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Aeromonas infection in fish

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Aeromonas infection in fish

Summary

  • Last modified
  • 09 November 2017
  • Datasheet Type(s)
  • Animal Disease
  • Preferred Scientific Name
  • Aeromonas infection in fish
  • Overview

  • Motile aeromonads of the Aeromonas hydrophila complex cause a haemorrhagic septicaemia in fish (

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Pictures

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PictureTitleCaptionCopyright
Eel (Anguilla japonica) with red-fin disease (haemorrhagic septicaemia) caused by Aeromonas hydrophila (courtesy of Dr Teruo Miyazaki).
TitleEel (Anguilla japonica) with red-fin disease
CaptionEel (Anguilla japonica) with red-fin disease (haemorrhagic septicaemia) caused by Aeromonas hydrophila (courtesy of Dr Teruo Miyazaki).
CopyrightT. Aoki
Eel (Anguilla japonica) with red-fin disease (haemorrhagic septicaemia) caused by Aeromonas hydrophila (courtesy of Dr Teruo Miyazaki).
Eel (Anguilla japonica) with red-fin diseaseEel (Anguilla japonica) with red-fin disease (haemorrhagic septicaemia) caused by Aeromonas hydrophila (courtesy of Dr Teruo Miyazaki).T. Aoki
Clariid catfish (Clarias batrachus) with ulcerative form of haemorrhagic septicaemia caused by Aeromonas hydrophila (courtesy of Dr Kriengsag Saitanu).
TitleClariid catfish (Clarias batrachus) with ulcerative form of haemorrhagic septicaemia
CaptionClariid catfish (Clarias batrachus) with ulcerative form of haemorrhagic septicaemia caused by Aeromonas hydrophila (courtesy of Dr Kriengsag Saitanu).
CopyrightT. Aoki
Clariid catfish (Clarias batrachus) with ulcerative form of haemorrhagic septicaemia caused by Aeromonas hydrophila (courtesy of Dr Kriengsag Saitanu).
Clariid catfish (Clarias batrachus) with ulcerative form of haemorrhagic septicaemiaClariid catfish (Clarias batrachus) with ulcerative form of haemorrhagic septicaemia caused by Aeromonas hydrophila (courtesy of Dr Kriengsag Saitanu).T. Aoki
Aeromonas hydrophila infection in Japanese eel. Intestine shows bacterial multiplication in the contents and mucous-desquamative catarrh (haematoxylin and eosin stain) (courtesy of Dr Teruo Miyazaki).
TitleAeromonas hydrophila infection in Japanese eel
CaptionAeromonas hydrophila infection in Japanese eel. Intestine shows bacterial multiplication in the contents and mucous-desquamative catarrh (haematoxylin and eosin stain) (courtesy of Dr Teruo Miyazaki).
CopyrightT. Aoki
Aeromonas hydrophila infection in Japanese eel. Intestine shows bacterial multiplication in the contents and mucous-desquamative catarrh (haematoxylin and eosin stain) (courtesy of Dr Teruo Miyazaki).
Aeromonas hydrophila infection in Japanese eelAeromonas hydrophila infection in Japanese eel. Intestine shows bacterial multiplication in the contents and mucous-desquamative catarrh (haematoxylin and eosin stain) (courtesy of Dr Teruo Miyazaki).T. Aoki
Aeromonas hydrophila infection in Japanese eel. Skin shows capillary haemorrhage in the dermal loose connective tissue and the thinned epithelium (haematoxylin and eosin stain) (courtesy of Dr Teruo Miyazaki).
TitleAeromonas hydrophila infection in Japanese eel
CaptionAeromonas hydrophila infection in Japanese eel. Skin shows capillary haemorrhage in the dermal loose connective tissue and the thinned epithelium (haematoxylin and eosin stain) (courtesy of Dr Teruo Miyazaki).
CopyrightT. Aoki
Aeromonas hydrophila infection in Japanese eel. Skin shows capillary haemorrhage in the dermal loose connective tissue and the thinned epithelium (haematoxylin and eosin stain) (courtesy of Dr Teruo Miyazaki).
Aeromonas hydrophila infection in Japanese eelAeromonas hydrophila infection in Japanese eel. Skin shows capillary haemorrhage in the dermal loose connective tissue and the thinned epithelium (haematoxylin and eosin stain) (courtesy of Dr Teruo Miyazaki).T. Aoki
Aeromonas hydrophila infection in Japanese eel. Kidney shows destroyed glomeruli accompanying exudation of serum and fibrin. Epithelia of renal tubules show necrosis and atrophy (haematoxylin and eosin stain) (courtesy of Dr Teruo Miyazaki).
TitleAeromonas hydrophila infection in Japanese eel
CaptionAeromonas hydrophila infection in Japanese eel. Kidney shows destroyed glomeruli accompanying exudation of serum and fibrin. Epithelia of renal tubules show necrosis and atrophy (haematoxylin and eosin stain) (courtesy of Dr Teruo Miyazaki).
CopyrightT. Aoki
Aeromonas hydrophila infection in Japanese eel. Kidney shows destroyed glomeruli accompanying exudation of serum and fibrin. Epithelia of renal tubules show necrosis and atrophy (haematoxylin and eosin stain) (courtesy of Dr Teruo Miyazaki).
Aeromonas hydrophila infection in Japanese eelAeromonas hydrophila infection in Japanese eel. Kidney shows destroyed glomeruli accompanying exudation of serum and fibrin. Epithelia of renal tubules show necrosis and atrophy (haematoxylin and eosin stain) (courtesy of Dr Teruo Miyazaki).T. Aoki
Comparison of deduced amino acid sequences among the AHH1, ASH4 and Vibrio cholerae El Tor haemolysin (Rader and Murphy, 1988). The same amino acid residues are indicated as o on the aligned sequences. VCH, V. cholerae El Tor haemolysin.
TitleComparison of deduced amino acid sequences
CaptionComparison of deduced amino acid sequences among the AHH1, ASH4 and Vibrio cholerae El Tor haemolysin (Rader and Murphy, 1988). The same amino acid residues are indicated as o on the aligned sequences. VCH, V. cholerae El Tor haemolysin.
CopyrightT. Aoki
Comparison of deduced amino acid sequences among the AHH1, ASH4 and Vibrio cholerae El Tor haemolysin (Rader and Murphy, 1988). The same amino acid residues are indicated as o on the aligned sequences. VCH, V. cholerae El Tor haemolysin.
Comparison of deduced amino acid sequencesComparison of deduced amino acid sequences among the AHH1, ASH4 and Vibrio cholerae El Tor haemolysin (Rader and Murphy, 1988). The same amino acid residues are indicated as o on the aligned sequences. VCH, V. cholerae El Tor haemolysin.T. Aoki
Comparison of deduced amino acid sequences among the AHH3, AHH4, AHH5, ASH3, ASA1, Aeromonas hydrophila aerolysin (Howard et al., 1987) and A. trota aerolysin (Husslein et al., 1988). The same amino acid residues are indicated as o on the aligned sequences. AHAER, A. hydrophila aerolysin; ATAER, A. trota aerolysin.
TitleComparison of deduced amino acid sequences
CaptionComparison of deduced amino acid sequences among the AHH3, AHH4, AHH5, ASH3, ASA1, Aeromonas hydrophila aerolysin (Howard et al., 1987) and A. trota aerolysin (Husslein et al., 1988). The same amino acid residues are indicated as o on the aligned sequences. AHAER, A. hydrophila aerolysin; ATAER, A. trota aerolysin.
CopyrightT. Aoki
Comparison of deduced amino acid sequences among the AHH3, AHH4, AHH5, ASH3, ASA1, Aeromonas hydrophila aerolysin (Howard et al., 1987) and A. trota aerolysin (Husslein et al., 1988). The same amino acid residues are indicated as o on the aligned sequences. AHAER, A. hydrophila aerolysin; ATAER, A. trota aerolysin.
Comparison of deduced amino acid sequencesComparison of deduced amino acid sequences among the AHH3, AHH4, AHH5, ASH3, ASA1, Aeromonas hydrophila aerolysin (Howard et al., 1987) and A. trota aerolysin (Husslein et al., 1988). The same amino acid residues are indicated as o on the aligned sequences. AHAER, A. hydrophila aerolysin; ATAER, A. trota aerolysin.T. Aoki

