What is it?
Piscirickettsiosis is caused by the bacterium Piscirickettsia salmonis, which has recently been classified within the gammaproteobacteria, family Piscirickettsiaceae.
The disease affects farmed salmon, trout and seabass and typically leads to mortality rates between 10 and 30 percent.
Piscirickettsiosis is an important disease problem in the Chilean salmon farming industry where mortality rates have reached up to 90 percent.
Where and when might it occur?
Outbreaks usually occur after fish are introduced to saltwater pens at water temperatures between 12˚C and 18˚C.
Transmission is mainly horizontal (fish to fish). Although P. salmonis has been isolated in reproductive organs of salmonids, vertical transmission has not been definitively demonstrated.
Bacteria are assumed to gain entry by breaching the physical barriers of the skin or gills. The invading bacteria then spread throughout the body via the blood (haematogenous spread), resulting in septicaemia.
Diagnosis
Fishing demonstrating a loss of appetite, emaciation, lethargic swimming, circling and swimming near the surface or at the sides of the net or pens is often a sign of the disease.
Increasing mortality is also seen.
Gross pathological signs are:
- progressive skin lesions that range from areas of raised scales, to white raised plaques, to shallow ulcers on flanks and head
- darkening of skin and pale gills
- swollen abdomen
- grey, swollen spleen and kidney
- mottled liver (due to diffuse haemorrhages) or ring-shaped white to pale-yellow lesions (granulomas and areas of necrosis)
- ascites (fluid in the abdominal cavity)
- signs of peritonitis, including generalised diffuse inflammation of abdominal organs, adhesions and increased volume of free abdominal fluid
- petechial (pinpoint) haemorrhages of the gastrointestinal tract, swim bladder and visceral fat.
Microscopic pathological signs are:
- vasculitis and necrosis of the liver and kidney; inflammatory macrophage infiltration
- Rickettsia-like organisms in macrophages and epithelial cells.
Source: Australian Government, Department of Agriculture, Fisheries and Forestry