Microsporidia of mammals--widespread pathogens or opportunistic curiosities?

The microsporidia are primitive eukaryotic parasites - well known in some invertebrates and in fish, and increasingly recognized in mammals. One species, Encephalitozoon cuniculi is widespread in rodents, lagomorphs and carnivores and has been reported in human and non-human primates. But although clinical expressions of E. cuniculi infections are well substantiated in carnivores, evidence for its pathogeniciry in primates is less clear. Indeed, serological evidence suggests that latent infections may be quite common in man. Another species, Enterocytozoon bieneusi has now been reported several times from AIDS patients, associated with a severe, intractable diarrhoea. Other records of microsporidia in mammals have also been associated with an immunoprivileged site or immunocompromized host. In this article Elizabeth Canning and Wafaa Hollister discuss the recent findings, and consider the likelihood that microsporidial infections of man will be increasingly revealed following immunosuppressive therapy. But will they be opportunistic infections, or manifestations of common parasites that are otherwise held at sub-patent levels?