| Loma salmonae is a microsporidian that infects salmonids of the genus Oncorhynchus and produces spore-filled xenomas in the gills. The resistance phenomenon of rainbow trout to L. salmonae has been documented, but the nature of this resistant state is unknown. The first objective examines how rapidly this resistance develops by using the exposure re-exposure model combined with a technique for measuring sphere-like structures. When fish received a primary oral challenge with live spores, partial resistance developed by week (wk) 2 postexposure (PE) and a complete resistance to re-challenge occurred at 8 wk PE. Fish which received their primary challenge in the form of non-viable spores administered intraperitoneally (IP), showed evidence of partial resistance by wk 3 and total resistance by wk 6 PE. The second objective of this study investigates if head kidney lymphocytes from recovered, exposed, and unexposed fish could respond to antigens from L. salmonae when measured in an in vitro lymphocyte proliferation assay. Lymphocytes from fish orally exposed to live spores reached a maximum proliferation at wks 6 and 8 PE, when stimulated in vitro with the soluble and particulate antigens (Ag) respectively. When fish recovered from an initial infection were re-stimulated at wk 14 with live spores, a peak in the proliferative response to the particulate Ag occurred only at wk 15 PE. Lymphocytes from fish IP exposed to dead spores had a high lymphoproliferation at wk 2 PE when in vitro stimulated with the particulate Ag rather than with the soluble Ag. Finally, lymphocytes from fish IP exposed to dead spores and then orally re-exposed with fresh spores at wk 22 PE, had the highest proliferative response when stimulated in vitro with both Ag. The third objective of this study was to examine the ultrastructural features of gills from fish experimentally infected with L. salmonae to explain the mechanisms of infection in gills. At 3 wk PE, meronts were recognized within blood cells and pillar cells (no xenomas) associated with the capillary channels, lamellae and lamellar arteries and inflammation was absent. In lamellar arteries, infected cells were underneath endothelium. (Abstract shortened by UMI.)... |
