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Toxoplasmosis
And Ocular Toxoplasmosis

Toxoplasmosis (tahks-oh-plaz-MOH-sus) is a common systemic disease that affects approximately one-fourth of the population at some time during their lives. It is caused by tiny one-celled parasitic organisms harbored by some animals, especially cats, and may be acquired from contact with them or by eating rare or poorly cooked beef. The symptoms of infection tend to be like the flu: fever, malaise, cough, and muscle aches lasting a week or so, so the illness is not particularly remarkable.

Most of the time the illness is not serious and is not likely to lean to any eye problems. But it is a different story when a pregnant woman becomes infected. The toxo-organisms tend to attack the growing fetus and sometimes these results in a miscarriage or still birth. Even if the infection in the fetus is mild, there can be some degree of liver, brain, or eye damage.

An infant born with this disease has what is called congenital toxoplasmosis; if the eyes have been involved, it is called ocular toxoplasmosis.  Ocular toxoplasmosis—though it may not be discovered until later in life—is still likely to have been present at birth, acquired from the mother.

No one born with congenital or ocular toxoplasmosis can pass it on to his/her children. Since the mother’s original toxoplasmosis makes her immune to another infection, she cannot pass the disease on to subsequent children.

Ocular Toxoplasmosis

At birth, an infant’s active infection may be healed, though there may have been some scarring inside the eye that will affect his/her vision. The reduced vision is not likely to be discovered until a child has a vision test, usually before beginning school.

The specific effects of ocular toxoplasmosis depend on where in the eye the damage has occurred. Typically, it involves the retina—the light sensitive tissue that lines the back of the eye—and the choroids—the layer of blood vessels and pigment and lies directly under the retina. Vision is most likely to be reduced when the macula—part of the retina responsible for sharp vision—has been scarred.

What is the Course of Ocular Toxoplasmosis?

After the active infection in the fetus has healed, some toxo-organisms remain within the retina inside small cysts. They may lie there, dormant for years, but the cysts can break open at any time and release active organisms. This “reactivation” creates a destructive retinal inflammation (retinitis) or inflammation of the retina and choroids (retino-choroiditis) typically adjacent to a healed scar. Over the next few weeks, the inflammation will most likely progress, but like the earlier infection, even severe inflammations usually heal in a few months. These too, will leave scars.

Over a lifetime, there may be no cycles of activation or very few, followed by quieting. No one can predict when or if a reactivation will occur. But repeated episodes can lead to other eye problems, such as vitreous floaters, glaucoma, or a cataract.

Symptoms of a Reactivation

The symptoms depend on exactly where the flare-up (reactivation) occurs. The most typical symptom is a gradual haziness or blurring of vision in one eye over a period of time. If the site is active retinitis is close to the macula, you may notice a rapid decrease in vision—over a few hours or days. But if a large macular scar has already impaired vision, a decrease in acuity will be less noticeable. If the active site is off to the side, you may notice only some increase in floaters or haze.

If other parts of the eye become involved, the eye may become red and uncomfortable and especially sensitive to bright light or sunlight.

Examination

Your vision will be checked and you will have a refraction test for glasses. Eyeglasses may not improve your vision because the problem is not with the optical parts of the eye. The information about the best level of vision obtainable is important for following the clinical course of this problem. During the exam, your pupils will be dilated (enlarged) with eye drops so that the inside of your eye can be studied with an ophthalmoscope. A special contact lens may be placed on your eye to allow the retina, macula and vitreous to be examined under high magnification with a slit lamp microscope. Different types of photographs may be taken. Pictures of the retina are useful in determining the extent of the problem and for evaluating the progression of scarring.

Treatment

Treatment may not be necessary when the retinal inflammation is mild and is located in the periphery, or when the macula is already scarred. In the case of scarring, even if medication were to halt the active inflammation, vision would not be improved by very much. But when the ocular inflammation is severe or if it is very near the macula so as to threaten vision, oral medications are given to try and safeguard as much vision as possible.

Several drugs are available for treating active flare-ups. These medications are potent and can produce serious side affects—to the blood cells or to the gastrointestinal tract—so they are prescribed with caution. Drugs are frequently given in combination to enhance their effectiveness and minimize side effects, and nutritional supplements may be prescribed to provide even further protection. In some cases, corticosteroids are added to help suppress excessive inflammation. If the front part of the eye is inflamed (uveitis and iritis), eye drop medications will also be prescribed. All pills and medications need to be continued until the eye responds, usually for about 6 weeks or more. You will be informed when you can stop taking the treatment medications.

If the inflammation does not respond as expected, the medications may need to be changed. Sometimes freeze burns (cryopexy) may be applied to the outside of the eye over the inflamed area, to help destroy the toxo-organisms. But this treatment is used only on rare occasions.

Eventually, whether treated or not, the active lesions will quiet down, hopefully before there has been a good deal of ocular damage. Both active and inactive forms of ocular toxoplasmosis need to be followed by regular eye examinations because the complications can be serious and may not show up until years later. Most patients, despite occasional flare-ups, remain relatively trouble fee throughout their lives.