Ocular toxoplasmosis

Toxoplasma gondii is a microscopic parasite, and in the eye it is probably the most common cause of infectious retinochoroiditis anywhere in the world. Retinochoroiditis means inflammation of two layers at the back of the eye at once: the retina, which captures light, and the choroid, the layer of blood vessels sitting right behind it that nourishes it.

The parasite gets there from the intestine. In the form called a tachyzoite it crosses the intestinal mucosa and travels through the blood and the lymph to distant organs, and the eye is one of them. It does not reach the brain and the eye on its own: certain cells of the body's own defense system, the dendritic cells and the macrophages, carry it across the blood-brain barrier, the filter that separates the blood from the nervous system.

What shows up at the back of the eye depends on the stage. In the active phase there is necrotizing chorioretinitis, meaning inflammation with tissue destruction, and vitritis, which is inflammation of the gel that fills the eye. In the inactive phase what remains are quiescent chorioretinal scars, with no activity. It leaves a mark on the retina that stays there even after the infection is no longer active.