Toxoplasmosis is an infection with the protozoan intracellular parasite Toxoplasma gondii. In the eye, Toxoplasma infections frequently cause significant inflammation and subsequent scarring which may temporarily or permanently impair vision.
Ocular toxoplasmosis is usually acquired by a congenital infection, transmitted from the mother to the fetus across the placenta during pregnancy. Typically newly pregnant women transmit the Toxoplasma organism to a fetus. Intrauterine transmission occurs in about 1/3 of pregnancies of acutely infected women. The overall infection rate appears to be about 1%.
The Toxoplasma organism is resides in the intestinal tracts in many animals, particularly cats. Infectious organisms are shed in cat feces, and are introduced into the body by ingestion. Sandboxes and cat litter boxes are likely reservoirs of the infection. Infection risk can be minimized by practicing good hygiene including hand washing (especially before preparing or eating food) and avoiding raw or undercooked meat. It is prudent for pregnant women to avoid handling cats, cat feces, and sandboxes.
More than 80% of newly infected persons experience no symptoms, and are unlikely to be aware of the infection. Symptoms may occur following an incubation period of one to two weeks after exposure and include mild fever, swollen glands, malaise, muscle and/or joint pain, headache, sore throat, and skin rash. The diagnosis can be confirmed by detecting antibodies to Toxoplasma in the blood. Swelling of the liver or spleen may be noted. In rare cases the lungs, brain, liver, or heart may be involved. The condition usually resolves without treatment within a few months.
Immunocompromised patients (including those with AIDS, cancer, or those taking immunosuppressive drugs) are at risk to acquire toxoplasmosis that can become a severe, even fatal, disease.
Most cases are generally self-limited, and rarely require treatment. If involvement of the internal organs is severe, treatment with antibiotics (usually pyrimethamine and a sulfa drug, or clindamycin) is considered. If the infection is recognized during pregnancy, spiramycin may be used to reduce the risk of maternal-fetal transmission.
Most cases of congenital toxoplasmosis are asymptomatic, and initially go unrecognized. Severe cases resemble other acute intrauterine infections such as rubella or cytomegalovirus. Low birth-weight, enlargement of liver or spleen, and jaundice are common. Evidence of retinal infection may be found in 80-90% of known infected babies.
The infection causes inflammation of a small patch of retina which typically spontaneously resolves [See figures 1 and 2]. However the infection sometimes leaves a localized pigmented scar (retina and underlying choroid) which contains the Toxoplasma organism in an inactive, encysted form. The chorioretinal scars are usually visually insignificant unless the scarring process involves the central portion of the retina (the macula).
The chorioretinal scars of congenital ocular toxoplasmosis are generally inert. However, the encysted Toxoplasma organisms can reactivate causing inflammation, pain, redness, sensitivity to light, blurred vision, and increased intraocular pressure. Examination during reactivation reveals a cloud of white blood cells overlying the whitened patch of inflamed retina. In severe cases, the view into the eye is quite cloudy, and the underlying acute inflammation can be only dimly perceived.
Mild cases which do not threaten the central retina (the macula) may resolve without treatment. In more severe cases, the duration of the inflammatory episode can be reduced by treatment with antibiotics. Commonly used antibiotics include pyrimethamine, trisulfapyrimidines, sulfadiazine, clindamycin, and minocycline. Folic acid is used with this regimen to prevent bone marrow suppression. Steroid eyedrops or systemic steroids may further reduce the inflammation.
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