The structure of the human microsporidium found by Yachnis and colleagues in two AIDS patients (Am. J. Clin. Pathol. 106: 535-43, 1996) (hereafter referred to as HMY) was investigated by light and transmission electron microscopy and compared with Thelohania apodemi Doby, Jeannes et Raoult, 1963, a microsporidian of small rodents. The fine structure of the HMY was found to be similar to that of Trachipleistophora hominis Hollister, Canning, Weidner, Field, Kench et Marriott, 1996. Characteristic is the presence of a thick layer of electron dense material on the outer lace of the meront plasmalemma, which is maintained during the whole life cycle and which later persists as an electron dense coat on the sporophorous vesicle (SPOV). However, HMY is distinguished from T. hominis during sporogony, as two types of SPOV and spores are formed in HMY. One type of SPOV contains thick-wallcd spores (usually 8 or more in number) with anisofilar polar filaments of 7 + 2 pattem, while the other type contains only two thin-walled spores with a smaller number (3-5) of isofilar polar filament coils. The HMY differs from T. apodemi which also forms SPOV with 8 spores inside, but the spores of which are larger in size and have 9 + 2 polar filament pattern.
Relatively few effective compounds are available for treating microsporidiosis in humans. In this study, several compounds were assayed for activity against Encephalitozoon intestinalis (Cali, Kotier et Orenstein, 1993) and Vittaforina corneae Shadduck, Meccoli, Davis ct Font, 1990 in vitro. Of the benzimidazoles tested, albendazole was most effective and the MIC50 values were 8.0 ng/ml and 55.0 ng/ml for E. intestinalis and V. corneae, respectively. Fumagillin and its analogue, TNP-470 were nearly equally effective against both E. intestinalis and V. comeae. The MIC50 values of fumagillin were 0.52 ng/ml and 0.81 ng/ml, and the MIC5() values of TNP-470 were 0.35 ng/ml and 0.38 ng/ml for E. intestinalis and V. comeae, respectively. In addition, 12 of 44 purines and pteridines with putative tubulin binding activity that were synthesized at Southern Research Institute (SRI), inhibited microsporidia) replication by more than 50% at concentrations that were not toxic to the host cells. Several chitin synthesis/assembly inhibitors inhibited growth of the microsporidia in vitro but were toxic for the host cells making it difficult to interpret the results. One exception was Iufcnuron, which caused no significant toxicity to the host cells and expressed approximate MICS0 values of 2.95 pg/ml and 6.3 pg/ml against E. intestinalis and V. comeae, respectively. These results warrant further studies on albendazole, fumagillin, TNP-470, lufenuron, and the selected SRI purines and pteridines for developing therapeutic strategies for microsporidiosis.