Ocular histoplasmosis

What is ocular histoplasmosis?

Histoplasmosis is an infection with the fungus Histoplasma capsulatum. It's a common problem in the USA, particularly in a region now known as the 'Histo belt', which includes Arkansas, Kentucky, Missouri, Tennessee and West Virginia, where as many as 90 per cent of the population have had the infection.

Causes and risk factors

The tiny spores of histoplasmosis are carried in the air and inhaled to cause an initial (or primary) lung infection. This is usually very mild and most people feel as though they have a slight cold. Their immune system soon deals with the infection and it may never even be properly diagnosed.

But some people go on to develop symptoms - usually of lung disease, although the fungus may spread to other organs - known as disseminated histoplasmosis and this can be fatal.

Very rarely the organism can spread to the eye to cause acute ocular histoplasmosis, which needs urgent treatment with antifungal medicine.

Symptoms

For most people, initial symptoms in the eye usually settle without needing treatment, leaving only tiny scars known as histo spots. However, months or years later the disease may re-emerge to cause a serious (but fortunately still rare) condition that affects vision.

This is known as presumed ocular histoplasmosis syndrome and it results in the growth of new and fragile blood vessels, which may leak and scar the delicate tissues of the retina.

The syndrome is thought to be linked to hypersensitivity to Histoplasma capsulatum, rather than a direct exposure of the eyes to the micro-organism, but some experts have found DNA or genetic material from the fungus in a layer of the eyeball known as the choroid, and suspect fungal spores may lodge here and cause problems.

Ocular histoplasmosis can cause blindness, although it mostly affects central vision and rarely involves peripheral vision so total blindness is rare. Anyone who has lived in an area where they may have been exposed to histoplasmosis and develops eye problems must be checked for the condition.

Treatment and recovery

Early treatment can limit the extent of damage from leaky blood vessels, but can't restore lost vision. The treatment consists of coagulation of the abnormal vessels using a powerful laser beam. This treatment itself can cause slight scarring of the retina but has been shown to reduce future vision loss by more than 50 per cent.

Sometimes steroid medicines are used. If active infection with the organism itself is suspected, then treatment with antifungal drugs may be recommended.

To get the best effect, the whole area affected by ocular histoplasmosis has to be treated.

Once a person has ocular histoplasmosis, they have it for life. It can recur and will need long-term monitoring.