Histoplasmosis is a disease caused when airborne spores of the fungus Histoplasma capsulatum are inhaled into the lungs, the primary infection site. This microscopic fungus, which is found throughout the world in river valleys and soil where bird or bat droppings accumulate, is released into the air when soil is disturbed by plowing fields, sweeping chicken coops, or digging holes.
Histoplasmosis is often so mild that it produces no apparent symptoms. Any symptoms that might occur are often similar to those from a common cold. However, histoplasmosis, even mild cases, can later cause a serious eye disease called ocular histoplasmosis syndrome, a leading cause of vision loss in Americans ages 20 to 40.
The fungus spores spread from the lungs to the eye, lodging in the choroid, a layer of blood vessels that provides blood and nutrients to the retina. Ocular histoplasmosis syndrome develops when fragile, abnormal blood vessels grow underneath the retina, forming a lesion known as choroidal neovascularization. If left untreated, this lesion can turn into scar tissue and replace the normal retinal tissue in the macula, which provides our sharp, central vision. When this scar tissue forms, visual messages from the retina to the brain are affected, and vision loss results.
Vision is also impaired when these abnormal blood vessels leak fluid and blood into the macula. If these abnormal blood vessels grow toward the center of the macula, they may affect a tiny depression called the fovea. The fovea is the region of the retina with the highest concentration of special retinal nerve cells, called cones, which produce sharp, daytime vision. Damage to the fovea and the cones can severely impair, and even destroy, this straight-ahead vision. Early treatment of ocular histoplasmosis syndrome is essential.