The ubiquitous nature of Aeromonas species in aquatic environments provides ample opportunity for animals, particularly fish and amphibians, to come into contact with, and to ingest organisms. Such contact may lead to infection which, depending on the species and the virulence of the strains encountered, may have life-threatening consequences.

Aeromonas hydrophila has been recovered from a wide-range of freshwater fish species worldwide, and occasionally from marine fish—e.g. ulcer disease of cod (Larsen and Jensen, 1977). However, conflicting views have been expressed over the precise role of A. hydrophila as a fish pathogen. Some researchers contend that the organism is only a secondary invader of previously weakened hosts, while others believe that A. hydrophila is a primary pathogen of freshwater fish (Eurell et al., 1978).

A. hydrophila has been associated with several disease conditions in fish, including tail rot, fin rot, and haemorrahagic septicaemias. Haemorrahagic septicaemia is characterized by the presence of small surface lesions, often leading to sloughing off of the scales, haemorrhaging in the gills and anus, ulcers, abscesses, exophthalmia (bulging eyes), and abdominal swelling (dropsy)--often seen in European carp culture. Internally, there may be the presence of ascitic fluid in the peritoneal cavity, anaemia, and swelling of the kidney and liver (Miyazaki and Kaige, 1985). Another condition caused by A. hydrophila, known as red-fin disease, is characterized by the presence of surface haemorrhages and scale erosion.

The etiologic agent is transmitted horizontally (between animals other than parents and offspring) but not vertically (from parent to offspring). The bacteria multiply inside the intestine, causing a haemorrhagic mucuous-desquamative catarrh (excessive mucous secretion). Toxic metabolites of A. hydrophila are absorbed from the intestine and induce poisoning. 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 Kage, 1985).

Aeromonas species produce many products that may be toxic to other cells. Some are released from viable cells in soluble form. Others may remain associated with the cell surface, and still others may be released upon cell death. Three of the extracellular proteins of Aeromonas species—that have been implicated in pathogenicity—have been cloned, sequenced, and characterized biochemically. These are aerolysin, GCAT (glycerophospholipid:cholesterol acyltransferase), and a serine protease (Howard and Buckley, 1986; Buckley et al., 1991; Rodriguez et al, 1992)

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 fungul or parasitic infection, contribute to physiological changes and heighten susceptibility to infection.

Prevention in Aquaculture (adpated from Warren, 1991)

Provide optimal environmental conditions for the species being reared, paying special attention to the maintenance of oxygen levels and the gentle handling of fish.

Prophylactic treatments are helpful when sorting, handling, or moving broodstock. Intraperitoneal injections of 10–30 mg per pound of body weight of aqueous chloramphenicol has reduced post-handling losses by 80–90 percent in Europe.

Antibiotics or disinfectants should be added to the water as a prophylactic measure when fish are transported in tank trucks or plastic bags. Acriflavin at 2–4 ppm has been used in routine fish transportation operations.

Chemotherapeutic agents are used for the treatment of A. hydrophila in fish farms. Isolates of A. hydrophila in fish have been found to be sensitive to chloramphenicol, florfenicol, tetracycline, sulphonamide, nitrofuran derivatives, and pyrodinecarboxylic acids (Aoki and Egusa, 1971; Endo et al., 1973; Katae et al. 1979; Fukui et al. 1987). In hatchery operations, terramycin has been effective when incorporated into pelleted fish feed and fed at 3.5 grams of active drug per 100 pounds of fish per day for 10 days (Warren, 1991). [Note: Antibacterial therapy will provide only short-term relief if adverse environmental conditions such as high water temperatures, low water flows, low oxygen levels, or crowding are not promptly corrected].

Bacteraemia (bacteria in the blood) is the most common pathogenic manifestation of Aeromonas in humans. Mild symptoms include fever and chills, but patients who become septic (overwhelming bacterial infection) with Aeromonas often exhibit abdominal pain, nausea, vomiting, and diarrhea.

Reports of Aeromonas wound infections have appeared increasingly in the literature. Unlike gastroenteritis, these infections can have fatal or serious debilitating outcomes, such as amputations. Aeromonas wounds fall into three categories, listed in order of increasing severity of damage caused: cellulitis, myonecrosis, and ecthyma gangrenosum.

Cellulitis, the most frequently encountered Aeromonas wound infection, is an acute inflammation of subcutaneous tissue characterized by redness and induration that may arise from injury or secondary to sepsis (Musher, 1980). Myonecrosis and ecthyma, the two less commonly seen types of Aeromonas infections, are typically found in patients that are immunocompromised. Myonecrosis, or bullous lesions, is characterized by the liquefaction of muscles with blackening of the tissue which may be gangrenous with gas formation. These patients require agressive antimicrobial therapy and debridement; those individuals that fail to respond to these measures may require amputation (Haburchak, 1996). The third type, ecthyma gangrenosum, is a cutaneous necrotic or gangrenous pustule that occurs secondary to sepsis. Lesions have an erythematous border surrounding a vesicle which can progress to necrosis of the soft tissue within 24 h. This type of infection is usually fatal (Musher, 1980).

• Aquatic Vaccine Unit at the Stirling Institute of Aquaculture

• Northwest Fisheries Science Center, NOAA

• Dr. Sin Yoke Min, National University of Singapore

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