Bacillus salmonicida, Bacterium salmonicida, and Bacterium trutta, AS, ulcer disease, erythrodermatitis

A. salmonicida can cause disease in fishes across a broad range of temperatures in marine and freshwater environments. It typically occurs in aquaculture and hatchery environments, but can also become established in the wild amongst introduced and native fish populations. 

Typical pathogenic infections of A. salmonicida has been documented in Chinook Salmon (Oncorhynchus tshwyatscha), Coho Salmon (Oncorhynchus kisutch), Brown Trout (Salmo trutta) and Lake Trout (Salvelinus namaycush) in the Great Lakes.  Juvenile Burbot (Lota lota) are highly susceptible to this pathogen (Natrah et al 2012). This disease has been reported in Atlantic Salmon (Salmo salar) in Ireland (Hiney et al. 1994) as well as the St. Marys River (R. Kinnunnen, Pers. Comm.).

Atypical pathogenic infections of A. salmonicida have symptoms similar to but  milder than the primary strain including lethargy, skin lesions, and respiratory distress.  Atypical infections are common in many other salmonid species such as Arctic char (Salvelinus alpinus), Brook Trout (Salvelinus fontinalis), Chum Salmon (Oncorhynchus keta), Grayling (Thymallus thymallus), Masou Salmon (Oncorhyncus masou), Pink Salmon (Oncorhynchus gorbuscha), Rainbow Trout (Oncorhynchus mykiss), and Sockeye Salmon (Oncorhynchus nerka).

Atypical pathogenic infections affect many additional species including Common Carp (Cyprinus carpio) and Crucian Carp (Carassius carassius) in which the bacteria causes erythrodermatitis, Chinese perch (Sinperca chautsi) in which it causes gill disease, as well as Goldfish (Carassius auratus), and Flounder (Platichthys flesus) in which it causes ulcer disease (Wiklund and Dalsgaard 1998, Lin et al. 2020, Ling et al. 2019). A. salmonicida has been reported to cause head ulcer disease in American Eel (Anguilla rostrata), European Eel (Anguilla anguilla), and Japanese Eel (Anguilla japonica) (Wiklund and Dalsgaard 1998).  Atypical pathogenic infections in species other than salmonids have also occurred in such hosts as Marbled Sole (Pleuronectes yokohamae), Japanese Flounder (Parlichthys olivaceus), and Spotted Halibut (Verasper variegatus) (Marsh 1902; Carson and Handlinger 1988; Mills et al. 1993; Cipriano et al. 1996; Bakke and Harris 1998; Cipriano and Bullock 2001; Crawford 2001; Martinez-Murcia et al. 2005; Kumagai et al. 2006).  Atypical Aeromonas  salmonicida is also associated with erosions on the mouth up to destruction of the jawbones in Sand Eels (Ammodytes lancea and Hyperoplus lanceolatus), Shotted Halibut (Eopsetta grigorjewi), and Eurasian Minnow (Phoxinus phoxinus) as well as impacts to the intestines in Common Wolffish (Anarhichas lupus).

Nonpathogenic infections have been observed in a diverse array of fishes, including Pacific herring (Clupea harengus pallasi), Bream (Abramis brama), Chub (Leuciscus cephalus), Crucian Carp (Carassius carassius), Dace (Leuciscus leuciscus), Roach (Rutilus rutilus), Rudd (Scardinius erythrophthalmus), Silver Bream (Blicca bjoerkna), Cod (Gadus morhua), Four Bearded Rockling (Enchelyopus cimbrius), Haddock (Melanogrammus aeglefinus), Tomcod (Gadus microgadus), Viviparous blenny (Zoarces viviparus), Whitling (Merlangius merlangus), Silver Perch (Bidyanus bidyanus), Spotted Wolffish (Anarhichas minor), Wrasse (Labrus berggylta), Goldsinny wrasse (Ctenolabrus rupestris),  European Perch (Perca fluviatilis), Smallmouth Bass (Micropterus dolomieui), Yellow Bass (Morone mississippiensis), Turbot (Scophthalmus maximus), American plaice (Hippoglossoides platessoides), Dab (Limanda limanda), Greenback Flounder (Rhombosolea tapirina), Halibut (Hippoglossus hippoglossus), Plaice (Pleuronectes platessa), Northern Pike (Esox lucius), and Sablefish (Anoplopoma fimbria) (Wiklund and Dalsgaard 1998).

