Taenia taeniaeformis (Cestoda) in the rat: ultrastructure of the host-parasite interface on days 8 to 22 postinfection.

The host-parasite interface was examined at the ultrastructural level 8 to 22 days postinfection (DPI) with metacestodes of Taenia taeniaeformis in the rat. Throughout this phase of development the parasite surface was invested with a dense surface coat of complex microtriches. At 8 to 14 DPI the plasma membrane of each microthrix extended beyond the distal end of the electron-dense tip, forming a slender tubular streamer over 10 microns long; by 18 DPI these had shortened and withered. Host cell processes interdigitated with the microtriches without evidence of harm to the parasite surface or the underlying tegument. The cells, on the other hand, became damaged, and their contents were shed into the matrix surrounding the microtriches. Lipid inclusions appeared within the parasites, and in the cytoplasm of surrounding inflammatory cells. By 22 DPI fibroblastic activity had resulted occasionally in the formation of a capsule surrounding a free-floating cysticercus, while in others intense granulocytic infiltration persisted with abutment and intermeshing of host cell and parasite surface processes; however there was still no evidence of any adverse effect on the microtriches, though many granulocytes were clearly pyknotic and degenerating. Evidently, the vigorous cellular response of the host is ineffective in either containing the expansion of the parasite or compromising the integrity of its surface membrane. The changing characteristics of the microtriches may be related to the need for dissolution of both intercellular matrices, and host cells as the vesicular organism rapidly increases in volume.