What are microsporidia?
They are tiny obligate intracellular spore-forming protozoa that are common parasites in many animals. Until the advent of AIDS,microsporidia were considered to be a rare cause of human infection. Up to 150 genera and over 1200 species have been described.

Life Cycle
A two stage developmental cycle follows infection
i) an initial proliferative phase (merogony) followed by
ii) a spore-forming phase (sporogony).
Spores are resistant to stress, surviving in the environment for a prolonged period of time

Detection
Spores are more easily detected than the intracellular proliferative stage
The coiled polar tube present in all microsporidial spores is pathognomonic, with infection occurring by direct invasion of host cells through eversion of this polar tube, allowing passage of spore contents (sporoplasm) into the host cell. Clinically, electron microscopy of a corneal biopsy is required for definitive diagnosis, although specular microscopy may be helpful in suggesting the diagnosis without biopsy.

Presentation
i) In AIDS
ii) In immunocompetant, if steroids given

Symptoms
Initially, bilateral, episodic photophobia and blurred vision to about 6/18. The vision deteriorates with progressive disease. The condition may be unilateral in patients who have been given topical steroids.

Signs
Mild conjunctivitis and indistinct intraepithelial corneal opacities which progress. They may initially be confused with adenoviral infection. Eventually, the corneal opacities become more yellowish, floccular and confluent. There may be an associated vitritis and vasculitis.

Differential diagnosis
Herpes simplex keratitis

Sources of infection
Animals
Water supplies
Topical immunosuppression

Routes of infection
Abrasions into which microsporidial spores are inoculated
Some form of immunosuppression
HIV
Topical steroid
Lower temperature of the eye

Diagnosis
Electron microscopy of corneal biopsies is the method of choice. PCR may be used to exclude herpes simplex, but is unlikely to be useful for detecting microsporidia because of the large numbers of primers that would be needed to detect all the different species.

Treatment
In the initial stages, the condition may improve by simply stopping topical steroids. In more advanced cases:-
i) topical fumagillin (70 µg ml/L)
ii) oral albendazole (400 mg twice daily)
iii) debulking microsporidial infection by penetrating keratoplasty with cryotherapy to the recipient edge has been performed successfully with no recurrence of disease. A full thickness graft is recommended as opposed to lamellar graft due to increased risk of recurrence in the residual tissue at the deep interface if a partial thickness graft is performed