Histoplasmosis is a disease that is caused by spores of a fungus called histoplasma capsulatum. This particular fungus is found in places were there is an accumulation of bird and/or bat droppings. When the ground is dug up for planting crops, sweeping chicken coops or digging holes, the spores of the fungus are released and can be breathed in through the lungs. This causes mild flu or cold-like symptoms and the body normally fights off the infection within a few days without treatment. Unfortunately, even a very mild case of histoplasma capsulatum infection can lead to ocular histoplasmosis syndrome which can lead to loss of vision.
Ocular histoplasmosis syndrome (OHS) develops by traveling through the blood stream from the lungs to the choroid. Once in the choroid, the fungus damages the interface between the choroid and the retinal pigment epithelium (RPE). This produces a detachment of the retina from the RPE at this point which can lead to new blood vessel growth (neovascularization). Neovascularization causes vision loss because the abnormal blood vessels that are formed do not have a tight outer layer which allows them to leak fluid and blood. If left untreated, the neovascularization can from a fibrovascular scar which will leave dead and/or damaged tissue in place of the normal, living tissue causing further vision loss.
There are usually little to no symptoms of OHS in early stages and those symptoms usually subside without any treatment. The only signs that OHS occurred are spots on the retina called "histo spots" where the infection occured. Histo spots do not generally affect vision, however, these spots have been associated with later neovascularization years or even decades after the primary infection.
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