Filamentous bacteria are complex group of micro-organisms, the classification of which is not fully agreed. For practical purposes they can be recognised by their shape, being delicate thread-like bacteria and the fact that on culture they are often highly pigmented (see picture). In fact fish suffering from filamentous bacterial stomatitis have striking bright yellow mats of these bacteria in and around their mouths. The larger mats are easily visible to the naked eye and appear somewhat like yellow paint. The species of bacterium involved is most often thought to be Flexibacter maritimus.
A variant of this problem (and it is not certain whether this is the same species of filamentous bacterium) seems to affect the dorsal fins of fish post transfer, starting as a relatively innocuous looking dorsal fin erosion but rapidly progressing to the base of this fin and undermining the skin around the fin. This skin then tends to slough away leaving extensive ulceration and muscle erosion.
Filamentous bacteria are generally secondary opportunistic bacteria and may simply be taking advantage of damaged tissue. On histology both of these types of problem seem to have a significant or predominant filamentous bacterial component, but other bacteria like various Vibrios and Moritella viscosa (the bacteria associated with "Winter Lesion") are certainly present. Although its not possible to dismiss the role of other bacteria or even viruses in the development of these conditions, clinically they appear to be different from "Winter Lesion" both in terms of location of the lesions and the fact that there appear to be less in the way of the internal haemorrhages caused by bacterial toxins that we tend to see with "Winter Sores".
Although some sites have experienced problems with fish that appeared to have no physical damage at the time of transfer we can assume that any damaged tissue is more likely to become a focus of infection.
Try to get non-specific fin erosions in freshwater to clear up before transferring fish to sea.
Try to ensure that fin (or skin) damage is limited during transfer by paying attention to netting/pumping methods and contact surfaces of pipes, wells and helicopter buckets.
Mouth damage can occur when fish burrow into nets as often occurs with fish in freshwater that are smolted (and ready to migrate) and in the sea when fish are being tormented by predators. Some sites have a practice of keeping the nets of newly transferred smolts pulled up for a few weeks. This should be avoided because fish are less likely to burrow into the nets if they have the full depth of the pen in which to escape from bird threat.
Competition for feed in both fresh and seawater could lead to aggression and physical damage.
Very often these conditions are present at such low levels within a population that treatment is not warranted. However be aware that the numbers of affected fish can escalate fairly quickly, so monitoring of numbers of affected fish and mortality trends is essential. Treating skin and mouth ulcerations with antibiotics can be frustrating, as very often the affected fish are not feeding. Besides this, skin has a relatively poor blood supply and good tissue levels of antibiotics are difficult to achieve.
Having said that we have had better success with more recent additions to the antibiotic arsenal. As ever the decision to treat should not be taken lightly and not without appropriate lab work to confirm the nature of the problem.
Source: Fish Vet Group - July 2005