Identity

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Preferred Scientific Name

  • Aeromonas infection in fish

International Common Names

  • English: Aeromonas hydrophila infection; Aeromonas infection of fish; bacterial eye disease; bacterial haemorrhagic septicaemia; bacterial hemorrhagic septicemia; eyeball disease in catfish; eye-disease of catfish; haemorrhagic septicaemia; hemorrhagic septicemia; motile aeromonad septicaemia; motile aeromonad septicemia; red fin disease; red sore disease; redfin disease

Overview

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Motile aeromonads of the Aeromonas hydrophila complex cause a haemorrhagic septicaemia in fish (Bullock et al., 1971; Egusa, 1978; Schäperclaus et al., 1992). This bacterium has been observed in numerous species of freshwater fish and occasionally in marine fish and in amphibians, reptiles, cattle and humans throughout the world (Bullock et al., 1971; Khardori and Fainstein, 1988). However, the most significant diseases occur in cultured freshwater fish. The bacterium is distributed widely in fresh water and bottom sediments containing organic material, as well as in the intestinal tract of fish (Aoki, 1974; Egusa, 1978; Hazen et al., 1978; Seidler et al., 1980; Kaper et al., 1981; Van der Kooij and Hijnen, 1988; Sugita et al., 1994, Dumontet et al., 1996).

