Human microsporidiosis and AIDS

Several protozoan parasites of the phylum Microspora, previously only rarely recognized in humans, are being detected with increasing frequency in patients with AIDS. These protozoa are small obligate intracellular parasites of the intestine, liver, muscles, cornea, and several other tissues. They are difficult to detect because they stain poorly, often evoke little or no tissue response, and require electron-microscopic methods for classification. Although there are no human isolates, serologic evidence in humans (using an animal pathogen [Encephalitozoon cuniculi] as the antigen) suggests that clinically silent infections may occur in humans, as they do in mice, rabbits, dogs, foxes, squirrel monkeys, and birds. Experiments in mice suggest that patients with impaired T cell functions may be vulnerable to clinical disease caused by these parasites. There is no known treatment.