Transmission and pathogenesis of Piscirickettsia salmonis in Atlantic salmon (Salmo salar)

Piscirickettsia salmonis is the etiological agent of Salmonid Rickettsial Septicaemia (SRS), a major disease affecting several species of salmonids cultured in salt water in Southern Chile. This organism to grow and produce a characteristic cytopathic effect (CPE) in several fish cell lines but not Lepomis macrochirus R. (BF-2). In a first experiment, Chinook salmon embryo (CHSE-214) and BB cells were inoculated with P. salmonis, incubated at $15\sp\circ$C for 78 days, and studied by light microscopy (LM) and transmission in CHSE-214, and after 45 days post-infection in BB cell line. Transmission electron microscopy (TEM) examination of BB cells after 78 days post-infection, revealed P. salmonis within membrane-bound vacuoles or free within the cytoplasm and the extracellular space. These results indicate that BB cells are susceptible to infection with P. salmonis but exhibit a slower pattern of invasion. In a second experiment, two susceptible Atlantic salmon fibroblasts (ASF) and CHSE-214, and one non-susceptible (BB) fish cell lines were infected with P. salmonis and studied by LM and TEM at 1, 7, and 14 days post infection. Results showed differences in the appearance of CPE and in the intracellular infection mechanisms of the pathogen among cell lines. Infected CHSE-214 cells had vacuoles containing P. salmonis whereas ASF cells had empty vacuoles and BB cells had empty vacuoles and P. salmonis free in the cytosol. An in vivo experiment was designed to compare three routes of inoculation (oral, gill surface and intraperitoneal), and to study the effect of physical contact as a risk factor in the horizontal transmission of SRS in Atlantic salmon raised in fresh water. (Abstract shortened by UMI.).