A distressing condition
Fish gills are susceptible to parasite, bacterial and fungal
diseases. Another common condition is hyperplasia, which if not resolved
is often fatal. Gill problems are a major cause of fish losses,
particularly, it seems, koi. It is a distressing condition to deal with,
both for the fish and owner, as too often the disease is at an advanced
stage before it becomes obvious that something is wrong. At such
a stage there is often little that can be done to save the
To understand why gill disease is such a threat, how it is caused, it
is important to have an understanding of the anatomy and function of the
gills. This will give us an insight into how conditions such as
hyperplasia arise and why parasite and bacterial gill diseases are such
a threat. With this understanding we will be better placed to spot early
clinical signs that something is amiss and decide how best to treat gill
The gills comprise of rows of long, thin primary filaments attached
to a gill arch - a bit like the teeth of a comb. This is what you see
when you look inside the operculum (gill cover). Running across the
primary filament, on both both sides, are a rows of tiny semi-circular
foldings - the secondary lamellae.
click on pictures to enlarge them
The delicate and complex structure of fish
The secondary lamellae and
their role in gaseous exchange
Normal gills at low magnification showing
primary gill filaments with the secondary lamellae branching out
from each side - the Christmas tree look! The viability of the
secondary lamellae is crucial to fish health
High magnification of healthy gill tissue.
The red blood cells pass in a single column through the
secondary lamellae where they pick up oxygen and dump carbon
dioxide and ammonia.
These can be seen in the diagrams and microphotographs above.
Deoxygenated blood from the heart is pumped into the primary filaments
via the afferent artery. It passes through the secondary lamellae where
gaseous exchange takes place - and then back down the efferent artery.
The oxygenated blood is then circulated around the body tissues before
returning to the heart and starting all over again..
Gaseous exchange - or how fish 'breath'
Gaseous exchange takes place by diffusion. This is facilitated by the
delicate structure of the secondary lamellae. The gill epithelium,
(the outer covering, a sort of skin) is extremely thin, usually just one
cell thick. Small molecules such as oxygen, carbon dioxide, ammonia etc.
can pass through the epithelium - from the water into the blood vessels
running immediately under the epithelium - almost as if there wasn't a
Anything that interferes with, damages or
obstructs this delicate structure is clearly going to be extremely
damaging to fish health.
Gill structure and disease
The fish gill has very little protection, other than the bony cover -
the operculum - and is susceptible to both physical and chemical damage.
A common response of the gill to irritation - whether from parasites,
suspended solids or ammonia, or indeed any other irritant - is hyperplasia. Hyperplasia is an abnormal
increase in the number of cells in an organ, in this case the gill
epithelium. This is similar to calluses that form on hands when
subjected to physical work such as digging. Hyperplasia is usually
accompanied by increased mucus production.
click on pictures to enlarge them
Chemical or physical irritation cause addition cells to grow as
a form of protection. Secondary lamellae clump together
affecting gaseous exchange and respiration
The effects of hyperplasia - higher
magnification. Gills are swollen and clumped
Extreme hyperplasia where both the secondary
lamellae and primary filaments are clumped together in a mass of
tissue. No gaseous exchange can take place and the fish
In addition to hyperplasia, the gill is also susceptible to bacterial
and fungal disease as a result of poor water quality, stress or
parasites. For this reason most gill disease is really a 'management
disease' resulting from poor conditions such as;
chemical irritants such as ammonia and
nitrite or high pH
high levels of parasites such as Trichodina,
Chilodonella, Flukes or Costia
high levels of dissolved or
particulate organic solids
low levels of oxygen or overcrowding
Gill disease diagnosis
The main problem with fish gills is that unlike the rest of the body,
we can't readily see what is happening. Often by the time it becomes
obvious that the fish is ill, the damage is advanced and untreatable, so
therefore an early diagnosis and treatment is vital.
The early signs are fish respiring heavily. You can judge this
by watching the operculum movements and comparing them to other
Fish laying on the bottom for long periods - general lethargy -
Fish tending to use one pectoral fin, keeping the other folded
back against the body
At a more advanced stage you may notice that the fish can't
fully close the operculum because of gill swelling.
Affected fish may segregate and stay alone - often near the
surface or water return
There may be strands of mucus trailing from the gills
At a really advanced stage - and usually too late for treatment
- the fish will lay on the bottom with its pectoral and dorsal
fins clamped to its body - literally waiting to die!
The other problem in advanced cases is treating against
parasites. The combination of excess mucus and hyperplasia forms a
secure shelter for any parasites between the secondary
lamellae making them very difficult to get at!
Right from the start I should stress that prevention is better
than cure and with good water management practices, most gill
disease is avoidable.
If there is the suspicion of a gill problem, is it just one
fish or are several affected? The latter will probably indicate an
Check the water for ammonia, nitrite pH and when was the last
time the system was cleaned? If several fish are affected, the
system should be cleaned and a substantial water change made,
somewhere between 50 -75%. Such is the seriousness of this type of
In minor cases simply providing optimum environmental
conditions may be enough. (Check out the
water quality pages and see if you come up to the five point
standard); Optimum conditions are mandatory if gill
disease is to be successfully treated.
Examine the fish for parasites. At this stage a skin scrape
from immediately behind the operculum will suffice.
For individual fish a salt bath on two consecutive days is a
good start. It won't exacerbate the problem and will help remove
any excess mucus or parasites.
If the salt treatment fails to work the next stage is probably
a gill biopsy to see what is going on. If this shows a parasite
problem then these will need to be treated. With regard to
treating gill disease, a combination of chloramine-T and
benzalkonium as separate treatments in a treatment tank - not the
pond - will help resolve gill problems provided that they are not
too advanced. See the treatment pages for details.
Potassium permanganate can be used, but it is often a kill or
cure treatment. It will rapidly reduce the parasite and bacterial
levels as well as reducing dissolved organics. The draw back is
that it will push the really sick fish over the top - mainly I
suspect because the permanganate forms a temporary precipitate
of manganese dioxide in the gills, affecting fish with
severe respiratory problems.
The outlook for more advanced cases where there is severe
hyperplasia and/or bacterial / fungal infection is not good. I
have had some success - not a lot - with intensive treatment of
chloramine-T and benzalkonium chloride together with antibiotic