Infectious abdominal dropsy in common carp has been attributed to the A. hydrophila group (Aeromonas punctata) and was first described by Schäperclaus (1930), who reported on this condition in cultured, wild and stocked carp in central and eastern Europe. The causative agent has since been shown to be rhabdovirus carpio (spring viraemia of carp) (Fijan, 1972; Wolf, 1988). During the 1960s, outbreaks of red fin disease, caused by A. hydrophila, occurred frequently in cultured eels in Japan (Hoshina, 1962; Egusa, 1978), with concurrent infections by Saprolegnia parasitica (Egusa, 1978). Currently, only sporadic outbreaks of A. hydrophila occur in cultured eels. Aeromonas hydrophila is typically recognized as an opportunistic pathogen or secondary invader (Austin and Austin, 1987).

Conversely, there have been reports of A. hydrophila acting as a primary pathogen in fish. Isolates differ greatly in their pathogenicity with some strains being highly virulent and others non-virulent. Eddy (1960) and Kou (1972a) reported that non-virulent or weakly pathogenic strains did not produce gas and acetone from glucose. Wakabayashi et al. (1981) recognized common and identical biochemical characteristics in virulent strains, in particular the production of elastase and enzymes involved in lysis of Staphylococcus. These characteristics were identical to the A. hydrophila biovar. hydrophila, following classification by Popoff and Véron (1976). Economically, the disease attributed to these bacteria is of greatest importance in cultured freshwater fish. Recent advances in biochemistry, molecular biology and virulence factors associated with A. hydrophila have led to new understanding of this bacterial group.

[Derived from: Woo, PTK and Bruno, DW, eds, 1999. Fish diseases and disorders, Volume 3: Viral, bacterial and fungal infections. Wallingford, UK: CAB International]

Hosts/Species Affected

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Most cultured and wild freshwater fish are susceptible to infection by Aeromonas hydrophila, but particularly cold-water fish, such as brown trout (Salmo trutta), rainbow trout (Oncorhynchus mykiss), chinook salmon (Oncorhynchus tshawytscha), ayu (Plecoglossus altivelis), carp (Cyprinus carpio), channel catfish (Ictalurus punctatus), clariid catfish (Clarias batrachus), Japanese eel (Anguilla japonica), American eel (Anguilla rostrata), gizzard shad (Dorosoma cepedianum), goldfish (Carassius auratus), golden shiner (Notemigonus crysoleucas), snakehead fish (Ophicephalus striatus) and tilapia (Tilapia nilotica) (Bullock et al., 1971; Egusa, 1978; Saitanu, 1986).

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Asia

AfghanistanPresentOIE, 2009
ArmeniaPresentOIE, 2009
AzerbaijanDisease not reportedOIE, 2009
BahrainDisease never reportedOIE, 2009
BangladeshPresentOIE, 2009
BhutanDisease not reportedOIE, 2009
CambodiaPresentOIE, 2009
ChinaRestricted distributionOIE, 2009
-Hong KongNo information availableOIE, 2009
IndiaPresentOIE, 2009
IndonesiaPresentOIE, 2009
IranRestricted distributionOIE, 2009
IraqPresentOIE, 2009
IsraelDisease not reportedOIE, 2009
JapanDisease never reportedOIE, 2009
JordanDisease never reportedOIE, 2009
KazakhstanDisease not reportedOIE, 2009
Korea, Republic ofNo information availableOIE, 2009
KuwaitDisease not reportedOIE, 2009
KyrgyzstanDisease not reportedOIE, 2009
LaosPresentOIE, 2009
LebanonDisease not reportedOIE, 2009
MalaysiaPresentOIE, 2009
MongoliaNo information availableOIE, 2009
MyanmarPresentOIE, 2009
NepalPresentOIE, 2009
OmanDisease not reportedOIE, 2009
PakistanPresentOIE, 2009
PhilippinesPresentOIE, 2009
QatarNo information availableOIE, 2009
Saudi ArabiaPresentOIE, 2009
SingaporeDisease not reportedOIE, 2009
Sri LankaDisease not reportedOIE, 2009
SyriaDisease not reportedOIE, 2009
TajikistanDisease not reportedOIE, 2009
ThailandPresentOIE, 2009
TurkeyNo information availableOIE, 2009
United Arab EmiratesNo information availableOIE, 2009
VietnamPresentOIE, 2009
YemenNo information availableOIE, 2009

