Bacterial fish diseases
Bacterial fish diseases and infections are very common in fish
keeping and are probably one of the hardest health problems to
deal with effectively. However, with a basic understanding of how
bacteria cause disease and how to recognize bacterial infections it is
possible to deal successfully with the majority of problems. The key to
successful treatment of bacterial disease is early, accurate diagnosis
and treatment. If treatment is delayed it can lead to substantial
losses. This is particularly relevant to koi ponds, which, for a variety
of reasons, are often susceptible to outbreaks of bacterial disease.
A typical bacterial
infection with a large area of ulceration just behind the
operculum (gill cover). The whole of the pectoral fin has been
totally eroded. Both the pelvic fins and anal fin are very red
and inflamed. The caudal (tail) fin has advanced fin rot
The fish survived with a combination of antibiotic injections
and topical treatments.
Pathogenic bacteria can spread disease throughout the fish's body if
they are absorbed through the gills or gut, or gain entry via the
skin. This is known as a systemic
infection. Other bacterial infections cause localised surface
disease such as fin rot and ulcers, however, if these are not resolved
they can lead to a systemic infection. In general there are four types
of bacterial infections that the hobbyist needs to be aware of:
Fin rot – usually resulting from
Bacterial body ulcers – open,
shallow to deep, lesions on the fish’s body
Bacterial gill disease – in
which the gills are the primary target
Systemic bacterial disease – in
which bacteria invade the fish’s body and damage internal
Diagnosing bacterial infections
Exhibiting my normal caution about “instant” diagnosis let me say
that a definite diagnosis of bacterial fish disease requires sampling
and culture of bacteria from the lesion or organ. The problem is that
many clinical signs of bacterial disease could also be caused by other
problems such as viruses, poor water quality or parasites and we
need to be aware of the dangers of jumping to conclusions. However,
taking these qualifications into account, typical (external) signs of
bacterial disease are;
inflamed areas on the body and fins, raised scales, skin ulcers,
exophtalmos (pop-eye), dropsy or swollen abdomen, fin rot.
Additionally, affected fish may be lethargic and anorexic
there may be lesions or haemorrhages in internal organs and/or a
build-up of often bloody fluid in the abdomen (ascites).
Any of these signs warrant immediate further investigation.
click on pictures to enlarge them
Koi carp with bacterial infection. There are
raised scales, with extensive inflammation and swelling.
If left untreated the infected area is liable to ulcerate
Bacterial infection in testis of a koi carp.
There were no external signs of disease
Where do bacterial infections come from?
It is important to our understanding of bacterial disease to realise
that there are essentially two types of pathogenic (disease-causing)
Primary or obligate pathogens –
These are pathogens that are not part of the normal aquatic flora
and are capable of causing disease in healthy individuals.
Typically, Aeromonas salmonicida
Opportunistic pathogens. These
are normally free-living, either in the water on the fish, but are
capable of adopting a pathogenic role under certain circumstances.
Many of these are saprophytes, normally living on dead organic
matter such as plant and animal remains or faeces. Typically, Aeromonas
hydrophilia, Pseudomonas and Vibrio. In
organically-rich water there may be millions of these
opportunistic bacteria in every litre of water, but under normal
circumstances, they do not pose a threat to healthy fish. This
situation can be directly compared to the normal microbial flora
of the human mouth and skin which are teeming with potentially
harmful ‘germs’ but rarely cause any problems unless the bugs
can get past our body defences.
As you might have guessed, in general most of the bacterial diseases
that affect ornamental fish are caused by opportunistic bacteria. Which
means that most bacterial infections result because of changes in the
bacteria / fish relationship. This is a very delicate relationship, but
generally when an opportunistic bacterium tries to establish an
infection the fish host is able to resist by a variety of defence
mechanisms. However, if the numbers or virulence of the pathogen
increases or the host resistance is reduced the pathogen may be able to
Host / parasite relationship: The balance between susceptibility and resistance
determines either good health or disease
How do opportunistic bacteria become pathogenic?
