(See also Overview of Intestinal Protozoan and Microsporidia Infections.)
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 routes of infection:
Ingestion
Inhalation
Direct contact with the conjunctiva
Microsporidia transmission occurs mostly through food including the fish and crustaceans. Transmission also occurs through water including seawater, drinking water, and other environmental sources.
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.
Microsporidia are emerging as important opportunistic pathogens. Infection also occurs in immunocompetent individuals. Microsporidiosis associated with HIV infection has diminished since the introduction of effective antiretroviral therapy (ART). The clinical manifestations of microsporidiosis are diverse and vary according to the causal species, immune status of the host, and route of infection. Microsporidia can infect the eyes, liver, biliary tract, sinuses, muscles, respiratory tract, genitourinary system, and central nervous system. Of these Enterocytozoon bieneusi-associated diarrhea is the most common. Disseminated infection can be fatal.
Symptoms and Signs of Microsporidiosis
Clinical illness caused by microsporidia varies with
The parasite species
The immune status of the host
Route of infection
In immunocompetent patients, microsporidia can cause asymptomatic infection or a self-limited watery diarrhea. Eye infections causing keratoconjunctivitis can also occur and have been increasingly reported in healthy individuals (1).
In patients with HIV, various microsporidia species cause chronic diarrhea, malabsorption, wasting, cholangitis, punctate keratoconjunctivitis, peritonitis, hepatitis, myositis, or sinusitis. Infections of kidneys and the gallbladder have occurred. Vittaforma corneum, Nosema ocularum, and several other species can cause ocular infections ranging from punctuate keratopathy with redness and irritation to severe, vision-threatening stromal keratitis.
Symptoms and signs reference
1. Tu EY, Joslin CE: Microsporidia and Acanthamoeba: the role of emerging corneal pathogens. Eye (Lond). 2012;26(2):222-227. doi:10.1038/eye.2011.315
Diagnosis of Microsporidiosis
Light or electron microscopy with special stains
Sometimes immunofluorescence or polymerase chain reaction (PCR) 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 assays are available in specialized laboratories. The Centers for Disease Control and Prevention (CDC) offers species-specific PCR assays for E. bieneusi, Encephalitozoon intestinalis, Encephalitozoon hellem , and Encephalitozoon cuniculi.
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 HIV, initiation or optimization of antiretroviral therapy (ART)
In patients with HIV, initiation or optimization of ART is important. Duration of antimicrobial therapy and outcome depend on the level of immune reconstitution with ART (1).
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.
E. intestinalis and other susceptible microsporidia. Such infections in immunocompetent patients may resolve spontaneously or after one week of treatment.
E. bieneusiE. intestinalis and other susceptible microsporidia species.
Oral fumagillin 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 United States.
Treatment reference
1. Panel on Guidelines for the Prevention and Treatment of Opportunistic Infections in Adults and Adolescents with HIV: Guidelines for the Prevention and Treatment of Opportunistic Infections in Adults and Adolescents with HIV: Microsporidiosis. National Institutes of Health, Centers for Disease Control and Prevention, HIV Medicine Association, and Infectious Diseases Society of America. Accessed April 2024.
Key Points
Microsporidiosis occurs mainly in immunocompromised patients, predominantly those with end-stage HIV, but keratoconjunctivitis is increasingly reported in otherwise healthy people.
Microsporidia spores can be acquired by a variety of routes and sources.
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 and PCR assays are available in specialized laboratories.
For patients with HIV, initiation or optimization of ART is of primary importance.