Africa

AlgeriaDisease not reportedOIE, 2009
AngolaDisease not reportedOIE, 2009
BeninPresentOIE, 2009
BotswanaDisease not reportedOIE, 2009
Burkina FasoPresentOIE, 2009
ChadNo information availableOIE, 2009
CongoNo information availableOIE, 2009
DjiboutiDisease not reportedOIE, 2009
EgyptDisease not reportedOIE, 2009
EritreaNo information availableOIE, 2009
EthiopiaPresentOIE, 2009
GabonDisease never reportedOIE, 2009
GambiaPresentOIE, 2009
GhanaPresentOIE, 2009
GuineaPresentOIE, 2009
Guinea-BissauPresentOIE, 2009
KenyaDisease not reportedOIE, 2009
LesothoDisease not reportedOIE, 2009
MadagascarDisease never reportedOIE, 2009
MalawiNo information availableOIE, 2009
MaliNo information availableOIE, 2009
MauritiusDisease not reportedOIE, 2009
MoroccoNo information availableOIE, 2009
MozambiqueDisease not reportedOIE, 2009
NamibiaDisease not reportedOIE, 2009
NigeriaDisease not reportedOIE, 2009
RwandaNo information availableOIE, 2009
SenegalPresentOIE, 2009
South AfricaAbsent, reported but not confirmedOIE, 2009
SudanPresentOIE, 2009
SwazilandNo information availableOIE, 2009
TanzaniaPresentOIE, 2009
TogoNo information availableOIE, 2009
TunisiaDisease not reportedOIE, 2009
UgandaNo information availableOIE, 2009
ZambiaPresentOIE, 2009
ZimbabweDisease not reportedOIE, 2009

North America

CanadaDisease never reportedOIE, 2009
GreenlandDisease never reportedOIE, 2009
MexicoDisease never reportedOIE, 2009
USAAbsent, reported but not confirmedOIE, 2009
-GeorgiaDisease not reportedOIE, 2009

Central America and Caribbean

BelizeDisease never reportedOIE, 2009
Costa RicaDisease never reportedOIE, 2009
CubaDisease never reportedOIE, 2009
Dominican RepublicNo information availableOIE, 2009
El SalvadorDisease never reportedOIE, 2009
GuadeloupeNo information availableOIE, 2009
GuatemalaDisease never reportedOIE, 2009
HaitiDisease never reportedOIE, 2009
HondurasNo information availableOIE, 2009
JamaicaDisease not reportedOIE, 2009
MartiniqueNo information availableOIE, 2009
NicaraguaNo information availableOIE, 2009
PanamaDisease not reportedOIE, 2009

South America

ArgentinaDisease not reportedOIE, 2009
BoliviaNo information availableOIE, 2009
BrazilPresentOIE, 2009
ChileDisease never reportedOIE, 2009
ColombiaDisease not reportedOIE, 2009
EcuadorDisease never reportedOIE, 2009
French GuianaDisease not reportedOIE, 2009
PeruDisease never reportedOIE, 2009
UruguayDisease never reportedOIE, 2009
VenezuelaNo information availableOIE, 2009

Europe

AlbaniaNo information availableOIE, 2009
AustriaDisease not reportedOIE, 2009
BelarusPresentOIE, 2009
BelgiumDisease not reportedOIE, 2009
BulgariaDisease not reportedOIE, 2009
CroatiaDisease not reportedOIE, 2009
CyprusDisease never reportedOIE, 2009
Czech RepublicDisease not reportedOIE, 2009
DenmarkDisease never reportedOIE, 2009
EstoniaDisease not reportedOIE, 2009
FinlandDisease not reportedOIE, 2009
FranceDisease never reportedOIE, 2009
GermanyDisease not reportedOIE, 2009
GreeceDisease not reportedOIE, 2009
HungaryDisease not reportedOIE, 2009
IcelandDisease never reportedOIE, 2009
IrelandDisease not reportedOIE, 2009
ItalyDisease not reportedOIE, 2009
LatviaDisease not reportedOIE, 2009
LiechtensteinDisease not reportedOIE, 2009
LithuaniaDisease not reportedOIE, 2009
LuxembourgDisease not reportedOIE, 2009
MacedoniaAbsent, reported but not confirmedOIE, 2009
MaltaDisease not reportedOIE, 2009
MontenegroDisease never reportedOIE, 2009
NetherlandsDisease never reportedOIE, 2009
NorwayDisease never reportedOIE, 2009
PolandDisease not reportedOIE, 2009
PortugalPresentOIE, 2009
RomaniaDisease not reportedOIE, 2009
Russian FederationPresentOIE, 2009
SerbiaNo information availableOIE, 2009
SlovakiaDisease not reportedOIE, 2009
SloveniaDisease never reportedOIE, 2009
SpainDisease not reportedOIE, 2009
SwedenDisease never reportedOIE, 2009
SwitzerlandDisease not reportedOIE, 2009
UKDisease never reportedOIE, 2009
UkraineDisease not reportedOIE, 2009

Oceania

AustraliaDisease never reportedOIE, 2009
French PolynesiaDisease never reportedOIE, 2009
New CaledoniaDisease never reportedOIE, 2009
New ZealandDisease never reportedOIE, 2009