Life History:

Like most bacteria, A. salmonicida reproduces by simple cell division, but plasmids can be transferred between individuals.  Plasmid transfer has been implicated in spread of antibiotic resistance (Attere et al 2017).  Reproduction occurs only within a suitable fish host.

The intestine may be the primary location of A. salmonicida in salmon, though infection via mucus, fins and gills may also occur (Hiney et al 1994). The bacteria then migrates to the bloodstream and kidneys.  The bacteria can go dormant and survive at least several days in water outside of the host (Allen-Austin et al. 1984) - 6 days on wet or dry nets, 8 days in seawater, 10 days in culture, 17 days in freshwater and 24 days in brackish water, 32 days in fish carcasses, 4 weeks in loamy sediment, 90 days in filtered seawater, 7 weeks in sandy sediment and 15 weeks in humic acid plus river sand (Cipriano and Bullock 2001).  This disease can be passed on via cohabitation, fish to fish contact, use of aerosols in aquaculture environments, ingestion of infected prey items, and cohabitation with infected mollusks (Marsh 1902; Post 1983; Arkoosh et al. 1998, 2004; Cipriano and Bullock 2001; Kozinska et al. 2002; Martinez-Murcia et al. 2005). Vertical transmission (host parent to offspring) is considered rare as infected eggs are unlikely to survive past the eyed-egg stage. Release of bacteria from dead fish can exceed 10^8 colony forming units per fish per hour (Rose et al. 2006).

There are various strains of the bacterium with differing and complex taxonomies, including a few subspecies (Studer et al. 2013, Cipriano and Bullock 2001). Forms vary genetically and biochemically, and both typical and atypical types of the bacteria can be either motile or non-motile.  Forms vary highly in virulence (Anderson 1972). Typical forms usually cause furunculosis in cold water salmonid species, and are particularly pathogenic to Brook Trout (Salvelinus fontinalis) or Char (Salvelinus alpinus). Atypical forms generally cause dermal ulcers and external pathological symptoms in other fish species, and may or may not lead to septicemia in the host. Interestingly, atypical forms of the species isolated from non-salmonids can sometimes cause typical symptoms in salmonids.

There are acute, chronic and carrier forms of furunculosis, and the impact of the disease increases with increasing environmental degradation and pollution. Originally considered cold-loving, many strains can grow at 20–37°C, although growth has also been recorded in the range of 6-18°C for other strains. Infection can occur at 11 °C or less for typical forms but there often will be no symptoms in host fish at such low temperatures. Mortality increases with temperature (Groberg et al. 1978). Optimum pH is around 6.4–8.0 but overall tolerance ranges from 5.3–9.0. Disease symptoms may be stress-induced (Hiney et al. 1994).

Summary of species impacts derived from literature review. Click on an icon to find out more...

Environmental	Socioeconomic	Beneficial

In salmonid hosts that contract furunculosis as a result of infection by A. salmonicida, symptoms can include furuncles, hemorrhaging, enlarged organs, erratic swimming, and lack of feeding (Cipriano and Bullock 2001, Crawford 2001). Because of this, infected fish may be more susceptible to predation (Lafferty and Morris 1996). However, the bacterium continues to be at very low prevalence (GLFHC 2006). In the Great Lakes and connecting tributaries, A. salmonicida has had a larger impact on native salmonids than on introduced salmonids. It primarily affects Atlantic Salmon (Salmo salar), Brook Trout (Salvelinus fontinalis), Lake Trout (Salvelinus namaycush), and Lake Whitefish (Coregonus clupeaformis). It secondarily affects other native non-salmonid species such as Northern Pike (Esox lucius), Yellow Perch (Perca flavescens), dace and minnows (Family Cyprinidae), catfish (Family Ictaluridae), sticklebacks (Family Gasterosteidae), and sculpins (Family Cottidae) (Crawford 2001, Mills et al. 1993). Aeromonas salmonicida has been found to be ubiquitous and established in wild populations throughout the Great Lakes and connecting tributaries (Bruneau et al. 1999).

