Furunculosis

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

• high mortality

Disease signs at the tank and pond level

• lethargy
• cessation of feeding
• swimming just below surface
• respiratory distress and random jumping from water immediately before an outbreak

Clinical signs of disease in an infected animal

• furuncles (or boils) causing deep abscesses, usually in adult salmon in the acute phase of infection
• haemorrhages on skin, mouth and fin bases (mainly paired fins)
• darkening of body colour
• exophthalmus (pop eye)

Gross signs of disease in an infected animal

• haemorrhages in muscle and internal organs
• enlarged spleen and focal necrosis of the liver
• stomach filled with mucus, blood and sloughed epithelial cells
• congested intestine
• fusion of gill lamellae

While classical furunculosis (as described above) occurs in both juvenile and adult salmon, peracute infections in juvenile salmon can result in death without the fish showing any clinical signs other than darkening of the skin.

Disease agent

Furunculosis is caused by the bacterium Aeromonas salmonicida subsp. salmonicida.

Host range

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

Presence in Australia

The strain of Aeromonas causing furunculosis in salmonids overseas (i.e. the typical strain) is not present in Australia. However, atypical strains are present (see Infection with Aeromonas salmonicida—atypical strains).

Epidemiology

• Furunculosis is highly contagious.
• The disease is considered by some to be the most commercially significant of all salmonid diseases. It occurs in fresh water and marine salmonid aquaculture worldwide, except in Australia and New Zealand.
• Overseas, this disease must be controlled on farms by medication or vaccination.
• Outbreaks would be expected to occur at temperatures above 10°C and may be precipitated by stress.
• Australia's relatively warm water temperatures may favour the establishment of A. salmonicida.
• Rainbow trout are relatively resistant to the disease, but are still considered susceptible.
• Transmission is via the water (mouth, gills and skin).
• Ingestion of tissue from infected salmonids can transmit infection to nonsalmonids nearby. Similarly, transmission to nonsalmonids can occur where fish used to feed these populations are taken from waters near an outbreak.
• Susceptibility to the disease increases with damaged mucus and skin, such as occurs when fish are handled with nets.
• Fish surviving disease outbreaks are recognised as carriers of the disease and may continue to infect the remaining population without themselves showing any outward signs of infection.

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.

Similar diseases

Viral haemorrhagic septicaemia, infection with Aeromonas salmonicida—atypical strains

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

www.daff.gov.au/animal-plant-health/aquatic/aquavetplan/furunculosis