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Encephalitozoon cuniculi has a worldwide distribution and is a common parasite of wild and domesticated rabbits, laboratory rodents and a wide spectrum of other mammals such as carnivores, ruminants, pigs, monkeys and man. Encephalitozoon parasites found in birds may belong to other species. E. cuniculi, widespread in conventional rabbit colonies in up to 76% of the stock, mostly causes latent infections and only rarely disease. Focal granulomatous encephalitis and nephritis are the main pathological changes. Natural transmission in rabbits is possible by the oral, tracheal and transplacental routes, but oral infection appears to be the most important way. In rabbits, spores of E. cuniculi are excreted in the urine in concentrations up to 106 spores/ml. The human cases of E. cuniculi infections include two cases of encephalitis with a favorable evolution in two immunocompetent children. In a 35-year old man with AIDS the E. cuniculi infection resulted in a granulomatous hepatitis. Recently, another case in a 45-year old patient with AIDS (Greek nationality) was described in Switzerland. An unusual peritonitis with a large granulomatous mass, containing developmental stages of E. cuniculi, were found at autopsy. In Sweden (S) and Great Britain (GB) high prevalences of serum antibodies against E. cuniculi antigen have been detected in persons with malaria (S: 38%, GB: 7%), Chagas disease (S: 15%), schistosomiasis (GB: 12%), filariases (S: 9%), neurological disorders (GB: 6%) and in travelers after a stay in the tropics (S: 12%) while non-exposed persons, animal dealers and some other groups were seronegative. The question is open whether E. cuniculi is more common in the tropics or whether tropical parasites cause immunodepression which supports microsporidian infections. Besides a systemic infection in a immunodefective child with Nosema conorii (Travelers' Diarrhea) and two cases of keratitis caused by Microsporidian species, the recent description of a myositis in an AIDS patient due to Pleistophora sp. is of special interest as parasite transmission from fish was suspected. Another species of this group is Enterocytozoon bieneusi. This parasite was first described in 1984 in France in a 29-year old AIDS patient who had suffered from heavy diarrhea. Microsporidia were found in enterocytes of the duodenum, jejunum and ileum. This parasite or similar forms were subsequently detected in at least 9 other patients with AIDS in the USA, Uganda and the Netherlands. The origin of this parasite is unknown. An infection similar to E. bieneusi in the enterocytes of a Callicebus monkey has been described.
Diagnosis is based on the direct detection of Microsporidia by histology after hematoxylin-eosin, Giemsa, Gram or other staining (spores are Gram-positive!) or by electron microscopy in autopsy or biopsy material. Serum antibody detection provides another tool for the in vivo diagnosis of encephalitozoonosis.
Information on chemotherapy of microsporidian infections is scanty. In experimental infections chloroquine and oxytetracycline reduced harvests of E. cuniculi spores only by 69% and 58%, respectively. Treatment of an AIDS patient suggested that either metronidazole or ganciclovir may have inhibited the microsporidian infection.