Bacterial disease, ulcers, fin-rot and stress
Two things are important to minimize the likelihood of serious
bacterial infections such as ulcers affecting our koi. Firstly, early
diagnosis and treatment is vital to limit the spread and severity of
infection, both in individual fish and throughout the pond. Secondly, an
understanding of the primary causes of bacterial disease will help
prevent problems occurring and getting out of hand.
However, I should stress that we should not look at these problems in
isolation and think that bacterial diseases just occur out of the blue.
Invariably they are preceded by stress factors or stressors - for
example, poor water quality or parasite infection - which create the
conditions for these diseases to develop in the first place.
As with any animal, koi have evolved sophisticated defence systems
against the bugs and germs that constantly surround them. A sample of
even the cleanest, clearest pond water will contain millions of
bacteria, many of which are the opportunistic type that can quickly take
advantage of any weak or stressed fish, in the same way that we are
surrounded by germs that have the potential to make us ill. But the
bacteria that cause most common ailments are naturally present in the
pond, and only become a problem when they overcome the fishes' weakened
Fish have two basic defence mechanisms to keep bugs and germs at bay.
First, the mucus that coats a healthy fish contains various chemical
substances that discourage parasites, fungi and bacteria. In addition,
the mucous layer is being continually sloughed off and renewed, making
it difficult for bugs to become established and proliferate. For bugs,
it's a bit like trying to climb a greasy lamp-post! It should be noted
that some chemical treatments, such as benzalkonium chloride, can remove
this protective coating and any irritants in the water - for example
raised ammonia levels - can result in a thickening of the mucus coat,
which creates ideal conditions for parasites and bacteria to multiply.
An active immune system
The other protection fish have in common with other animals is an
active blood-based immune system. White blood
cells circulating around the body detect 'foreign' invaders
in the blood and tissues and by various methods attack and destroy them.
There are two important qualifications to add to this
over-simplification of the immune system. First, it is strongly
temperature dependent, being far more effective at higher temperatures,
and becoming virtually non-existent at temperatures below 10oC to 12oC.
The second consideration is that it is affected by stress.
A stressed fish will have a greatly reduced immune response compared
to a healthy, unstressed fish. Because stress has this disruptive effect
on the immune system any form of prolonged stress is potentially
dangerous and often a precursor to health problems. It is this constant
need to consider all factors that makes diagnosis and treatment so
difficult because, as we can see, it would make no sense to treat a
stressed fish suffering from a parasite or bacterial infection until we
had first rectified or removed the cause of the stress.
Ponds are ideal breeding grounds for bacteria, especially if the
water is slightly polluted with fish waste, decomposing food and other
organic matter. A build-up of decomposing organic matter provides ideal
conditions for bacteria to multiply. Very few bacteria are pathogenic
(disease-causing) and the majority of those which cause problems are
opportunistic. So provided the background level of bacteria is kept at a
reasonable level by routine pond and filter cleaning and maintenance,
and the fish are fit and well, there should be few opportunities for
bacteria to cause problems.
There are times of the year that are inherently stressful for koi
-during early spring, for example, before their immune system has really
woken up, and at spawning time which is very stressful for both males
It is worth noting that young fish approaching puberty are likely to
be susceptible to diseases due to hormonal changes.
Fish are susceptible to a variety of infections. Some diseases show
no outward signs and do their damage inside the body, attacking vital
organs and systems. Luckily, this type of infection is rare, as the
first outward signs are normally a number of seemingly healthy fish
dying for no apparent reason. The most common infections encountered by
owners are usually associated with ulceration and fin-rot.
The first signs of a infection are usually the appearance of red
spots or red areas on the body. These inflamed areas can appear anywhere
on the body, often starting with inflammation near a scale.
Common sites are around the mouth and at the base of fins, especially
fins on the lower part of the body, the ventral and anal fins.
Unfortunately, these initial signs are often missed and the infection is
not noticed until a larger open ulcer appears.
An ulcer is a break in the skin, often circular but not always, which
is usually inflamed, looking very red and angry. Because the appearance
of a infection is not always noticed until fairly advanced, koi-keepers
tend to think the 'sudden' appearance of an open wound is due to the
fish cutting itself on a pipe or rocks, They may leave the wound to get
better but in the case of a bacterial infection it simply gets
Fish do occasionally cut themselves but it is rare and the appearance
of any open wound should be taken seriously, particularly if there are
signs of inflammation. These open lesions usually become infected with
other bacteria and fungi which obscure the underlying cause of the
Fins are another main site for infections. The initial signs are at
the edges of the fins. Often the caudal (tail) and pectoral (paddler)
fins become opaque and frayed, sometimes becoming inflamed (red). In
some instances the resulting 'fin-rot' appears as a discrete bite shape
or a noticeable reddening. All fins are susceptible to infection, and
this is usually stress related. Infections to the lower anal and ventral
fins are relatively common so inspect the 'undercarriage' during your
regular health checks. Again, it is rare for this type of damage to
occur from fish biting each other or knocking against walls and pipes.
The above are all typical signs of bacterial infection. For a
definitive diagnosis it may be necessary to have a bacteriological swab
taken and sent to a laboratory for identification. This type of test
will hopefully isolate the primary pathogen and determine the best
antibiotic treatment to combat the infection. Without doubt, bacterial
problems can be fatal but if caught at an early stage, simply improving
environmental conditions may effect a cure.
It is important to realise that, with bacterial infections, affected
fish can often create a reservoir of infection and spread the disease
throughout the pond. Unless the problems are minor and resolved within a
few days with the use of a proprietary treatment, it is likely that
professional or other expert help will be needed. It
is vital that bacterial problems are diagnosed and treated promptly as
any delays in treatment simply makes matters worse.
Severely affected fish will usually need individual treatment and
possibly isolation. If one or more fish are displaying signs of
infection it is crucial that all fish are examined. Most outbreaks of
bacterial disease tend to affect several fish, particularly when the
cause is an underlying stressor such as poor water quality. In this
situation it is important that all affected fish, no matter how minor
their lesions, are noted and treated at the same time otherwise the
infection will continue to spread throughout the pond, despite your
efforts to control it.