Piscirickettsiosis

Signs of disease

Important: animals with disease may show one or more of the signs below, but disease may still be present in the absence of any signs.

Disease signs at the farm level

• increasing mortality

Disease signs at the tank and pond level

• lethargy
• circling
• cessation of feeding
• emaciation
• swimming at the sides of net-pens

Clinical signs of disease in an infected animal

• multiple small white spots on skin
• raised skin patches progressing to shallow ulcers on flanks and head
• darkening of skin
• ascites (swollen abdomen from accumulated fluid)

Gross signs of disease in an infected animal

• pale gills
• grey, swollen spleen and kidneys
• mottled to spotted liver with large pale necrotic lesions
• pinpoint haemorrhages of the stomach organs and flank muscle

Disease agent

Piscirickettsiosis is caused by the bacterium Piscirickettsia salmonis, which has recently been classified within the gammaproteobacteria, family Piscirickettsiaceae.

Host range

Fish known to be susceptible to piscirickettsiosis:

Piscirickettsiosis is only known to affect aquaculture stock and has not been recorded in fish from the wild.

* naturally susceptible (other species have been shown to be experimentally susceptible)

Presence in Australia

EXOTIC—not present in Australia.

Epidemiology

• Outbreaks usually occur at 12–18^oC, an unusually high water temperature for salmonids.
• Little is known about the ecology of this relatively recently identified agent.
• This disease results in septicaemia, with bacteria infecting the entire body through the blood.
• Causes 10–90 per cent mortality in seawater pen-raised coho salmon in Chile. Mortality is considerably less elsewhere.

Differential diagnosis

The differential diagnostic table and the list of similar diseases appearing at the bottom of each disease page refer only to the diseases covered by this field guide. Gross signs observed might well be representative of a wider range of diseases not included here. Therefore, these diagnostic aids should not be read as a guide to a definitive diagnosis, but rather as a tool to help identify the listed diseases that most closely account for the gross signs.

Further images

Sample collection

Because of uncertainty in differentiating diseases using only gross signs, and because some aquatic animal disease agents might pose a risk to humans, you should not try to collect samples unless you have been trained. Instead, you should phone your state or territory hotline number and report your observations. If samples have to be collected, the agency taking the call will advise you on what you need to do. Local or district fisheries/veterinary authorities could advise you on sampling.

Emergency disease hotline

For your state or territory emergency disease hotline number, see Whom to contact if you suspect a disease.

Further reading

The currently accepted procedures for a conclusive diagnosis of piscirickettsiosis are summarised at www.oie.int/eng/normes/fmanual/A_00030.htm