Pathology

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Pathogenesis and immunity

A variety of possible virulence factors of Aeromonas hydrophila have been suggested, including lipopolysaccharides (endotoxins), extracellular products (ECP), siderophores, the ability of attachment to host cells and surface proteins. The ECP include a cytotoxin, enterotoxin, haemolysins, protease, haemagglutinin and acetyl cholinesterase (Cahill, 1990; Gosling, 1996; Howard et al, 1996). Aeromonas hydrophila enters through the epithelium of the intestinal tract of fish. Enterotoxins of A. hydrophila cause fluid to accumulate in ligated rabbit ileal loops. Enterotoxins are divided into two types, cytotonic and cytotoxic. Boulanger et al. (1977) isolated two different haemolysins, an alpha- and a beta-haemolysin, both of which have been implicated in the pathogenesis of infection. Allan and Stevenson (1981) investigated the production of protease and haemolysin in the ECP of A. hydrophila strains and showed a close correlation between the quantity of haemolysin and toxicity to fish. Two biologically similar but immunologically distinct haemolysins were purified by Asao et al (1986) and Kozaki et al. (1987, 1989) These haemolysins (cytotoxic enterotoxin) had enterotoxic activity and caused fluid accumulation in infant mice, in mouse intestine and in rabbit ligated ileal loops. Purified haemolysin also displayed cytotoxicity to Vero cells and lethal toxicity to mice. The molecular weight of the haemolysin was estimated at 48,000-50,000 kDa and biological activity was inactivated with heating for 5 min at 65°C (Asao et al., 1984).

Cross-reaction of A. hydrophila haemolysin and cholera toxin has been reported, and a specific synthetic oligonucleotide probe for regions of A + B subunits of cholera toxin was found to hybridize with chromosomal DNA from some strains of A. hydrophila. The enterotoxin purified by Rose et al. (1989) had a molecular size of 52,000, but only 25 residues of the N-terminal sequences were found to be identical to the haemolysin of A. hydrophila (aerolysin) (Howard et al., 1987). The amino acid residues differed significantly between enterotoxin and aerolysin over the vast majority of the protein. The structure of proaerolysin was determined by X-ray crystallography at 2.8 Å (Parker et al., 1994). The aerolysin secretion system indicates that it is a haemolysin which is directed outside the cell through periplasm (Wong and Buckley, 1993). Five haemolysin genes (AHH1, AHH2, AHH3, AHH4 and AHH5) were cloned from A. hydrophila into a E. coli vector (Aoki and Hirono, 1991; Hirono and Aoki, 1991; Hirono et al., 1992). Each was classified into three groups, depending on their nucleotide sequences. AHH1 belonged to one group (group 1), which contained part of a homologous sequence of the haemolysin genes of Vibrio cholerae El Tor and Vibrio vulnificus (Rader and Murphy, 1988). There were two highly conservative regions, and the location of the cysteine residue was conserved. These regions may be important in the control of haemolytic activity. The other group (group 2) included AHH3, AHH4 and AHH5, the previously reported aerolysin gene from A. hydrophila and A. trota (Howard et al., 1987; Husslein et al., 1988). Chopra et al. (1993) cloned a cytolytic enterotoxin gene from A. hydrophila. The gene was also classified into group 2 by the nucleotide sequence. The remaining gene, AHH2, is placed in group 3. From colony hybridization analysis, using the cloned haemolysin genes, it was found that AHH1 (group 1) and AHH5 (group 2) were widely distributed among aeromonads. It is interesting that all tested strains of A. salmonicida have AHH1 and AHH5 genes. One strain of A. hydrophila has two or three haemolysin genes (Hirono and Aoki, 1991; Hirono et al., 1992). A significant qualitative as well as quantitative difference in the protease components of ECP was produced by A. hydrophila and A. sobria, which were pathogenic for fish (Nieto and Ellis, 1991). The role of protease in the virulence of A. hydrophila is currently controversial. Wakabayashi et al. (1981) described most of the virulent strains of A. hydrophila biovar. hydrophila as having a high proteolytic activity. A single protease purified from A. hydrophila was lethal to carp and was dermonecrotic to guinea-pigs. Thune et al. (1982) and Lallier et al. (1984) found A. hydrophila protease to be lethal. Lallier et al. (1984) also showed that both virulent and weakly virulent strains were dermonecrotic in the guinea-pig, and a dermonecrotic factor was observed in sonicated cells of an A. hydrophila strain isolated from eel (Shimizu, 1968). Chabot and Thune (1991) characterized three proteases and found no correlation between either qualitative or quantitative protease production and virulence in age-0 channel catfish. A metalloprotease with a molecular size of 38 kDa and a serine proteinase with a molecular size of 22 kDa was purified from A. hydrophila strain B52. These were stable at 56°C for 10 min and had a lethal effect in fish, with a median lethal dose (LD50) of 150 ng g-1. Only the serine protease possessed cytotoxic activity (Rodriguez et al., 1992). Leung and Stevenson (1988) found two distinct types of extracellular protease: thermostable metalloprotease and thermolabile serine protease. The relationship between these and the two purified by Nieto and Ellis (1986) is not known, but they were also lethal to fish. Protease from A. hydrophila enhanced haemolysin activity (Howard and Buckley, 1985), but Lallier et al. (1984) found that purified haemolysin from A. hydrophila was not lethal to fish. A novel zinc-proteinase was purified and characterized from A. hydrophila (Loewy et al., 1993), and Rivero et al. (1990) cloned an extracellular protease gene from A. hydrophila. Aeromonas hydrophila strains, which had haemolytic activity, enterotoxin productivity and cytotoxic ability, were not virulent for rainbow trout (Santos et al., 1988).

