Toxoplasma gondii is a protozoan (single cell organism) that lives within other cells (parasite). The definitive host (where organism can reporduce) is a cat, but other organisms, including humans, can contract the disease. In humans, there are two types of disease that affect the eyes, congenital and acquired toxoplasmosis. Toxoplasmosis is the most common cause of posterior uveitis, which is inflammation in the back part of the eye. Twenty to seventy percent of US adults are seropositive, indicating past exposure to the organism.
This is the most common type of ocular toxoplasmosis seen in ophthalmology clinics. It is due to maternal infection while pregnant. Depending on the trimester of pregnancy, multiple systemic effects can be seen. This often is seen as bilateral retinal scars, often involving central vision.
This form may be more common than originally thought. Many postnatal toxoplasmosis infections are asymptomatic and rarely result in visual problems (estimated at 3%). Exposure to oocysts from cats or through ingestion of raw meat can lead to acquired disease. This often is seen on a routine eye exam as a retinal scar in one eye.
Symptoms vary, but usually consist of unilateral floaters or blurred vision when the disease becomes active. Inactive disease rarely causes visual symptoms unless scarring is near the central retina or macula.
Toxoplasmosis causes a necrotizing (tissue-destroying) retinitis, most commonly in posterior pole. Often, there will be an old scar with an area of reactivation at the edge of the scar. This is seen as a focal retinitis, vitreous cells, and vasculitis.
|Fundus photograph of a patient with active toxoplasmosis adjacent to an old scar. The active disease below the scar appears white with indistinct borders due to retinal tissue edema (swelling) and destruction. The scar is the pigmented area at the edge of the photo. Inflammation in the vitreous causes an overall clouding of vision as well.|
Recent advances have taught us that there are at least 3 strain types. Antigenically, all strains are indentical. Which ones cause ocular disease are not known at this time.
In cases of suspected toxoplasmosis, we will have blood drawn to look for antibodies to Toxoplasma gondii. All positive dilutions are considered significant if a patient has fundus lesions compatible with toxoplasmosis. Some laboratories have newer technology such as the polymerase chain reaction (PCR), that has a 60% sensitivity for active toxoplasmosis when fluid is drawn from the anterior chamber of the eye or vitreous.
Treatment is not always indicated and does not result in the elimination of the organism from the eye. It is felt that the cyst form of the organism persists for life and can reactivate later. Certain lesions, such as macular threatening lesions, or lesions between the macula and nerve have a high chance of causing visual loss and are almost always treated. Those associated with severe inflammation (vitritis) and any lesion in patients with AIDS or other immune deficient state are treated as well. Small, peripheral lesions that do not threaten central vision may often be observed.
While there is no consensus on the best treatment, there is agreement that treatment should last a minimum of six weeks. The classic treatment is a triple drug regimen consisting of sulfadiazine, pyramethamine (Daraprim), and folinic acid. However, side effects are common and blood tests have to be performed to monitor for toxicity. Common side effects of some of these medicines are listed below
- Sulfadiazine - Stevens-Johnson syndrome, kidney stones and bone marrow suppression
- Pyramethamine - Bone marrow depression, especially platelets
- Clindamycin - Pseudomembranous colitis (Treatment: vancomycin)
Because of this, newer alternatives include clindamycin, minocycline, atovaquone (Mepron), Bactrim DS, and azithromycin. We often use Bactrim DS twice a day because of its easy dosing and good safety profile.
Oral corticosteroids are often used after the antibiotics have been started to reduce inflammation. Anywhere from 20-100 mg a day are prescribed based on body weight and severity of inflammation. There are multiple potential side effects of steroid use, but predominantly in patients with long-term use. Side effects of oral prednisone may include the following:
- Eyes - Glaucoma, cataracts
- Systemic - Osteoporosis, weight gain, nervousness
Ocular toxoplasmosis is a relatively common cause of intraocular inflammation. Visual acuity can be unaffected to severely affected, depending on the area and severity of involvement. Because recurrences can be seen, the goal of any treatment is to control the infection and limit the resulting scarring. This should help limit the amount of permanent visual loss.