Histoplasmosis is caused by spores of the microscopic fungi Histoplasma capsulatum. These spores are inhaled into the lungs that are the primary site of the infection. Histoplasma capsulatum lives in river valleys and soil with accumulation of bird or bat droppings. When the soil is disturbed by ploughing fields, digging or turning or when chicken coops or bird cages are cleaned, the tiny spores of the fungi are released in air and are inhaled.
Generalized histoplsmosis is a mild condition with symptoms similar to a bout of common cold or the condition may show no apparent symptoms at all. The body’s immune system normally overcomes the infection in a few days without treatment. However, even in mild cases there may be a serious complication if the eyes are infected as in the ocular histoplasmosis syndrome. 1-6
Causes of ocular histoplasmosis
Ocular histoplasmosis occurs due to spores of the fungi Histoplasma capsulatum. The spores enter the lungs first and spread to the eye. Within the eye they lodge in the choroid. Choroid is a layer within the retina (the photo-sensitive back wall of the eye ball) of blood vessels that provides blood and nutrients to the retina.
This leads to formation of fragile, abnormal blood vessels under the retina and this forms an abnormal growth under the retina called the choroidal neovascularization (CNV). This CNV lesion, as it heals, turns into a scar that damages a significant area of the retina causing loss of vision at that area. Vision may also be lost when these abnormal blood vessels bleed, break or leak into the retina and damage the vital areas for vision of the retina.
The retina is responsible for collecting visual information and transmitting it to the brain via nerves. There are vital areas of the retina that are important for vision. The central part of the retina is called the macula. It is responsible for focussed central vision that is important for reading or performing a task like driving or writing. If there is a scar tissue or abnormal leakage of fluid at this spot, vision may be severely affected.
The center of the macula is another extremely sensitive area called the fovea. Fovea is a tiny depression in the normal macula where there is the highest concentration of special retinal nerve cells called the cones. These cones are responsible for transmitting visual signals like color vision and focussed vision.
Damage to the fovea and the cones (due to histoplasmosis spots over the macula, or scars from ocular histoplasmosis), lead to damage to straight-ahead vision.
Who is at risk of ocular histoplasmosis?
In the United States, the highest incidence of histoplasmosis occurs in an area called the “Histo belt”. This mainly comprises of the river valley. Here nearly 90% of the adult population has been infected by histoplasmosis at some point or another in their lifetime.
In most cases they are unaware of this past infection. Thus they need to get their eyes checked for histoplasmosis spots. Approximately 62% of the adult population living in this region are carriers. It affects men and women equally. Most carriers of the infection have no symptoms at all.
This region includes all of Arkansas, Missouri, Kentucky, Tennessee, and West Virginia as well as Alabama, Indiana, Iowa, Illinois, Kansas, Maryland, Mississippi, Louisiana, Ohio, Oklahoma, Nebraska, Texas, and Virginia.
Histoplasmosis may be a cause for trouble if the infected individual has a weakened immune system for example in those with HIV AIDS.