Yadav et al. (1992) noted that the fish cell lines, especially the BB (brown bullhead, Ictalurus nebulosus) cells were sensitive targets for A. hydrophila cytotoxins, even when assayed in conditions vastly different from those of mammalian cells. Cytotoxin-producing strains were frequently associated with epizootic ulcerative syndrome (EUS)-infected fish, compared with healthy fish. Cytotoxic A. hydrophila strains have a role in the pathogenicity and progression of EUS (Yadav et al., 1992).

Aoki and Holland (1985) observed that iron-binding proteins with a molecular size of 68-70 kDa in A. hydrophila were induced under iron-limiting conditions. Enterobactin, the catecholate siderophore produced by a strain of A. hydrophila (Andrus and Payne, 1983), and amonabactin, a novel phenolate siderophore in A. hydrophila 495A2, were identified (Barghouthi et al., 1989, 1991). Esteve and Amaro (1991) found the hydroxamate-type siderophores produced by A. hydrophila. The iron-uptake system was similar in A. hydrophila and A. sobria isolated from European eels (Anguilla anguilla) and in A. salmonicida. These authors suggested that siderophore produced by A. hydrophila could be important and play a role in virulence for acquisition of iron from the host. Recently, the amonabactins were synthesized and their spectroscopic properties were elucidated (Telford et al., 1994). The receptor cell surface of A. hydrophila can bind to iron-containing proteins - lactoferrin, transferrin, ferritin, cytochrome C and haemin - of the host (Kishore et al., 1991; Ascencio et al., 1992), demonstrating a relationship between lactoferrin binding and siderophore production by the bacteria. A carbohydrate-reactive outer-membrane protein, which may contribute as an adhesive mechanism, was isolated from A. hydrophila strain A6 (Quinn et al., 1993, 1994). This protein is related to the colonization strategies of aeromonads associated with human enteric disease. However, the protein has not been proved to relate to pathogenicity in fish.

Two cytotonic enterotoxin genes were cloned from A. hydrophila; one of the enterotoxins was heat-labile at 56°C, while the other was heat-stable (Chopra et al., 1994).

The role of haemolytic activity and cytotoxic activity in contributing to pathogenicity to fish still needs to be elucidated.

The attachment of a pathogen to the epithelial tissue is the first step in the host disease process. The W pili of A. hydrophila strain Ae6 is the colonization factor for the intestine, as well as a haemagglutinin (Hokama and Nakasone, 1990; Hokama et al., 1990). Haemagglutinating activity was also detected in A. hydrophila (Atkinson and Trust, 1980; Corral et al., 1990). Aeromonas hydrophila exhibited aggregative adherence to HEp-2 cells (Neves et al., 1994), fish tissue culture and mucus-coated glass slides (Krovacek et al., 1987). Adherence and haemagglutination were not significantly correlated with virulence in fish (Corral et al., 1990).

De Meuron and Peduzzi (1979) have suggested that the K-antigen is a pathogenicity factor. Virulent strains of A. hydrophila possessed an S-layer on the cell surface (Dooley et al., 1986; Dooley and Trust, 1988; Ford and Thune, 1991, 1992).

Recently, Rodriquez et al. (1993) purified acetylcholinesterase from ECP of A. hydrophila and showed it to be lethal to fish. Glycerophospholipid : cholesterol acyltransferase (GCAT) and lipopolysaccharide complex enhanced the lethal exotoxicity and cytolysin of A. salmonicida (Lee and Ellis, 1990). A gene encoding GCAT was cloned from A. hydrophila (Thornton et al., 1988). Experimental vaccination for prophylaxis against infection of A. hydrophila has been examined (Stevenson, 1988). Fish immunized either intramuscularly or intraperitoneally with vaccine showed protection against challenge. The agglutinating antibody titre increased in the serum of immunized fish (Song et al., 1976; Ruangpan et al., 1986; Karunasagar et al., 1991).

Immersion vaccination of channel catfish using polyvalent sonicated antigens of A. hydrophila provides protection (Thune and Plumb, 1982). Lamers et al. (1985) noted that agglutinating antibody was recognized in the serum of carp immunized with A. hydrophila bacterin, following a second immersion with this vaccine. However, fish vaccinated by immersion or orally showed questionable protection.