Aeromonas salmonicida has moderate socio-economic impact in the Great Lakes.
Previously considered exclusive to fish hosts, rare cases of A. salmonicida infecting humans have been reported in recent years (Salehi et al 2019). Left unchecked, Aeromonas salmonicida can have devastating impacts on captive (aquaculture and hatchery) operations.  Infectious diseases cost the Canadian aquaculture industry over $400 million annually, with furunculosis accounting for approximately 10% in annual losses (Nash et al. 2006). The proportion of these losses specific to the Great Lakes region has not been reported. Likewise, the economic impact of pathogen screening requirements for the baitfish and aquaculture industries are likely to be significant, but have not been documented.

There is little or no evidence to support that Aeromonas salmonicida has significant beneficial effects in the Great Lakes.
In lab environments, the A. salmonicida bacterium can be pathogenic to zebra mussels (Dreissena polymorpha). In the wild, mussel populations may act mainly as reservoirs for A. salmonicida, and further research is necessary to understand the ecology of the interaction between these species (Gu and Mitchell 2002, Maki et al. 1998).

Regulations (pertaining to the Great Lakes)

The Great Lakes Fishery Commission lists Aeromonas salmonicida as a restricted pathogen - hatchery stocks are routinely tested for the pathogen and fish exhibiting clinical symptoms are not to be transferred, stocked or released.

New York, Pennsylvania, Michigan, Illinois, Wisconsin, and Minnesota have instated similar baitfish regulations to control the spread of furunculosis and other fish pathogens. Those of New York include that bait harvested from inland waters for personal use is only permitted to be used within the same body of water from which it was taken and cannot be transported overland (with the exception of smelt, suckers, alewives, and blueback herring). Once transported, baitfish cannot be replaced to its original body of water (NYSDEC 2012). Live or frozen bait harvested from inland New York waters for commercial purposes is only permitted to be sold or possessed on the same body of water from which it was taken and cannot be transported over land unless under a permit and or accompanied by a fish health certification report. Bait that is preserved and packaged by any method other than freezing, such as salting, can be sold and used wherever the use of bait fish is legal as long as the package is labeled with the name of the packager-processor, the name of the fish species, the quantity of fish packaged, and the means of preservation (NYSDEC 2012). Certified bait may be sold for retail and transported overland as long as the consumer maintains a copy of a sales receipt that contains the name of the selling vendor, date sold, species of fish sold, and quantity of fish sold. Bait that has not been certified may still be sold but the consumer must maintain a sales receipt containing the body of water where the bait fish was collected and a warning that the bait cannot be transported by motor vehicle. Bait sold for resale require a fish health certification along with a receipt that contains the name of the selling vendor, date sold, species of fish sold, and quantity of fish sold, which must be kept for 30 days or until all bait is sold (NYSDEC 2012).

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.

Control

Control methods are focused on control in hatcheries and aquaculture as well as on limiting spread during fish handling (e.g., during catch and release monitoring) There are very few control methods suitable for wild populations; voluntary AIS Baitfish HACCP protocols include inspection of harvested bait (do not harvest for use or sale if any indication of disease).

Biological
While supplements of lactic acid bacteria (Carnobacterium spp.) given with fish feed do not protect against A. salmonicida infection (Gildberg et al. 1995), such probiotic supplementation can decrease mortality in Atlantic Salmon (Salmo salar) and Rainbow Trout (Oncorhynchus mykiss) infected with A. salmonicida (Irianto and Austin 2002). Carnobacterium strain K1 colonizes the intestinal tract of Rainbow Trout and inhibits A. salmonicida growth (Jöborn et al. 1997).

