Phylogenetic relationships of Pleistophora-like microsporidia based on small subunit ribosomal DNA sequences and implications for the source of Trachipleistophora hominis infections

The microsporidian Trachipleistophora hominis was isolated in vitro from th e skeletal muscle of an AIDS patient. Since its discovery several more case s of myositis due to Trachipleistophora have been diagnosed but the source of infection is unknown. Morphologically, T. hominis most closely resembles Pleistophora and Vavraia, which undergo polysporous sporogony in sporophor ous vesicles, but differs from these genera in the mode of formation of spo roblasts and in the morphology of the sporophorous vesicles. Alignment and analyses of the small subunit ribosomal DNA sequences of T. hominis and sev eral other polysporoblastic genera indicated that its closest phylogenetic relationships were with species of the genera Pleistophora and Vavraia, in line with morphological predictions. The type species of the latter two gen era are Pleistophora typicalis and Vavraia culicis; these are parasites of fish and mosquitoes, respectively. These results suggest two possible route s and sources of infection to AIDS patients, these being perorally by inges tion of inadequately cooked fish or crustaceans or percutaneously during a bloodmeal taken by a haematophagous insect. Support for an insect source ha s been provided by recent detection of a microsporidium from mosquitoes in human corneal tissue.