Clinical Features:
Human microsporidiosis represents an important and rapidly emerging opportunistic disease, occurring mainly, but not exclusively, in severely immunocompromised patients with AIDS.  Additionally, cases of microsporidiosis in immunocompromised persons not infected with HIV as well as in immunocompetent persons also have been reported.  The clinical manifestations of microsporidiosis are very diverse, varying according to the causal species with diarrhea being the most common.

*Two reports of E. bieneusi in respiratory samples have also been published, one in 1992 and the other in 1997.

Laboratory Diagnosis:
There are several methods for diagnosing microsporidia:

• Light microscopic examination of the stained clinical smears, especially the fecal samples, is the easiest and quickest method of diagnosing microsporidial infections even though it does not allow identification of microsporidia to the species level.  The most widely used staining technique is the Chromotrope 2R method or its modifications.  This technique stains the spore and the spore wall a bright pinkish red.  Often, a belt-like stripe, which also stains pinkish red, is seen in the middle of the spore.  This technique, however, is lengthy and time consuming and requires about 90 minutes.  A recently developed “Quick-Hot Gram Chromotrope technique” however, cuts down the staining time to less than 10 minutes and provides a good differentiation from the lightly stained background fecal materials so that the spores stand out for easy visualization.  The spores stain dark violet and the belt-like stripe is enhanced.  In some cases dark staining Gram positive granules are also clearly seen.  Chemofluorescent agents such as Calcofluor white are also useful in the quick identification of spores in fecal smears.  The spores measure from 0.8 to 1.4 µm in the case of Enterocytozoon bieneusi, and 1.5 to 4 µm in Brachiola algerae, Encephalitozoon spp., Vittaforma corneae, and Nosema spp.
• Transmission electron microscopy (TEM) is still the gold standard and is necessary for the identification of the microsporidian species.  However, TEM is expensive, time consuming, and not feasible for routine diagnosis.
• Immunofluorescence assays (IFA) using monoclonal and/or polyclonal antibodies are being developed for the identification of microsporidia in clinical samples.
• Molecular methods (mainly Polymerase Chain Reaction, PCR) is an alternative method for the laboratory diagnosis of microsporidiosis.  PCR is available only in research laboratories and has been successfully used for the identification ofBrachiola algerae, Enterocytozoon bieneusi, Encephalitozoon intestinalis, Encephalitozoon hellem, and Encephalitozoon cuniculi.  The principal drawback is that it does not work well on formalin-fixed samples stored for long term.

Treatment:
The treatment of choice for ocular microsporidiosis (Brachiola algerae, Encephalitozoon hellem, E. cuniculi, Vittaforma corneae) is oral albendazole* plus topical fumagillin.  Corneal infections with V. corneae often do not respond to chemotherapy and may require keratoplasty.  Oral fumagillin has been effective to treat Enterocytozoon bieneusi infections, but it has been associated with thrombocytopenia.  Albendazole* is the drug of choice to treat gastroenteritis caused by  Encephalitozoon intestinalis and to treat disseminated microsporidiosis (E. hellem, E. cuniculi, E. intestinalis, Pleistophora sp., Trachipleistophora sp., Brachiola vesicularum) and skin and deep muscle infection (Brachiola algerae).  For additional information, see the recommendations in The Medical Letter (Drugs for Parasitic Infections).

* This drug is approved by the FDA, but considered investigational for this purpose.