What is ocular toxoplasmosis?

Ocular toxoplasmosis is an inflammation of the eye caused by an infection of the retina by a parasite called Toxoplasma gondii. The retina is a thin layer of delicate nerve tissue that lines the inside wall of the eye like the film in a camera. In the eye, light is focused onto the retina, which “takes the picture” of objects you look at and sends the image to the brain.

What causes ocular toxoplasmosis?

Toxoplasma gondii infects humans and animals throughout the world. Twenty to fifty percent of adults in the United States test positive for exposure, but few people experience symptoms. The most common presentation resembles mononucleosis with symptoms of fever, fatigue, and swollen lymph glands. Because the toxoplasma organism can become inactive and form a protective microscopic cyst within human cells, it can cause relapses of infection and spread to new areas of the body months to years after the initial infection. If the parasite spreads through the blood stream to the eye, ocular toxoplasmosis may threaten blindness.

How is toxoplasmosis contracted?

The most common ways to contract infection are as follows: 1.) Hand-to-mouth transmission of the toxoplasma organism in cat feces e.g. cleaning the litter box. 2.) Eating undercooked meat, chicken, or eggs from infected animals. 3.) Eating unwashed fruits and vegetables that are contaminated by animal feces. 4.) Hand-to-mouth transmission from handling infected raw meat or animal products. 5.) Transmission from infected mother to child through the placenta during pregnancy. 6.) Rarely, Toxoplasma infection may acquired through contaminated drinking water or dust in the air.

Toxoplasma gondii completes its life cycle by producing millions of oocysts (eggs) in the intestines of the cat. These infectious eggs leave the cat in the feces and may lie dormant in the ground for up to two years. These eggs may infect an animal that may eat from the ground or a human who eats unwashed, contaminated fruits and vegetables from the ground. Once inside a human or animal, the eggs “hatch”, multiply, and spread throughout the body. When the immune system attacks the parasite, it becomes dormant and “hibernates” inside cells throughout the body. In months or years in the future, it may become active to cause infection again.

What are the symptoms of ocular toxoplasmosis?

If toxoplasma reaches the eye, early symptoms include seeing new tiny floating spots (floaters), pain, redness, tearing, light sensitivity, and blurred vision. Late symptoms include permanent blind spots in the vision and, rarely, blindness. These symptoms are not specific for ocular toxoplasmosis, but they are especially important to recognize in people with known toxoplasma scars in the retina. If such symptoms are promptly reported to the doctor, treatment may minimize permanent damage to the eye.

What treatment is available?

Sulfa antibiotics (Septra or Bactrim) are frequently used for several weeks to treat active infection. In some cases of toxoplasmosis, another antibiotic called clindamycin is used. This medicine may rarely cause a severe bloody diarrhea, which should be reported to the doctor immediately for effective treatment. Prednisone pills and similar eyedrops may be used to decrease inflammation. Rarely, antibiotics may be injected into the eye.  Often, the pupil of the eye is dilated with eyedrops during the active infection to prevent scarring and to relieve pain.

It is not uncommon for untreated ocular toxoplasmosis to result in loss of vision. It is rare to experience serious side effects from medication used to treat toxoplasmosis. Armed with the knowledge to report side effects promptly to the doctor, you can minimize the chance of suffering from any permanent ill-effects from treatment.

Finally, notify your doctor if you had a toxoplasma infection in the past as it may be important to avoid intraocular steroid injections to prevent a return of infection.

By Scott E. Pautler, MD

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