Merck Manual

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Chelsea Marie

, PhD, University of Virginia;

William A. Petri, Jr

, MD, PhD, University of Virginia School of Medicine

Reviewed/Revised Jun 2022

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) or with topical fumagillin and oral albendazole for eye disease.

Microsporidia are obligate intracellular spore-forming parasites that are fungi or closely related to them. Microsporidia used to be classified as protozoa.

At least 15 of the > 1,400 species of microsporidia are associated with human disease. Spores of the organisms are acquired by the following:

  • Ingestion

  • Inhalation

  • Direct contact with the conjunctiva

  • Animal contact

  • 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 Human Immunodeficiency Virus (HIV) Infection Human immunodeficiency virus (HIV) infection results from 1 of 2 similar retroviruses (HIV-1 and HIV-2) that destroy CD4+ lymphocytes and impair cell-mediated immunity, increasing risk of certain... read more Human Immunodeficiency Virus (HIV) Infection and, to a lesser degree, in those with other immunocompromising conditions. Encephalitozoon species including E. bieneusi and E. (formerly Septata) intestinalis can cause chronic diarrhea in patients with AIDS and CD4 cell counts of < 100/mcL. Depending on the species and immune status of the host, some Microsporidium, Nosema, Vittaforma, and other genera can infect the eyes, liver, biliary tract, sinuses, muscles, respiratory tract, genitourinary system, central nervous system, and occasionally cause disseminated disease.

The incidence of microsporidiosis in persons with AIDS has decreased substantially with the widespread use of effective antiretroviral therapy.

Symptoms and Signs of Microsporidiosis

Clinical illness caused by microsporidia varies with

  • The parasite species

  • The immune status of the host

In immunocompetent patients, microsporidia can cause asymptomatic infection or a self-limited watery diarrhea. There are reports of eye infections causing keratoconjunctivitis.

In patients with AIDS, various microsporidia species cause chronic diarrhea, malabsorption Overview of Malabsorption Malabsorption is inadequate assimilation of dietary substances due to defects in digestion, absorption, or transport. Malabsorption can affect macronutrients (eg, proteins, carbohydrates, fats)... read more , 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 Interstitial Keratitis Interstitial keratitis is chronic, nonulcerative inflammation of the mid-stroma (the middle layers of the cornea) that is sometimes associated with uveitis. The cause is usually infectious.... read more Interstitial Keratitis .

Diagnosis of Microsporidiosis

  • Light or electron microscopy with special stains

  • Sometimes immunofluorescence or polymerase chain reaction (PCR)–based assays

Infecting organisms can be demonstrated in specimens of affected tissue obtained by biopsy or in stool, urine, cerebrospinal fluid (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.

Immunofluorescence assays (IFA) and PCR-based assays are available in specialized laboratories.

Transmission electron microscopy is currently the most sensitive test, but it is not feasible for routine diagnosis.

Molecular methods are used for speciation.

Treatment of Microsporidiosis

  • For patients with AIDS, initiation or optimization of antiretroviral therapy (ART)

  • For gastrointestinal, skin, muscle, or disseminated microsporidiosis, oral albendazole or fumagillin (where available), depending on the infecting species

  • For keratoconjunctivitis, oral albendazole and topical fumagillin

In patients with AIDS, initiation or optimization of ART Antiretroviral Treatment of HIV Infection Because disease-related complications can occur in untreated patients with high CD4 counts and because less toxic drugs have been developed, treatment with antiretroviral therapy (ART) is now... read more Antiretroviral Treatment of HIV Infection is important. Duration of antimicrobial therapy and outcome depend on the level of immune reconstitution with ART.

The antimicrobial treatment of microsporidiosis depends on the infecting microsporidia species, the immune status of the human host, and the organs involved. Data on therapeutic options are limited. Consultation with an expert is recommended.

Albendazole, a benzimidazole-type broad-spectrum anthelmintic, is used to treat infections from certain microsporidia, but it can have serious adverse effects including liver injury (hepatitis) in 10% of patients and, rarely, low white blood cell count.

Albendazole (400 mg orally 2 times a day in adults or 7.5 mg/kg 2 times a day in children for 2 to 4 weeks) is often effective in controlling diarrhea in patients with enteric or disseminated infections due to E. intestinalis and other susceptible microsporidia. Such infections in immunocompetent patients may resolve spontaneously or after one week of treatment.

Albendazole has minimal efficacy for the treatment of E. bieneusi. Albendazole (400 mg 2 times a day in adults or 7.5 mg/kg 2 times a day in children for 2 to 4 weeks) has been used to treat skin, muscle, or disseminated microsporidiosis due to E. intestinalis and other susceptible microsporidia species.

Oral fumagillin 20 mg 3 times a day 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 orally 2 times a day plus fumagillin eye drops. Topical fluoroquinolones, as well as topical voriconazole, 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.

Key Points

  • Microsporidiosis occurs mainly in immunocompromised patients, predominantly those with AIDS, but keratoconjunctivitis is being increasingly reported in otherwise healthy people.

  • Microsporidia spores can be acquired by ingestion, inhalation, direct contact with the conjunctiva, animal contact, or person-to-person transmission.

  • Manifestations vary widely depending on the organism and the patient's immune status, but chronic diarrhea, malabsorption, wasting, cholangitis, punctate keratoconjunctivitis, peritonitis, hepatitis, myositis, or sinusitis may occur.

  • Diagnose using light or electron microscopy with special stains; immunofluorescence assays and PCR-based assays are available in specialized laboratories.

  • For patients with AIDS, initiation or optimization of ART is of primary importance.

  • Albendazole and oral or topical fumagillin may be useful, depending on the infecting species and organs involved; oral fumagillin is not available in the US.

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