Short-term bathing of presmolt Atlantic Salmon infected with furunculosis with siderophore-producing Pseudomonas fluorescens is another successful biological control (Gram et al. 1999, Smith and Davey 1993). Bath treatments with V. alginolyticus (Austin et al. 1995) can also lead to a reduction in mortality (Verschuere et al. 2000). Presence of the freshwater algae Chlorella sacharophila and Chlamydomonas reinhardtii protected juvenile Burbot from A. salmonicida (Natrah et al. 2012), these and other algae are being investigated for potential control applications.

Administration of bacteriophage to infected fish may also help control outbreaks. In a study that administered the bacteriophage HER 110 to A. salmonicida HER 1107 infected Brook Trout (Salvelinus fontinalis), A. salmonicida populations declined by six log units (base 10) in 3 days. Further tests within fish populations are necessary to better understand the implementations of this alternative therapy (Imbeault et al. 2006).

The U.S. FDA-approved vaccine cefuroxime administered to brood stock prior to spawning has proven to be very effective at reducing A. salmonicida prevalence (GLFHC 2006). Very good to excellent results controlling and preventing out-breaks of furunculosis have also been obtained using an autogenous vaccine; this treatment was accompanied by an immune-enhancing feed administered for three weeks prior and three weeks post vaccination, as well as stock thinning to reduce overall stress (GLFHC 2006).

Selective breeding for disease resistance has had success for hatchery-reared Brook Trout and Brown Trout lines and potential for selective breeding has been noted for Atlantic Salmon (Cipriano and Bullock 2001).

Chemical

Povidone iodine (>30 minutes 50mg iodine/mL) is the compound of choice for disinfecting trout and salmon eggs for hatcheries (Cipriano and Bullock 2001).
Antimicrobial agents used to treat A. salmonicida infections include thiophenicol, furazolidone (Herman 1968), oxytetracycline (Heo and Seo 1996, Herman 1968, Wiklund and Dalsgaard 1998), sulphamerazine, tetracycline and a combination of trimethoprim (Heo and Seo 1996, Wiklund and Dalsgaard 1998) and sulphonamide (McCarthy and Roberts 1980). Others include flumequine (Michel et al. 1980), oxolinic acid (Hastings and McKay 1987, Heo and Seo 1996, Wiklund and Dalsgaard 1998), florfenicol (Inglis et al. 1991b), amoxicillin (Inglis et al. 1992), enrofloxacin (Stoffregen et al. 1993), chloramphenicol, neomycin, nitrofurantoin and ciprofloxacin (Heo and Seo 1996, Wiklund and Dalsgaard 1998). Feed containing terramycin and romet are effective in treating A. salmonicida (MDNR 2012). However, an increase in antimicrobial resistance was recognized in the United States beginning in 1967 (Wood 1967). In many fish farms, antibacterial agent resistance among different species and strains of bacteria has become a major problem (Schmidt et al. 2000). Antimicrobial resistance by A. salmonicida has been discovered with the following agents: sulphonamides (Herman 1968), oxytetracycline (Grant and Laidler 1993, Austin et al. 1998, Inglis et al. 1991a, Tsoumas et al. 1989), a combination of sulphonamide and trimethoprim (Grant and Laidler 1993, Tsoumas et al. 1989), oxolinic acid (Grant and Laidler 1993, Hastings and McKay 1987, Höie 1992, Inglis et al. 1991a, Oppegaard and Sörum 1994, Tsoumas et al. 1989), nalidixic acid (Ortega et al. 2006), flumequin (Höie 1992), and amoxicillin (Grant and Laidler 1993).

Aeromonas salmonicida is capable of transferring plasmids that confer drug resistance from one strain to another, which has the potential to result in new and more virulent strains of the disease evolving and appearing among salmonid populations (Bakke and Harris 1998).

Physical

Aeromonas salmonicida can adhere to surfaces.  Stainless steel exhibits the least adhesion and should be preferentially used for fish handling equipment Carballo et al. 2000) to avoid cross contamination.

Other
Minimizing fish stress can reduce the risk of disease outbreak (FTS 2012).

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.