Catfish immunized intraperitoneally by injection with the acid extract of the S-layer protein of A. hydrophila were protected from the homologous, virulent strain (Ford and Thune, 1992). Serological types of A. hydrophila are heterogeneous and a polyvalent vaccine is thought to be necessary for prevention of the infection.

Topics for further study

There is significant interest in the pathogenicity of A. hydrophila to fish. Several virulence factors, including the production of endotoxin, ECP, siderophores and surface proteins, and the ability of attachment to host cells, are under study, but their relative importance has not yet been elucidated. Cahill (1990) considers that proteases are most important in fish pathogenicity, but further study on adhesins, siderophores and surface proteins is needed to understand their role in the pathogenesis of disease caused by A. hydrophila.

Diagnosis

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The gross signs of disease are haemorrhagic septicaemia and fin rot. The aetiological agent can be grown on brain-heart infusion medium, tryptosoy agar (tryptone soya agar), nutrient agar and MacConkey agar with incubation at 20-25°C for 24-48 h. Numerous selective media have been developed for the isolation and presumptive identification of Aeromonas hydrophila or motile aeromonads (Moyer, 1996), including Rimler-Shotts medium (Shotts and Rimler, 1973), modified peptone beef-extract glycogen agar (McCoy and Seidler, 1973), Rippey-Cabelli (membrane filter method (mA)) agar (Rippey and Cabelli, 1979), MacConkey's agar supplemented with trehalose (Kaper et al., 1981) and starch-ampicillin agar (Palumbo et al., 1985). Davis and Sizemore (1981) reported that Rimler and Shotts medium and Rippey-Cabelli agar were not suitable for A. hydrophila. Arcos et al. (1988) compared six media for selective isolation of A. hydrophila and showed that mA agar gave the best recovery rate and also an acceptable specificity, but its selectivity was low. An API-20E test strip is used widely for identification of the Enterobacteriaceae (Kaper et al., 1979). Toranzo et al. (1986) indicated that the importance of biochemical characteristics must be backed up by standardized testing.

For diagnosis, the aetiological agent is isolated on nutrient agar at 30°C from kidney of a fish with haemorrhagic septicaemia. Isolates are motile Gram-negative bacilli. Wakabongo et al. (1992) found that only four tests - aesculin hydrolysis, acetoin production, lysine decarboxylation and gas from glucose - were sufficient to distinguish A. hydrophila, A. caviae and A. sobria. A number of A. hydrophila-lytic bacteriopahges were isolated from river water, river mud, sewage and human clinical origin (e.g. stool, urine) and phage typing was attempted (Chow and Rouf, 1983; Demarta and Peduzzi, 1984; Altwegg et al., 1988; Merino et al., 1990; Fukuyama et al., 1991, 1992). A comprehensive phage-typing system for A. hydrophila will provide a useful tool for epizootiological and ecological studies. Aeromonas hydrophila has been identified by the gel-diffusion technique (Bullock, 1966), direct fluorescent antibody technique (Lewis and Allison, 1971), indirect fluorescent antibody technique (Lewis and Savage, 1972), immunoblotted sodium dodecylsulphate (SDS)-polyacrylamide gel electrophoresis (PAGE) (Mulla and Millership, 1993) and enzyme-linked immunosorbent assay (Merino et al., 1993). These methods are of limited value, because many different serological types of A. hydrophila are distributed in fish farms (Eddy, 1960; Bullock, 1966).

Lucchini and Altwegg (1992) differentiated ribotyping of restriction genomic DNAs of aeromonads, using different fragments of a 16S rDNA gene of E. coli as a probe and achieved identification of most Aeromonas strains to species level. This method is easier than DNA-DNA hybridization, because only minimal amounts of genomic DNA are needed and several strains can be analysed on a single gel. Pulsed-field gel electrophoresis is a rapid and discriminatory technique for the typing of A. hydrophila where a common origin of infection is suspected (Talon et al., 1996).

Deoxyribonucleic acid probe hybridization technology is now becoming available for the direct detection and identification of microorganisms. The colony hybridization technique (Grunstein and Hogness, 1975) can be applied easily to identify and count the causative organism. Fish-pathogenic bacteria, such as Vibrio anguillarum (Hirono et al., 1996) and Pasteurella piscicida (Zhao and Aoki, 1992a), can be detected by probe hybridization with their specific DNA nucleotide sequence or cryptic plasmid. However, there are many common DNA fragments between A. hydrophila and Aeromonas salmonicida (Miyata et al., 1995), which makes it unlikely that this probe technique will be successful for this species.

The sensitivity obtained using hybridization with non-radiolabelled probes is lower than with radiolabelled probes (Zhao and Aoki, 1992a). However, radiolabelled probes are generally unacceptable for use in diagnosis and further development of detection procedures based on non-radiolabelling is still required.

