Bacterial ulcers a common, but difficult problem
Bacterial ulcers are a common fish disease problem, particularly with
ornamental pond fish such as goldfish and koi. They are one of the most
difficult problems to deal with, especially if large numbers of fish are
affected. Dealing effectively with bacterial ulcers can be
time-consuming and really requires a high degree of skill and
experience. However, unlike bacterial gill diseases, the success rate
can be very high even with quite severe and advanced cases.
Typical cases of ulcers on koi carp. The area
of erosion in both cases is quite extensive. The lesions are
deep, exposing the muscle underneath. The margin of inflammation
can be clearly seen around the lesion. Both fish recovered
following combination topical treatment and antibiotic
What are ulcers?
An ulcer is defined as a break in the skin extending through all the
layers, which fails to heal and is often accompanied by inflammation.
The typical skin ulcer has a central area of tissue erosion that may be
shallow or deep – in many cases exposing the underlying muscle. Around
the edge of the lesion is usually an area of whitish necrotic (dead)
tissue and debris, with a large outer zone of inflammation extending
into healthy tissue. The scales surrounding the lesion may be raised or
infected. Without treatment it gets progressively bigger. In most, but
not all, cases they are caused by opportunistic bacteria already in the
pond or tank.
Severe ulceration on the body of a koi. A
close view of the photo shows many infected scales around the
lesion which had to be removed to facilitate healing. This fish
recovered after three weeks of intensive treatment
on pictures to enlarge them
How are ulcers caused?
They can be caused by damage to the skin from parasites or chemicals
(for example exposure to high levels of ammonia or nitrite or high/low
pH). Fish can also be susceptible if their normal defences are
compromised by stress. High numbers of opportunistic bacteria, such as
would be found in water with a high organic content or other diseased
fish, also increase the risk of infection.
If just one is affected it is probably due to a health problem
specific to that individual. However if several are affected it is quite
likely that there is an underlying environmental problem. In this quite
common situation it is no good simply throwing in some anti-bacterial
treatment and hoping. It is important to establish and resolve the
underlying cause, be it water quality or parasites, as well as treating
the bacterial infection.
It is hard to give a general answer because it really depends on the
circumstances surrounding each case. If just one individual is affected
and assuming that it has been diagnosed at an early stage, simply
dealing with any other health problems and treating the lesion topically
If several are affected it is vital that the system and all of the
fish are examined, either in or out of the water, to determine the full
extent of the spread of the infection.
My own experience is that the chances of survival are greatly
improved by a thorough debridement of the lesion together with topical
treatment. This is an important step irrespective of any other
Treat all the affected fish
It is often the case that there are many more infected fish than
initially thought. It is important that all of those affected are
treated at the same time otherwise the untreated fish will in turn get
worse as well as acting as a reservoir of infection. This is often the
situation where an outbreak of bacterial disease goes on for months,
despite continually treating the most obviously affected individuals. To
the owner it looks like fresh outbreaks despite his best efforts, when
it is really just an ongoing chronic infection.
As far as treatments are concerned this is really a matter of
judgment based on the severity of the situation. The options are topical
treatments, medicated food, bath treatments (short duration or long
term) or antibiotic injections. Whatever course of treatment is decided
the aim should be to resolve the problem within a matter of 2-3 weeks.
If there is not a marked improvement during that time then either the
treatment plan needs reappraising or there are still underlying problems
that haven’t been tackled.
Aim for a 90% survival rate or higher
I should stress the importance of tackling bacterial diseases such as
ulcers in a systematic, thorough way. Simply applying dartboard
medication, in which a range of different treatments is tried in
succession, is unlikely to work and in all probability will simply make
matters worse. When dealing with bacterial ulcers we should be looking
for at least a 90% survival rate. If the survival rate is significantly
less than this it is most likely due to inappropriate treatment rather
than bacterial infection