(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:
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 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, 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.
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)
In patients with AIDS, initiation or optimization of ART 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.
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 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.
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.
