Koi parasites and fungus
These are two problems that regularly affect koi health, Both can be
dealt with fairly simply when they are at an early stage. Fungus, if
allowed to spread, will quickly overwhelm fish. Large numbers of
parasites can cause physical damage to both skin and gills and are often
the precursor to bacterial disease. Diagnosis and treatment is often a
chicken and egg situation.
Diagnosis is really 'detective work'. The clues are usually there for
the careful observer and if you look carefully enough. If you are
methodical and try to eliminate as many possibilities as you can,
without jumping to conclusions, then you may well be led to the correct
conclusion. At the very least, you will have gathered important
observations that will assist someone else, more expert than you, in
reaching a diagnosis or coming to a conclusion. There are two main
problems. The first is that there are many clinical signs that can have
more than one cause. For example, 'flashing' and leaping out of the
water could be caused by irritation from parasites, but it may just as
well indicate a water quality problem. For this reason it is important
to carry out regular water testing and to keep a record of readings in
order to assist in arriving at the correct diagnosis.
The other difficulty is deciding whether the apparent cause of poor
health is the primary or secondary factor. For example, a fish that is
unwell is more prone to parasites, which may begin to multiply and
become a health problem in their own right. But this would be a secondary infection brought
about by the original problem. This is what I meant when I compared
making a diagnosis with being a detective. It is often very easy to jump
to conclusions and thereby overlook the initial cause.
However, bearing in mind these potential complications, we can, for
the sake of simplicity, divide health problems into three general
The most common fungi to affect fish is Saprolegnia. Often it
is first noticed as white or grey cotton wool-like tufts sticking out of
the skin or fins of fish. It is sometimes tinged green with trapped
algae. Fungus is nearly always a secondary problem, meaning it has
occurred because of some other initial damage. The primary cause may be
bacterial, parasitic, water quality or the fish may have physically
damaged it's skin or fins in some way. It can be a serious problem
because the fungus will spread and invade healthy tissue. Treatment with
a proprietary anti-fungus compound will normally clear the problem, if
applied early enough. but it is just as important to treat the
underlying primary cause to prevent the fungus reappearing.
Fish attract a whole range of microscopic parasites, some of which
live on the outside of the body on the skin, fins and gills, while
others invade the internal body tissues and organs. Those living on the
outside are known collectively as ectoparasites, while internal
parasites are called endoparasites. Although some endoparasites
can be quite serious, it is fair to say that most parasitic problems in
ponds involve ectoparasites. The latter vary in size and structure, from
minute single- celled protozoa, such as Costia and Trichodina,
to worm-like skin and gill flukes which may be up to 1 mm long (see the
main health section for more details).
In small numbers they are usually no problem but, given their
enormous reproductive capabilities and the right conditions, they can
rapidly multiply and overwhelm the fish. In severe infestations fish can
become literally smothered. Large numbers can endanger the health of
fish because of the damage they do to delicate skin and gill surfaces.
For instance. flukes and leeches have attachment devices - normally
suckers and/or hooks - which enable them to stay attached to the fish
and move freely over the body. The damaged tissues then provide ideal
entry sites for secondary fungal or bacterial infections.
As you can probably imagine. the presence of large numbers of
parasites is extremely irritating and one of the main signs of a
ectoparasite infestation is the sight of fish rubbing themselves against
a solid object, often bottom drains, in an attempt to ease this
irritation. This rubbing action, during which the fish often turn on
their sides, is called flashing. Other signs of irritation are fish
leaping out of the water and swimming at speed around the pond,
apparently in great irritation.
The other response is an increase in mucus production, often leading
to 'slime disease', whereby the mucus forms a grey/white layer over
parts of the body. If fish lie motionless in the pond or gasp at the
water surface, or hang around water returns, this may also indicate a
parasitic problem, but it is also indicative of several other serious
problems, and the beginner would be well advised to seek more expert
help in this situation.
Although the above signs of irritation are useful indicators of an
ectoparasite disease, it should be remembered that poor water quality,
especially high ammonia or nitrite levels, will cause the same response
or 'clinical signs'. In reality, the only
certain way to make a diagnosis by carrying out a microscopic
examination of a skin scrape, which has the advantage of
determining the actual species involved and the severity of the
Other specific problems worthy of mention are 'white spot', which is
fairly easily cured but can be fatal if not rectified quickly. This is
easy to identify, as the fish looks as though it has been sprinkled with
fine grains of salt. Small, white cysts up to 1 mm in diameter are
clearly visible over the body and fins of the fish.
When the trophonts inside these cysts mature, the cysts rupture,
releasing the mature trophonts into the water. The damage done as the
cyst ruptures can provide a site for secondary infection. Meanwhile, the
mature white spot trophont reproduces, releasing hundreds of 'swarmers'
or tomites into the water to re-infect the fish. Larger ectoparasites,
such as Argulus (fish louse) and Lernaea (anchor worm),
are visible without magnification.
These are two real nasties which should be treated as soon as
possible. Argulus is up to 10 mm across and is often overlooked
because at a glance it appears to be just a small black mark. However,
if your fish are 'examined' regularly these lice should be easily
spotted and if watched for a few moments these black' spots will be seen
to move. If Argulus is suspected, a close examination in a
shallow bowl will quickly settle any doubts. The first sign of Lernaea
is what appears to be fleshy pieces of cotton, up to 20 mm long,
attached to the skin or fins of the fish. Closer examination shows the
body of the anchor worm, with its head buried into the flesh of the
fish, with a trailing body and two egg sacs at its tail end.
With the exception of a few species, such as white spot, Argulus and
Lernaea, most ectoparasites are not harmful in small numbers,
being similar to fleas on a cat or dog. The major danger comes when, for
some reason, the number gets out of control. Without the aid of a
microscope it is difficult to determine just how severe the infestation
is. However, it would be a mistake to treat the pond with an
anti-parasite treatment every time a fish rubbed against the side or
jumped out of the water. Equally, it would be wrong to ignore things if
the fish were obviously suffering from great irritation and were
constantly leaping around or tearing around the pond, rubbing along the
bottom and sides.