The polymerase chain reaction (PCR) (Mullis and Faloona, 1987) is useful for detecting fish pathogens from diseased fish and their environment. This technique is available for A. salmonicida (Miyata et al., 1996), Edwardsiella tarda (Aoki and Hirono, 1995), P. piscicida (Aoki et al., 1997) and V. anguillarum (Hirono et al., 1996). However, recently, Cascón et al. (1996) found a specific PCR primer set for the detection of A. hydrophila hybridization group 1.

The DNA fingerprinting method, AFLP (amplified fragment length polymorphism), is a valuable high-resolution genotype tool for classification of Aeromonas species (Huys et al., 1996).

List of Symptoms/Signs

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SignLife StagesType
Finfish / Haemorrhagic lesions - Skin and Fins Aquatic:All Stages Sign
Finfish / Skin erosion - Skin and Fins Aquatic:All Stages Sign

Disease Course

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Diseased fish usually display cutaneous haemorrhage of the fins and trunk, and the condition is often refered to as 'red fin disease' (Hoshina, 1962).

The bacteria multiply inside the intestine, causing a haemorrhagic mucous-desquamative catarrh. Toxic metabolites of Aeromonas hydrophila are absorbed from the intestine and induce a toxaemia. Capillary haemorrhage occurs in the dermis of fins and trunk and in the submucosa of the stomach. Hepatic cells and epithelia of renal tubules show degeneration. Glomeruli are destroyed and the tissue becomes haemorrhagic, with exudates of serum and fibrin (Miyazaki and Jo, 1985; Miyazaki and Kaige, 1985).

European carp infected with A. hydrophila show severe tail and fin rot and visible haemorrhage and ulceration of the body surface. Widespread proliferation of bacteria is usually observed in the intestine. In some reports (Fijan, 1972; Wolf, 1988), the histopathological phenomena associated with the rhabdovirus infection haemorrhagic septicaemia of carp have been erroneously attributed to motile aeromonads (Bullock et al., 1971). Aeromonas hydrophila is widely distributed in the intestinal tract of cultured fish and the water and sediments of freshwater ponds which are rich in organic materials. Virulent strains of A. hydrophila in these environments are possible sources of infection. Outbreaks of disease are usually associated with a change in environmental conditions. Stressors, including overcrowding, high temperature, a sudden change of temperature, rough handling, transfer of fish, low dissolved oxygen, poor nutritional status and fungal or parasitic infection, contribute to physiological changes and heighten susceptibility to infection. This applies to fish of all ages.

Epidemiology

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The aetiological agent is transmitted horizontally but not vertically. It is distributed widely in water and sediments of ponds and can be transmitted by discharge from the intestinal tract and external lesions on the skin (Aoki, 1974; Egusa, 1978; Hazen et al., 1978; Seidler et al., 1980; Kaper et al., 1981; Van der Kooij and Hijnen, 1988; Sugita et al., 1994; Dumontet et al., 1996). Parasite damage and fungal infection of the epidemics may allow entry and spread of bacterial infection (Egusa, 1978).

Aeromonas hydrophila has been recognized as a pathogen not only of amphibians, reptiles and snakes but also cattle and humans (Eddy, 1960; Khardori and Fainstein, 1988). It has been implicated as the causative agent of clinical infections in humans, including septicaemia and peritonitis (Janda and Abbott, 1996). Recently, its role as a psychrotrophic spoiler of meat, seafood and vegetables has been recognized (Palumbo, 1996). No confirmed cases of A. hydrophila food poisoning have been reported, but its association with gastrointestinal illness suggests that it plays a role in food-borne disease. The epidemiological relationship among A. hydrophila isolated from fish, human and environmental resources is particularly difficult to assess.

Aeromonas hydrophila is cosmopolitan in distribution. Infection has been seen in many freshwater fish, e.g. ayu, carp, channel catfish, eel, gizzard shad, salmon, snakehead fish and trout (Bullock et al., 1971; Egusa, 1978; Saitanu, 1986).

Impact Summary

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CategoryImpact
Animal/plant collections None
Animal/plant products None
Biodiversity (generally) Negative
Crop production None
Environment (generally) None
Fisheries / aquaculture Negative
Forestry production None
Human health None
Livestock production None
Native fauna Negative
Native flora None
Other None
Rare/protected species None
Tourism None
Trade/international relations None
Transport/travel None

Zoonoses and Food Safety

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Aeromonas hydrophila has been recognized as a pathogen not only of amphibians, reptiles and snakes but also cattle and humans (Eddy, 1960; Khardori and Fainstein, 1988). It has been implicated as the causative agent of clinical infections in humans, including septicaemia and peritonitis (Janda and Abbott, 1996). Recently, its role as a psychrotrophic spoiler of meat, seafood and vegetables has been recognized (Palumbo, 1996). No confirmed cases of A. hydrophila food poisoning have been reported, but its association with gastrointestinal illness suggests that it plays a role in food-borne disease. The epidemiological relationship among A. hydrophila isolated from fish, human and environmental resources is particularly difficult to assess.

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