OHS is a major cause of visual impairment in the eastern and central United States where 90 percent of adults have been exposed to histoplasma capsulatum. This common fungus is found in molds from soil enriched with bat, chicken or starling droppings and yeasts from animals.

Although the fungus is not found directly in the eye, people with OHS usually test positive for previous exposure to histoplasma capsulatum.

Histoplasmosis is usually mistaken for a cold. The symptoms are very similar. The body's immune system normally overcomes the infection in a few days. The only evidence of histoplasmosis is histo spots, tiny scars on the retina. Generally histo spots do not affect vision, but for unknown reasons, some people can have ocular complications years or decades later.

Doctors believe that the histoplasmosis spores travel from the lungs to the eye where they settle in the choroid, the layer of tiny blood vessels that provides blood and nutrients to the retina, the light-sensing layer of cells lining the back of the eye.

Ocular histoplasmosis develops when fragile, abnormal blood vessels grow under the retina. The abnormal blood vessels form a lesion known as choroidal neovascularization (CNV). If left untreated, the CNV lesion can turn into scar tissue and replace the normal retinal tissue in the macula.

The only proven treatment for OHS is a form of laser surgery called photocoagulation. The laser's small, powerful beam of light destroys the abnormal blood vessels, as well as a small amount of the retinal tissue. Treatment is not necessary unless the new vessels are in the macula, the part of the retina responsible for acute central vision.

Although only a tiny fraction of people infected with the histoplasmosis virus develop OHS, if you have been exposed to histoplasmosis you should be sensitive to any changes in your eyesight.