Infection with Aeromonas salmonicida - atypical strains

Depending on the species affected, infection with atypical strains of Aeromonas salmonicida is known by such names as goldfish ulcer disease; carp erythrodermatitis; ulcer disease of flounder, eel and salmon; or marine aeromonad disease of salmonids (MAS) in Tasmania.

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 tank and pond level

• sluggishness
• loss of appetite
• increased mortality
• abnormal swimming and disorientation

Clinical signs of disease in an infected animal

• white raised patches on the skin that progress to ragged-edged red ulcers
• haemorrhages on skin and fin bases (usually the paired fins)
• fingernail-sized ulcers that might be anywhere on the fish
• ulcers can extend through the stomach wall, leading to protrusion of intestines
• ulcers most often on the upper side of the lateral line behind the head or at the base of the tail fin
• pale gills with pinpoint haemorrhaging

Gross signs of disease in an infected animal

• haemorrhages in muscle and internal organs

Disease agent

Atypical strains of the Aeromonas salmonicida bacterium differ from the typical strain causing furunculosis in salmonids, in that atypical strains affect mainly nonsalmonids (wild and cultured, marine and fresh water). Infection with atypical A. salmonicida does not necessarily result in the acute mortality and septicaemia characteristic of the typical furunculosis strain, but manifests more as external lesions and ulceration, often involving secondary infection. An exception to this is infection by MAS in Tasmania; in such cases, Atlantic salmon are most commonly affected and clinical presentation is similar to furunculosis.

Four strains of atypical A. salmonicida are recognised worldwide—the subspecies achromogenes, masoucida, smithia and pectinolytica. Of these, all except A. salmonicida pectinolytica cause disease in fish. A new atypical strain that has recently been described in Australia, A. salmonicida biovar Acheron, causes MAS in Atlantic salmon.

Host range

Fish found to be susceptible to infection with atypical A. salmonicida:

Laboratory experiments indicate that all trout and salmon species, as well as many nonsalmonids, are potentially susceptible to atypical strains of A. salmonicida. For example, Atlantic salmon and striped trumpeter have been found to be infected by cohabitation with infected flounder. Rainbow trout have been shown to be relatively resistant to atypical strains.

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

Presence in Australia

Atypical A. salmonicida has been officially reported from New South Wales, Queensland, South Australia, Victoria (goldfish ulcer disease) and Tasmania (greenback flounder biovar and Acheron biovar). Movement controls are in place to prevent the spread of goldfish ulcer disease to Western Australia and Tasmania. The Acheron biovar has been reported only from Tasmania and is limited to an isolated production area.

Epidemiology

• Transmission is generally through water.
• Susceptibility to the disease increases with damaged mucus and skin, such as occurs when fish are handled with nets.
• Outbreaks would be expected to occur at water temperatures above 10°C (i.e. summer months) and may be precipitated by stress.
• Secondary infection with other bacteria often results.
• 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.
• Diagnosis based on clinical or external signs of disease is difficult because clinical signs vary and skin ulcers are often infected with opportunistic bacteria and fungi. A definitive diagnosis requires laboratory examination.

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

Epizootic ulcerative syndrome, spring viraemia of carp, channel catfish virus disease

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.int-res.com/abstracts/dao/v56/n1/p31-42

www.nfmikro.net/Vintermotet04/Foredrag/Gudmundsdottir.htm

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