Piscirickettsia salmonis 1238 Microscopic lesions caused by P. salmonis in naturally acquired infections can be found in numerous organs and tissues characteristic of a systemic or septicaemic condition. Although the sequential development of the naturally acquired infections has not been reported, initial infections presumably commence when the physical barriers of the skin and/or gills are breached by the bacterium (Almendras et al. 1997; Smith, Pizarro, Ojeda, Contreras, Oyanedel & Larenas 1999). Replication of the bacterium results in raised discoloured areas on the skin that may then progress to shallow ulcers, as observed under field conditions and following experimental applications of the bacterium with filter paper patches to the skin (Smith, Contreras, Larenas, Aguillon, Garcés, Perez & Fryer 1997). Direct dispersion of the bacterial suspension onto the gills also initiates local infections that spread via the blood (haematogenous) and then via major vessels to the parenchyma of numerous organs (Almendras et al. 1997). A serosal spread of the bacterium is more characteristic of infections induced by intraperitoneal injections of the bacterium. In injected fish, the capsules of the major organs in the peritoneum are sites of replication prior to invasion of the parenchyma (Almendras et al. 1997). In the later stages of infection the internal and microscopic pathological changes observed in fish exposed via different routes become similar, most likely because septicaemia eventually occurs even in cases of the serosal spread of the bacterium (Cvitanich et al. 1991). The septic nature of infections is demonstrated by the presence of infected macrophages visible in blood smears from heavily infected fish (Cvitanich et al. 1991) (Fryer and Hedrick, 2003). Salmon Rickettsial Disease, Piscirickettsiosis The immune response of salmonids to P. salmonis infections remains poorly understood, but more recent studies, including successful laboratory vaccine trials, provide optimism for this approach to control of the disease. The humoral response following natural or experimental exposures to P. salmonis is weak, although anti-P. salmonis antibodies have been detected in the serum of convalescent coho salmon (Fryer and Hedrick, 2003). The transmission of P. salmonis in natural infections is horizontal or fish-to-fish and no vector is required. The reservoir for the bacterium is unknown but may include one or more species of fish or other aquatic animals. The principal hosts for P. salmonis are salmonid fish. Coho salmon was the species involved in the first recorded outbreaks of piscirickettsiosis in Chile. The disease has also been reported in rainbow trout, cherry salmon, Oncorhynchus masou (Walbaum), and Atlantic salmon in Chile (Fryer and Hedrick, 2003). Piscirickettsia salmonis is the first Gram-negative, intracellular bacterial pathogen isolated from fish and is a significant cause of mortality in salmonid fish (Fryer and Hedrick, 2003). Baboon Papio cynocephalus 9556 Bank vole Clethrionomys glareolus 447135 Bear Ursus americanus 9643 Birds Passeroidea 175121 Brown Trout Salmo trutta 8032 Buffalo Bison bison 9901 Carnivores Vulpes 9625 Cat Felis catus 9685 Catfishes Siluriformes 7995 Cattle Bos taurus 9913 Chicken Gallus gallus 9031 Chimpanzee Pan troglodytes 9598 chinchillas Chinchillidae 10150 Copper Pheasant Syrmaticus soemmerringii 9067 Deer Cervus elaphus 9860 Deer mouse Peromyscus maniculatus 10042 Dog Canis familiaris 9615 Ducks Anas 8835 Ferret Mustela putorius furo 9669 Fish Hyperotreti 117565 Gerbil Gerbillina 10045 Goat Capra hircus 9925 Gray wolf Canis lupus 9612 Guinea pig Cavia porcellus 10141 Hamster Mesocricetus auratus 10036 Horse Equus caballus 9796 Human Homo sapiens 9606 Macaque Macaca fascicularis 9541 Mongolian Gerbil Meriones unguiculatus 10047 Monkey Platyrrhini 9479 Mouse Mus musculus 10090 None None Parrot Psittacidae 9224 Pig Sus scrofa 9823 Rabbit Oryctolagus cuniculus 9986 Rainbow trout Oncorhynchus mykiss 8022 Rat Rattus 10114 Raven Corvus corax 56781 sei whale Balaenoptera borealis 9768 Sheep Ovis aries 9940 Squirrel Spermophilus richardsonii 37591 Tree shrew Tupaiidae 9393 Trouts, salmons & chars Salmoninae 504568 Turkey Meleagris gallopavo 9103 Vole Microtus ochrogaster 79684 Water buffalo Bubalus bubalis 391902 Arthrobacter Live Culture Vaccine (USDA: 1K11.00) Novartis Animal Health US, Inc VO_0001819 Live vaccine Licensed USA Fryer and Hedrick, 2003 journal Fryer JL, Hedrick RP 2003 26 5 251-262 Journal of fish diseases