Ocular histoplasmosis is a rare complication of a common fungal infection. In its most severe form, it leads to abnormal blood vessel growth that damages the retina, causing vision loss. New treatments can halt the progressive damage caused by ocular histoplasmosis.
 

What is histoplasmosis?
Histoplasmosis is usually a mild illness caused by inhaling airborne spores of the microscopic fungus Histoplasma capsulatum. The lung is usually the main organ affected by histoplasmosis.
 

Activities such as ploughing fields, digging, construction work and gardening can release fungal spores into the air from the soil. The fungus feeds on bird droppings and can be transmitted in bat droppings3 so is likely to be found in areas where these accumulate in the soil or in areas such as chicken coops and caves.
 

Histoplasmosis is a particular risk for people with occupations such as farming, construction or archaeology in the areas where the disease is widespread.
 

Few people notice when they have an acute histoplasmosis fungal infection. If symptoms are present, they can include:

  • Shortness of breath, cough and other lung problems particularly in people with underlying lung disease.
  • Fevers, headache, severe joint pain and other symptoms can be experienced if histoplasmosis spreads to other organs.

Disseminated histoplasmosis is rare and can be fatal: risk factors for it include being immunocompromised, through AIDS/HIV or because of the immunosuppressive drugs taken to prevent rejection of an organ transplant. The very old and the very young are also at particular risk.

However mild the original infection, it can still give rise to ocular histoplasmosis years later, which is a common cause of vision loss in Americans aged between 20 and 40.
 

Development of ocular histoplasmosis
Fungal spores spread from the lungs to the eye, causing ocular histoplasmosis syndrome.  The histoplasmosis spores lodge in the choroid, the layer of blood vessels and connective tissue between the light-sensitive retina and the sclera, the white of the eye.

Ocular histoplasmosis can involve:

  • Histo spots: these are tiny areas of retinal scarring resulting from inflammation caused by the fungus. They only tend to affect vision if they are in the central area, the macula, where they can produce blind spots. Only about 5% of people with histo spots are at risk of vision loss.
  • Choroidal neovascularisation: rarely, abnormal, fragile blood vessels grow underneath the retina and pose a threat to vision.

Formation of these new and abnormal blood vessels can have serious consequences:

  • Scar tissue can form in the macula if these blood vessels form a choroidal neovascularisation lesion that is then left untreated.
  • Leakage of fluid and blood from the vessels can damage the eye and reduce vision.
  • Destruction of sharp, straight-ahead vision: if the abnormal blood vessels damage the fovea, the small depression in the centre of the macula that has the highest concentration of retinal cone cells, fine central vision is lost. Peripheral vision is rarely affected so ocular histoplasmosis does not cause total blindness.

Symptoms and diagnosis of ocular histoplasmosis
Ocular histoplasmosis often has no symptoms and abnormal blood vessels can grow later, sometimes years after the histo spots have formed, causing distorted vision and blind spots.1 People who may have been exposed to the fungus and develop eye symptoms should be checked for ocular histoplasmosis.
 

Histo spots are often visible in a detailed eye examination and the growth of new abnormal blood vessels is suggested by retinal swelling. Two tests are often used to diagnose ocular histoplasmosis and to establish whether the fovea is involved:

  • A dilated eye examination, in which the pupil is made larger with eye drops to allow an ophthalmologist to examine the retina.
  • Fluorescein angiography, in which a dye injected in the arm travels in the blood to the retina, allowing the lesions and blood vessels to be detected when the back of the eye is scanned.