I should stress that we are taking a somewhat over-simplified view of
a complex subject, but in general there seems to be three major ways
that infections occur
Damage to the integument (skin) resulting from an abrasion,
parasite activity or chemical damage (for example from ammonia,
nitrite or high/low pH). My own experience is that small cuts or
abrasions on healthy fish rarely become infected – in much the
same way that we do not get infections every time we cut
ourselves. I mention this because there is often a tendency to
diagnose obvious bacterial infections as “cuts” or scrapes and
hope they get better.
The fish’s normal defences are depressed as a result of
environmental stress or disease. This enables opportunistic
bacteria to take advantage. This is similar to the situation when
we catch colds and other diseases if we are run-down. Fish can
become stressed for a variety of reasons, the most common reasons
being poor water quality, parasites, overcrowding, thermal stress.
A substantial increase in the numbers of opportunistic bacteria
such as we would find in systems with other infected fish and /or
high levels of decomposing organic matter. This is typically the
situation when the disease starts to spread and several fish are
affected. Under these conditions, especially if fish are still
stressed, it seems that some opportunistic bacteria can become
primary pathogens through weight of numbers.
It is because of these complications, together with tremendous
reproductive potential of most bacteria, that a seemingly minor problem
can rapidly snowball out of control leading to high numbers of infected
fish and possibly many losses.
Treating bacterial infections
It is hard to lay down hard and fast rules about treating bacterial
diseases. It mostly comes down to the experience of the person on the
spot in deciding how severe the situation is and what course of action
is required. However, before any treatment is started it is important to
try and establish the underlying cause – particularly if several fish
are affected, as this would suggest some form of environmental stress.
The reason why this is important is that few, if any, treatments
completely destroy all the pathogenic bacteria. What they do is knock
the bacteria back for a short time, giving the fish’s own defences
chance to start fighting back and the wound-healing process chance to
Clearly, the fish will only be able to fight back if conditions are
improved and all efforts are made to keep the numbers of bacteria as low
as possible. So really any anti-bacterial treatments should be seen as
supportive treatment for the fish’s own immune system and what we are
trying to do is to tilt the balance of health back in the fish’s
There are broadly four treatment methods for bacterial infections.
They can be used individually or in combination for an enhanced effect
Short-term baths for a few hours each day using anti-bacterial
products such as chloramine-T, potassium permanganate or
Long-term pond or tank treatments usually using a proprietary
anti-bacterial or anti-ulcer product
On their own baths are only likely to be effective for relatively
minor bacterial infections such as fin rot or situations when just one
or two fish have minor ulcerations. This supposes that any underlying
problems are resolved or treated. Baths are unlikely to resolved severe
This involves cleaning the lesion, usually with an antiseptic
solution and applying a waterproof ointment or cream such as Orabase or
similar. Topical treatments, if used properly can be very effective in
dealing with minor to moderate cases – providing that the infection is
not systemic. I would normally expect to see signs of improvement –
that is a reduction in inflammation or the first visible signs of skin
growth – within seven days of the start of treatment. If there is no
sign of healing within a week to ten days a change of treatment is
These are specially prepared foods containing antibiotics. In the UK
they are only available with a veterinary prescription. While the use of
medicated food has obvious advantages, the disadvantages are the slight
delay while the food is ordered and processed, very sick fish may not be
eating and there is widespread resistance to the most commonly used
antibiotics. It is possible to prepare your own medicated food by
top-dressing ordinary food with an appropriate antibiotic.
This usually involves a course of five injections over a ten-day
period in conjunction with topical treatments. It is important to use
the appropriate antibiotic (established by laboratory antibiotic
sensitivity tests). This procedure does require a degree of knowledge
and skill. As it usually involves sedating the fish for every treatment
it can be very time-consuming, particularly if large numbers of fish are
involved. The positive side is that there is a very high success rate
(typically > 90%)– even in the most severe cases.