Aeromonas salmonicida (furunculosis) is the persistent sequential occurrence of furuncles (or boils) over a period of weeks or months caused by the organism aeromonas salmonicida. It is a bacterial disease of freshwater and marine fishes. There are actually several subspecies of the bacterium which are classified according to their laboratory culture properties as well as the clinical signs they produce.
This disease is common in koi (ornamental carp) where it is referred to as "koi ulcerative disease." Affected fish usually have shallow or deep ulcers somewhere on the body, but may exhibit other signs such as exophthalmia (pop-eye), areas of ecchymosis (bloody spots), and a distended abdomen. Infected fish with open sores appear to spread the disease to other fish, and subclinical carriers may exist, shedding bacteria in their feces.
There is frequently a history of recent introduction (within the past month) of new fish to the pond or aquarium. The problem is especially prevalent during the spring and fall months.
In the ulcerative form of the disease, necrosis (cell death) and hemorrhage (bleeding) of the skin occurs, and underlying bone and muscle are commonly exposed. These ulcers can expose the fish to invasion by secondary bacterial, fungal, and protozoal pathogens. If untreated, these fish commonly die due to the presence of the primary and secondary pathogens, as well as fluid imbalance.
Your veterinarian can make a diagnosis of furunculosis based on microbiological culture with the help of a diagnostic laboratory. However, A. salmonicida is difficult to culture, and samples are commonly contaminated and overgrown by other bacteria such as A. hydrophila, another fish bacterial pathogen.
Frequently, a diagnosis is made on the history and clinical signs. Differential diagnoses include mycobacteriosis and trauma. In cases of systemic disease, bacteria may be cultured from the blood, kidneys, or spleen. Attempts at culturing open ulcers are often unrewarding because of contamination of the lesion with other bacteria.
When the condition is identified early, and the fish are treated appropriately with antibiotics, the prognosis is fair to good, although surviving fish may have permanent scars.
When possible, injectable antibiotic treatment of clinically affected fish, along with oral treatment of other fish in the pond or aquarium, is recommended. Antibiotics of choice include enrofloxacin, trimethoprim-sulfamethoxazole, and amikacin. You should only use these and other antibiotics under the direction and supervision of a licensed veterinarian who has examined your fish.
The best way to prevent furunculosis is to quarantine all new aquarium pond fish for at least one month. This practice, at the very least, should identify infected fish before they have a chance to spread this bacterial disease to your established fish population.
Other important preventative steps include maintaining excellent water quality, performing frequent water changes (at least 25 percent each month), not overcrowding your aquatic system, and maintaining steady temperatures and an adequate air supply.