve on its own.
When treatment is required for large lesions or those near important structures, doctors prescribe a combination of medications. The “classic triple therapy” includes the antiparasitic drug pyrimethamine and the antibiotic sulfadiazine to halt the parasite’s proliferation. Because pyrimethamine can interfere with folic acid use, patients are also given leucovorin to manage this side effect.
Corticosteroids like prednisone are also used to control inflammation. Steroids are started shortly after antimicrobial therapy begins to ensure the parasites are under control before suppressing the immune response. Treatment lasts about four to six weeks, and other antibiotics like clindamycin or azithromycin may also be used.
Prognosis and Recurrence
The long-term outlook depends on the infection’s location and severity. While treatment stops the active infection, the healing process leaves a permanent, non-functional scar on the retina. This scar creates a permanent blind spot (scotoma). A scar on the periphery of the retina may go unnoticed, but one on the macula can cause a permanent loss of central vision.
Ocular toxoplasmosis has a high rate of recurrence. Dormant parasite cysts near retinal scars can reactivate at any time, triggering a new episode of inflammation, often next to a previous scar. Reactivation can occur years or even decades after the initial infection for reasons that are not fully understood.
Each recurrence risks further retinal damage and vision loss. Individuals with a history of the disease should have long-term follow-up with an eye care specialist. Prompt recognition and treatment of a recurrence are necessary to preserve as much vision as possible.