Microsporidiosis is infection with microsporidia. Symptomatic disease develops predominantly in patients with AIDS and includes chronic diarrhea, disseminated infection, and corneal disease. Diagnosis is by demonstrating organisms in biopsy specimens, stool, urine, other secretions, or corneal scrapings. Treatment is with albendazole or fumagillin (depending on the infecting species and clinical syndrome), with topical fumagillin added for eye disease.
Microsporidia are obligate intracellular spore-forming protozoan parasites. At least 15 of the > 1200 species are associated with human disease. Spores of the organisms are acquired by ingestion, inhalation, direct contact with the conjunctiva, animal contact, or person-to-person transmission. Inside the host, they harpoon a host cell with their polar tubule or filament and inoculate it with an infective sporoplasm. Intracellularly, the sporoplasm divides and multiplies, producing sporoblasts that mature into spores; the spores can disseminate throughout the body or pass into the environment via respiratory aerosols, stool, or urine. An inflammatory response develops when spores are liberated from host cells.
Little is known about routes of transmission to humans or possible animal reservoirs. Microsporidia probably are a common cause of subclinical or mild self-limited illness in otherwise healthy people, but only a few cases of human infection were reported in the pre-AIDS era—perhaps because overall awareness of microsporidial infection was less. Recently, microsporidial keratoconjunctivitis has become increasingly reported in immunocompetent people.
Microsporidia have emerged as opportunistic pathogens in patients with AIDS and, to a lesser degree, in those with other immunocompromising conditions. Encephalitozoon bieneusi and E. (formerly Septata) intestinalis can cause chronic diarrhea in patients with AIDS and CD4+ cell counts of < 100/μL. Microsporidian species can also infect the biliary tract, cornea, muscles, respiratory tract, GU system, and, occasionally, the CNS.
Symptoms and Signs
Clinical illness caused by microsporidia varies with the parasite species and the immune status of the host. In patients with AIDS, various species cause chronic diarrhea, malabsorption, wasting, cholangitis, punctate keratoconjunctivitis, peritonitis, hepatitis, myositis, or sinusitis. Infections of kidneys and the gallbladder have occurred. Vittaforma (Nosema) corneum and several other species can cause ocular infections ranging from punctuate keratopathy with redness and irritation to severe, vision-threatening stromal keratitis.
Infecting organisms can be demonstrated in specimens of affected tissue obtained by biopsy or in stool, urine, CSF, sputum, or corneal scrapings. Microsporidia are best seen with special staining techniques. Fluorescence brighteners (fluorochromes) are used to detect spores in tissues and smears. The quick-hot Gram chromotrope technique is the fastest. Immunoassay and PCR-based assays hold promise for the future.
Transmission electron microscopy is currently the most sensitive test and is used for speciation.
For GI microsporidiosis, albendazole (400 mg po bid for weeks in adults) may be effective in controlling diarrhea due to E. intestinalis. The drug reduces the number of organisms in small-bowel biopsies but does not eliminate infection. Albendazole 400 mg po bid for weeks has been used to treat skin, muscle, or disseminated microsporidiosis due to E. intestinalis and many other microsporidial species. Albendazole 400 mg po bid in combination with itraconazole 400 mg po daily has been used for Trachipleistophora and Anncaliia infections. Albendazole is not active against E. bieneusi and V. corneum. In patients with AIDS, duration of therapy and outcome depend on the level of immune reconstitution with ART.
Oral fumagillin 20 mg tid for 14 days has been used for intestinal E. bieneusi infection, but it has potentially serious adverse effects, including severe reversible thrombocytopenia in up to half of patients. Oral fumagillin is not available in the US.
Ocular microsporidial keratoconjunctivitis can be treated with albendazole 400 mg po bid plus fumagillin eye drops 3 mg/mL (2 drops q 2 h for 4 days, then 2 drops qid). Topical fluoroquinolones have been effective in some patients. When topical and systemic therapy are ineffective, keratoplasty may be useful. Outcome is typically very good in immunocompetent patients; in patients with AIDS, it depends on the level of immune reconstitution with ART.
Last full review/revision August 2013 by Richard D. Pearson, MD
Content last modified September 2013