External & Internal Signs

Clinical Signs & Gross Pathology
SRS can cause high economic losses to the farmers affected, particularly in Chile, as typical cumulative mortality averages 20% during the 18-month saltwater production time to harvest. Affected fish are lethargic, dark in colour, anorexic, anaemic with mottled focal lesions within the liver, show respiratory problems, and swim near the surface. The first signs observed are often haemorrhages and lesions of the skin. The lesions range from small areas to shallow ulcers up to 2 cm in diameter. Internally, the kidney is swollen and the spleen enlarged. Petechial haemorrhages are found on the swim bladder and viscera. Diagnostic ring-shaped, cream-coloured lesions are present on the livers of chronically infected fish. However, in acute cases, death may be the only gross sign of disease.

Histological changes have been classified into the broad category of necrosis and inflammation. Inflammatory cells, fibrosis, a generalized coagulative necrosis, tubular degeneration and necrosis of the endothelium infiltrate the liver, spleen, intestine and haematopoietic cells of the kidney. Moribund fish are anaemic and haematocrit is often 20% to 50% of normal. The rickettsial organism infects a variety of cells, including circulating macrophages, in which they can replicate and cause cell lyses. It also enters brain tissue, thus affecting swimming ability. The mechanisms by which P. salmonis can enter target cells, avoid intracellular killing and survive inside the host are unclear.

An initial diagnosis of piscirickettsiosis can be made from gross lesions and is supported by the examination of tissue sections. Confirmation of the diagnosis requires isolation and/or serological identification of the causative organism. Kidney tissue from affected fish is aseptically removed, homogenized and inoculated on a cell monolayer with an antibiotic-free growth media. P. salmonis has been cultured in many (mostly salmonid) fish cell lines maintained in buffered Eagle's minimum essential medium (MEM) supplemented with 10% foetal bovine serum. Optimal in vitro growth occurs at 15 – 18 °C but is retarded above 20 °C and below 10 °C. [Due in part to this thermal range, there is no indication that P. salmonis or other RLO of fish cause disease in humans or other mammals.] Typically, P. salmonis isolation and growth is determined by the gradual appearance of a typical cytopathic effect (CPE) in cell monolayers. The first signs of a CPE consist of the formation of cell clusters about 10 days post-inoculation. The infected cells in the clusters typically round up and develop one or more large vacuoles within the cytoplasm. Inoculated cell cultures should be observed for up to 28 days before they are considered negative.

An indirect fluorescent antibody technique (IFAT) and immunohistochemistry have been developed as alternative procedures to detect P. salmonis. These latter techniques are faster and more specific than histochemical staining. However, they require additional specialized equipment and are more expensive. The detection of P. salmonis in cultivated salmonids via a nested PCR using universal primer is coming on stream and will be important for diagnosis of this disease.

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