Human microsporidiosis

At least 10 species of microsporidia are known from humans, most of them being found only once. Six of them belong to genera exclusive to man: Brachiola, Enterocytozoon, Septata, Vittaforma, and Trachipleistophora. Before the advent of HIV infection and AIDS, microsporidia had only occasionally been observed in man. Later, microsporidia were commonly observed in AIDS patients, and they were often considered to be opportunistic

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pathogens. The first human case of sufficiently substantiated microsporidial infection was reported in 1959 (Matsubayashi et al. 1959). Nearly 25 years later the first human case was observed in Scandinavia (Bergquist et al. 1984). Up to 1994 totally 10 sufficiently documented cases of human microsporidial infection in persons not infected with HIV have been reported (Weber et al. 1994). The first Scandinavian case connected with AIDS was observed in 1992 (Hojlyng et al. 1993). As a result of antiretroviral therapy no cases of microsporidiosis have been observed in HIV-positive persons in the northern countries in the last few years.

The sources of microsporidia infecting humans and the modes of transmission are still uncertain. Whether other animals are involved in the transmission to man is unknown, but one of the species, Enterocytozoon bieneusi, has recently been found in faecal material from pigs (Deplazes et al. 1996). The following species are the most important and best known microsporidia pathogenic to man. Encephalitozoon cuniculi, described by Levaditi et al. (1923), was, in 1922, the first microsporidium to be recognized as a parasite of mammals. Contrary to the microsporidia of invertebrates, this species has a wide host range, including hares, rabbits, mice, various carnivores, elephants, and primates, including man. Brain and kidneys are the primary sites of infection, and it has been proven that the parasite is transmitted across the placenta. Few cases of E. cuniculi infection have been observed in connection with HIV infection. In AIDS patients the parasite causes widely disseminated infection involving nearly all organ systems. A second Encephalitozoon species, E. hellem, described by Didier et al. (1991), was isolated from the eyes of AIDS patients with keratoconjunctivitis. Encephalitozoon hellem is known to provoke systemic infection, also comprising the bronchial epithelium and the kidney tubules. Brachiola vesicularum, described by Cali et al. (1998), was isolated from an AIDS patient. It causes myositis with muscular weakness and pain. Infection has not been found in tissues other than muscle cell cytoplasm and fibres.

Enterocytozoon bieneusi, described by Desportes et al. (1983), is the smallest microsporidium reported to infect humans (spore length 1-1.6^m). This is the most commonly diagnosed of the microsporidian species infecting man. It is apparently persisting in populations in light infections which are below the threshold of detection. It cannot be excluded that E. bieneusi is a natural parasite of man with worldwide distribution. The infection is restricted to the gut where enterocytes of the small intestine are destroyed, causing severe diarrhoea. This species has also been identified from immunocompetent and HIVnegative patients with self-limited traveller's diarrhoea (Sobottka et al. 1995). Septata intestinalis, described by Cali et al. (1993) and recently transferred to the genus Encephalitozoon by Hartskeerl et al. (1995), has mainly been identified in HIV-infected patients. It is mainly a parasite of the gut, where it invades enterocytes throughout the intestine, but disseminates to other epithelial cells, for example in the kidneys, liver, or respiratory tract.

The new genus Vittaforma was established by Silveira and Canning (1995) for a species previously known as Nosema corneum. It was originally isolated from the cornea of an immunocompetent patient. Trachipleistophora hominis, described by Hollister et al. (1996), was originally found in skeletal muscles of AIDS patients, but has later also been observed in cornea scrapings and washings from the nasopharyngeal tract. A second species, T. anthropophthera, was recently found in brain, kidneys, liver, and a variety of other organs in an AIDS patient (Vavra et al. 1998). This is a dimorphic species, yielding two kinds of morphologically different spores occurring together.

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