The Disease

SRS (a.k.a. Salmon Rickettsial Syndrome or Piscirickettsiosis or Coho salmon septicaemia or Huito disease) is considered to be the most important disease problem in the Chilean salmon farming industry, with economic losses of over US$100 million in some years. SRS was first reported, from Chile, in 1989, but (Pisci)rickettsia-like organisms (RLO) are now frequently associated with disease syndromes in both salmonid and non-salmonid fish from both fresh and saltwater worldwide. During 1989, this disease was considered to be the cause of death of an estimated 1.5 million Coho salmon, many near market-size. A year later, the disease was also found to occur in Atlantic salmon and up to 90% mortality was seen on some farms. Outbreaks of SRS in other countries have not reached the levels of the Chilean outbreaks. For example, variable and inconsistent mortality of 0.6 - 15% has been reported in Canada and Norway.

Causative Agent
SRS is caused by the Gram-negative bacterium, Piscirickettsia salmonis. This was the first "rickettsia-like" bacterium to be recognized as a pathogen of fish. P. salmonis is a non-motile, obligate intracellular bacterium, pleomorphic but predominately coccoid, and 0.5-1.5 µm in diameter. It is currently placed in the class Gammaproteobacteria; order Thiotrichales; and family Piscirickettsiacaea, and has a closer relationship to, e.g., Legionella and Coxiella, than to members of the genera Rickettsia. P. salmonis replicates within membrane-bound cytoplasmic vacuoles in selected fish cell lines and in the cells of tissues throughout infected fish.

P. salmonis is the first of the RLO of fish to be fully characterized. Since it's recognition, the impact of RLO in fish has become increasingly apparent. Growing awareness of the emergence of these intracellular organisms has led to the discovery of rickettsial diseases among diverse species of fish from different geographic locations and aquatic environments. The source, reservoir, and mode of transmission of many of these agents, as well as consistently effective methods of disease prevention and control, remain to be established.

Disease reprinted courtesy of OIE Diagnostic Manual for Aquatic Animal Diseases, OIE (World Organisation for Animal Health), Paris, France.