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Import Risk
Analysis on
Live Ornamental
Finfish

Page 93

78

4.2.5 AEROMONAS SALMONICIDA,

TYPICAL (FURUNCULOSIS) AND ATYPICAL STRAINS

Exotic strains of A. salmonicida are considered in this

section. Because goldfish ulcer disease (GUD) is

present in Australia and is not the subject of mandatory

controls nationwide, no further consideration is given to

the GUD biovar of ‘atypical’ A. salmonicida.

Geographic distribution

‘Typical’ A. salmonicida has a wide distribution but has

not been reported in Australia, New Zealand, or South

America (reviewed in Hiney and Olivier 1999). However,

Shotts (1994) reported the agent’s presence in North

and South America, Europe, Asia and Africa. ‘Atypical’

strains of A. salmonicida have been reported in various

parts of the world including Canada, the United States,

Japan, central and northern Europe (including the Nordic

countries), Australia, South Africa, the Mediterranean,

Yugoslavia and Italy (Wiklund and Dalsgaard 1998).

Hiney and Olivier (1999) observed that there appears

to be a strong correlation between strains of ‘atypical’

A. salmonicida and their geographical region of origin.

Host range and prevalence

‘Typical’ A. salmonicida is reported from both salmonids

and non-salmonids, and causes several diseases

including furunculosis of salmonids. Bernoth (1997) in

her review of this pathogen listed several non-salmonid

hosts, including ornamental cyprinids (probably koi carp),

minnow (Phoxinus phoxinus), stickleback (Culaea

inconstans) and paddlefish (Polyodon spathula) from

freshwater; and wrasse (Labrus bimaculatus and

Ctenolabrus rupestris), coalfish (Pollachius virens)

and Atlantic cod (Gadus morhua) from marine waters.

Of the species listed on Schedule 6, ‘typical’

A. salmonicida non-experimental infection has only

been reported from Labrus bimaculatus, a marine

coldwater wrasse (Labridae).

There is little information on the role of non-salmonid

carriers in the epidemiology of ‘typical’ A. salmonicida

infection of salmonids. In a study of cleaner wrasse,

Hjeltnes et al (1995) concluded that the transmission

of A. salmonicida from wrasse to salmonids under cage

culture conditions would be a rare event. Wrasse are

in the family Labridae, all species of which are included

on Schedule 6.

The reported host range for ‘atypical’ A. salmonicida is

wide. Of Schedule 6 species, ‘atypical’ A. salmonicida

has been isolated from apparently healthy goldsinny

wrasse (Ctenolabrus rupestris), a coldwater finfish

species (Frerichs et al 1992) and clinically infected

goldfish (Carassius auratus) (reviewed in Hiney and

Olivier 1999).

A. salmonicida is usually isolated from

temperate/coldwater species. Except for goldfish,

temperate species are not commonly traded in the

ornamental finfish industry in Australia.

Detection methods

Diagnosis of A. salmonicida is usually based on culture

of bacteria recovered from internal organs (especially

kidney) and from skin lesions if present. Problems may

be encountered with culture of fastidious strains as well

as contamination by opportunistic pathogens (Hiney and

Olivier 1999). ‘Typical’ subspecies are, on the whole,

quite homogeneous in genetic and biochemical profile

(Hiney and Olivier 1999).

Covert infection with ‘atypical’ A. salmonicida may occur.

The finding in Australia in 1974 of the GUD biovar is

thought to have followed from the importation of sub-

clinically infected goldfish (Humphrey and Ashburner

1993). Similarly, ‘typical’ A. salmonicida present at

extremely low levels in carrier fish may also be difficult

to detect (Shotts 1994).

The American Fisheries Society recommends using IFAT

on tissues and culture of bacteria recovered from

intestinal material and kidney tissue to detect ‘typical’

A. salmonicida in asymptomatic fish (Shotts 1994).

Standard serological tests for ‘typical’ A. salmonicida

could be modified and used for detecting ‘atypical’

strains (Hiney and Olivier 1999). Stress testing based on

heat treatment and injection of immunosuppressants is

more sensitive but may take up to 18 days to complete

(Hiney et al 1997). A. salmonicida-specific enrichment

media have been developed and there are several PCR-

based tests (both for pure culture and direct detection in

tissues) with varying degrees of sensitivity (Byers et al

C H A P T E R 4 : R I S K A S S E S S M E N T

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