Mark Kelly: Armidale Rural and Referral Hospital, Armidale, NSW

Microsporidia are ubiquitous obligate intracellular organisms that were once thought to be parasites but are now considered to be fungi.1,2 Recent work suggests that microsporidiosis may be a zoonotic infection.3 Although recognised as a veterinary disease for many years, microsporidiosis was first described as a human disease in 1985 in a patient with acquired immune deficiency syndrome (AIDS) and unexplained diarrhoea.4 As with cryptosporidiosis, microsporidiosis is a rare manifestation of human immunodeficiency virus (HIV) disease in the era of effective combination antiretroviral therapy (cART) yet continues to be problematic in areas of the world with limited access to cART.5

The small intestine is the most common site of microsporidiosis. Enterocytozoon bieneusi and Encephalitozoon intestinalis are the most common microsporidial species identified in patients with HIV infection. E. bieneusi infection is usually confined to the small intestine (and occasionally involves the biliary tree). It causes 80% of microsporidial diarrhoea. E. intestinalis infects the small intestine, but can also cause more widespread disease. The capacity of the latter species to infect lamina propria macrophages is thought to explain its capacity for disseminatated disease.6

Infection occurs following ingestion or inhalation of microsporidial spores. Transmission of spores is thought to be primarily through faecal-oral or urinary-oral routes. Encephalitozoon keratoconjunctivitis may occur through contact with aerosols or surfaces contaminated with urine containing microsporidia.7

Clinical presentation

The most common clinical manifestation of microsporidiosis in patients with AIDS is diarrhoea. Like cryptosporidia, microsporidia can cause self-limiting diarrhoea in immunocompetent patients.8 Microsporidial infection accounts for up to 20% of diarrhoeal illness in patients with AIDS.9 Typically, patients have a CD4 T lymphocyte cell (CD4) count of < 50/μL.1 Immunodeficient patients present with chronic (initially intermittent) non-bloody diarrhoea that may be accompanied by fever, anorexia and weight loss. E. bieneusi may cause cholangitis or cholecystitis. Uncommon non-gastrointestinal manifestations of microsporidial infection include sinusitis, keratoconjunctivitis, hepatitis, peritonitis, adrenalitis, nephritis, pituitary gland infection, osteomyelitis, encephalitis, myositis and pneumonia.10

Microsporidial disease has been reported to present as an ‘unmasking’ immune reconstitution inflammatory syndrome (IRIS) in a very immunodeficient patient (CD4 counts < 20 cells/μL) who had unrecognised microsporidial infection before commencing cART and presented with diarrhoea and diffuse abdominal pain one month after commencing cART. While diarrhoea and detection of microsporidia in stool specimens resolved after albendazole therapy, abdominal pain associated with diffuse peritoneal and small bowel granulomas continued and only resolved following immune reconstitution.11


Stool microscopy is the investigation of choice in the diagnosis of microsporidiosis. As microsporidial shedding may be intermittent, at least three stool specimens are required. Special stains, such as the modified Ziehl-Nielsen, Warthin- Starry, fluorescent and trichrome stains have varying specificity and sensitivity for different species of microsporidia. These stains, however, do not provide species-specific identification, which requires either electron microscopic analysis of a small bowel biopsy or polymerase chain reaction (PCR) analysis.12 Electron microscopy is considered the gold standard, but is not routinely available. The use of monoclonal antibodies and oligonucleotide microarray assays to detect pathogenic species of microsporidia in human clinical specimens have been reported recently and may increase the diagnostic yield.13,14


No specific therapy is required in patients with self-limited microsporidial enteritis in whom CD4 counts are > 200 cells/μL. For patients with lower CD4 cell counts, the most important management strategy is achieving immune recovery following the initiation of cART.15 HIV protease inhibitors such as lopinavir, ritonavir and saquinavir have been shown to have antimicrosporidial effects in vitro, however there are no reports of improved responses using a protease inhibitor-based antiretroviral regimen.16

Albendazole (400 mg twice a day) is successful in the treatment of E. intestinalis infection but other microsporidial species have variable responses to this drug.17,18 Although clearance of microsporidia has been reported with cART alone, treatment with albendazole is recommended to reduce the duration of symptoms. Fumagillin (20 mg taken three times a day) is active against E. bieneusi and has been demonstrated to eradicate the organism,19 but is not licensed in Australia. Uncontrolled studies have suggested that thalidomide may reduce stool frequency and lead to weight gain in patients with microsporidial diarrhoea.20 The potential mechanisms are unclear as thalidomide has no direct antimicrosporidial effect.21 The following drugs have been used with variable success to treat microsporidiosis: metronidazole, atovaquone, azithromycin, itraconazole, sulfa-based drugs and nitazoxanide.

Specific dietary intervention is advantageous in patients with microsporidiosis and malabsorption. Medium-chain triglyceride-based diets, as opposed to long-chain triglyceride-based diets, reduce stool frequency and lead to weight gain.22 Octreotide may provide symptomatic reduction in stool frequency and volume.


Therapeutic strategies for primary prophylaxis have not been defined. Patients with microsporidiosis caused by species other than E. intestinalis frequently relapse following cessation of albendazole therapy in the absence of cART-induced immune recovery. These patients may require long-term suppressive therapy with albendazole or an alternative agent. The role of fumagillin in this context has not been studied.


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