Aeromonas salmonicida

Cell structure and metabolism

A. salmonicida is a facultative anaerobe, which means it is capable of making ATP by aerobic respiration if oxygen is present, but is also capable of switching to fermentation when oxygen is not present. It does not ferment sucrose or lactose, using glucose in this pathway, instead; glucose fermentation creates gas. The bacterium grows optimally at temperatures between 22 and 25 °C. The maximum temperature at which it can grow is 34.5 °C. After about a 24-hour growth period, the bacterial colonies reach about the size of a pin point. The colonies also have a brown pigmented color that appears after it has been growing for 48–72 hours.

Host range

• Salmon
• Trout
• Cyprinids
• Pike
• Perch
• Bullheads
• Turbot
• Halibut

Epizootiology

A. salmonicida, an airborne pathogen, can travel 104 cm from its host into the atmosphere and back to the water, thus making it difficult to control. The bacterium can maintain its pathogenicity in freshwater conditions for 6–9 months, and in saltwater conditions for up to 10 days without a host. Several direct count methods and other detection methods have found the organism does not lose or reduces its titer concentrations.

Transmission of furunculosis mainly occurs through fish-to-fish contact by the skin or by ingestion. Rainbow trout have been found to carry A. salmonicida up to two years after initial infection without re-exposure. Chemically immunosuppressed fish compared with temperature-stressed fish had a 73% mortality as opposed to a 33% mortality rate, respectively. Naturally occurring trout infections consisted of a 5–6% mortality rate per week with an 85% rate in untreated populations. Some clinical furunculosis survivors of an infected trout population became A. salmonicida carriers. When comparing furunculosis epidemics with depressed oxygen levels, when oxygen concentrations were decreased to less than 5 mg/L, A. salmonicida concentrations increased. While observing chum salmon in a density of 14.7 fish per square meter, 12.4% were infected with A. salmonicida, whereas, densities at 4.9 fish per square meter were infection-free. Additionally, A. salmonicida concentrations were considerably elevated in water with low dissolved oxygen (6–7 mg/L), compared to water with higher dissolved oxygen (10 mg/L). High density-low oxygen water resulted in survival rates that were roughly 40% less than in those consisting of low density-high oxygen conditions.

Pathology

The bacterium is pathogenic for fish, and causes the disease known as furunculosis. The symptoms the fish show are external and internal hemorrhaging, swelling of the vents and kidneys, boils, ulcers, liquefaction, and gastroenteritis. Furunculosis is commonly known as tail rot in fish and is common in goldfish and koi. Infected fish with open sores are able to spread the disease to other fish.

It is also one of several bacteria that can cause bald sea urchin disease. Since A. salmonicida cannot grow at 37 °C, it is not pathogenic in humans.

Clinical symptoms and disease diagnosis

Furunculosis is classified into four categories based on severity: acute, subacute, chronic, or latent. When fish are infected, they become listless and weak until they die. Other characteristics observed include anorexia and lethargic movement, and they may exhibit a darkened pigment. Deep or shallow ulcers, exophthalmia, bloody spots, distended abdomen, and petechia at the base of the fin may also occur. Internally, the infected fish may suffer from gastroenteritis, hemorrhagic septicemia, edematous kidney, and an enlarged spleen. The liver may appear pale in color and the spleen may be darkened. The peritoneal cavity may also be bloody and inflamed.

Bacteria must be isolated to positively identify the disease. Isolates are retrieved from muscle lesions, kidney, spleen, or liver, and then grown on trypticase soy agar and brain-heart infusion medium incubated at 20–25 °C. Colonies of A. salmonicida appear hard, friable, smooth, soft, and dark in color.

While cultural procedures produce good results, serological procedures produce more rapid results by using serum agglutination, fluorescent antibody, or enzyme linked immunosorbent assay on infected tissue or cultured bacteria. Mooney et al. developed a DNA probe with polymerase chain reaction to detect A. salmonicida DNA; results were successful in 88% of wild Atlantic salmon.

Detection

A. salmonicida tests negative for indole formation, coagulase, hydrolysis of starch, casein, triglycerides, and phospholipids, hydrogen sulfide production, citrate use, phenylalanine, and the Voges–Proskauer (butanediol fermentation) test. It tests positive for oxidase, lysine decarboxylase, methyl red, gelatin hydrolysis, and catalase.