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Questions and Answers Concerning Furunculosis (pdf, 26kb)

Furunculosis is a disease of wild and cultured salmonids (trout and salmon), caused by the obligate bacterium, Aeromonas salmonicida. The disease causes bacterial septicemia, and is often fatal in farmed fish if not treated promptly with antibiotics. There is no human health risk associated with this fish disease.

Furunculosis is named for the raised muscle lesions resembling skin boils (furuncles), which occur in chronically infected fish (See illustration on Fish Diseases page). In the laboratory we can test for furunculosis by isolating the bacteria in culture media.

Fish infected by A. salmonicida often exhibit lethargy, darkened skin and hemorrhage at the bases of fins. In acute disease, few external signs may be noticed and heavy mortality can occur. The bacterium has a wide host range, infecting many species of freshwater and marine fish, and it is considered geographically ubiquitous throughout North America. The disease furunculosis is most common in trout and salmon, although it has been reported in several baitfish species and it's been shown to cause ulcerative skin lesions in several non-salmonid species.

In September of 2013, furunculosis was found for the first time at the Pequest Trout Hatchery in brown and brook trout, which impacted the 2014 trout stocking season. It is believed that the bacterium was introduced to the hatchery from a bird. Birds are a major source for introducing diseases in fish hatcheries by feeding on diseased fish in the wild and introducing the pathogen to the hatchery either by diving into the raceways or through the feces. The disease was treated with several fish approved antibiotics (Florfenicol and Oxytetracycline).

In order to reduce the risk of spreading the bacterium from the hatchery to wild fish, a fish health policy (pdf, 16kb) for hatchery fish was established. Prior to stocking all lots of trout were retested for the presence of the bacterium; those fish that were effectively treated and did not show evidence of the bacterium were stocked into non-trout waters.

It is known that some fish may remain carriers of the bacterium even after effective antibiotic treatment, therefore these fish were not stocked into trout waters in order to conserve natural trout populations. Fish that tested positive for the bacterium were not stocked into any state waters. The reoccurrence of furunculosis in the brook trout at the hatchery during spring, 2014, led to the humane euthanasia of about 205,000 brook trout.

A number of preventative actions have been taken and are ongoing at Pequest to help eliminate the bacterium from the hatchery and to avoid the disease in future years. All hatchery raceways have been steam-cleaned and disinfected, the remaining brook and brown trout have been vaccinated to help protect them from the disease, the hatchery will raise mainly rainbow trout for the next few years since this is a species with natural resistance to the disease, and options are being explored to cover the outdoor raceways in which the trout are raised to increase biosecurity of the facility.

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Last Updated: May 1, 2014