What is ocular histoplasmosis?

Histoplasmosis is an infectious condition caused by the fungus Histoplasma capsulatum. It is caused when the spores of the fungus are inhaled into the lungs. The lungs are the primary infection site.

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 fungi are released in air. 1-5

Symptoms of the infection

Histoplasmosis is often so mild that there may be no apparent symptoms. Commonly symptoms appear to be similar to a common cold. 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. This is called ocular histoplasmosis syndrome. Among Americans ocular histoplasmosis is a leading cause of vision loss between ages 20 and 40.

Causes of ocular histoplasmosis

Ocular histoplasmosis occurs due to spores of the fungi Histoplasma capsulatum. It is speculated that the spores enter the lungs first and spread to the eye. Within the eye they lodge in the choroid. The 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. The retina is responsible for collecting visual information and transmitting it to the brain via nerves.

Infection with the fungus causes fragile, abnormal blood vessels to grow 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. Ocular histoplasmosis commonly affects the central and most vital parts of the vision and rarely affects the sides of the retina responsible for peripheral vision. It is for this reason that early detection and treatment of the condition is vital.

Who is at risk of ocular histoplasmosis?

In the United States the highest incidence of histoplasmosis occurs in an area called the “Histo belt”. Here nearly 90% of the adult population has been infected by histoplasmosis. This region includes:

• Arkansas
• Missouri
• Kentucky
• Tennessee
• West Virginia
• Alabama
• Indiana
• Iowa
• Illinois
• Kansas
• Maryland
• Mississippi
• Louisiana
• Ohio
• Oklahoma
• Nebraska
• Texas
• Virginia

Those living in these areas may be unaware of their histoplasmosis infection and need to get their eyes checked for histoplasmosis spots.

Symptoms of ocular histoplasmosis

Early stages usually have no symptoms. In fact early histoplasmosis spots in the eyes may not lead to any vision loss. The loss of vision occurs much later after the initial infection has healed. In later stages the symptoms of ocular histoplasmosis may appear if the abnormal blood vessels affect the centre of the retina.

Diagnosis of ocular histoplasmosis

Diagnosis involves careful examination of the retina using an ophthalmoscope. This is a hand held light device that is used to shine a light into the back of the eye. For better view of the whole of the retina the pupils are dilated.

If abnormalities are detected further tests like fluorescein angiography is advised. In this test a dye is into the patient's arm. It travels to the blood vessels of the retina. This allows a better view of the blood vessels of the retina and the CNV lesion.

Treatment of ocular histoplasmosis

Ocular histoplasmosis is best treated using laser surgery or laser photocoagulation. In this a powerful beam of laser light is used to destroy the CNV and the abnormal blood vessels as well as some of the retina overlying the abnormal spots.

Although this damages small areas of the retina and causes focal vision loss, the therapy hopes to protect central important parts of the retina. Sometimes steroid medicines are used and if there is active infection treatment with antifungal drugs may be recommended.