Skin flukes (Gyrodactylus ) and gill flukes (Dactylogyrus
) and fish health
Skin and gill flukes are common fish parasites and in small numbers
probably cause little harm. In large numbers both skin and gill
flukes represent a serious threat to fish health. Flukes can cause
lesions and tissue damage as well as producing side affects such as
hyperplasia of both skin and gill epithelium and creating entry sites
for secondary infections.
The connection between flukes and bacterial fish diseases such as
ulcers is well established. Although Gyrodactylus and Dactylogyrus
are classed as skin and gill flukes respectively, they are not exclusive
to these areas and flukes of either persuasion may be found at either
What are flukes?
Flukes are small worm-like parasites up to 2mm in length. Technically
they are monogenean trematodes, which describes their biological
classification (trematodes) and the fact they only need one host to
complete their life cycle (monogenean) whereas many parasitic trematodes
need two or more hosts and are thus digeneans!
Skin Flukes (Gyrodactylus)
click on pictures to enlarge them
Skin fluke, Gyrodactylus, showing the
array of hooks on its opisthohaptor. It uses these to cling to
the fish and move around in a jerking fashion
The same parasite showing a developing embryo
already armed with hooks!
A fish's view of that fearsome opisthohaptor.
The hooks can cause tissue damage and lead to secondary
Flukes have an array of hooks at one end of their body which they use
to anchor themselves to the fish and to move around in a jerky,
caterpillar-like fashion. These hooks can be seen clearly in the photos
on the above. The gyrodactylids differ from dactylogyrids in that Gyrodactylus
has a v-shaped head, no eyespots and is viviparous (live-bearing),
whereas Dactylogyrus has a scalloped head, distinct eyespots and
is oviparous (egg-laying).
Cause and reproduction of flukes
It is generally agreed that heavy fluke infestations affecting more
than one fish are usually caused by overcrowding, poor water quality or
polluted water - that is water with a high organic content and stress.
Under ideal circumstances, these parasites can rapidly multiply.
The photo above show a gyrodactylid embryo virtually fully formed
inside its mother- however, the baby of that yet unborn fluke is already
forming. With this head start - three generations in one - the doubling
time for Gyrodactylus can be as little as 24-hours! It is much
the same with dactylogyrids, which, in warm water and ideal conditions
can reproduce very rapidly and quickly overwhelm any sick or stressed
Damage and diagnosis
The fish response to the irritation caused by flukes is similar to
any other irritant, whether it is parasites or chemical. Initially there
will be rubbing and flashing. As the disease reaches a more advanced
stage the fish will become lethargic - which is the point when some
people think the problem has gone away. At a very advanced stage the
fish will isolate itself and spend long periods laying on the bottom
with its fins clamped to its body. Other signs may be skin cloudiness
resulting from excess mucus production, skin hyperplasia, or focal
reddening. A definite diagnosis can only be made via a skin scrape or
gill biopsy. In large numbers, flukes will kill fish either directly, or
indirectly through secondary infections.
Right from the start let me say that in many cases flukes are hard to
treat and complete eradication is virtually impossible. For individuals
affected with flukes, consecutive salt baths over 2-3 days can be
useful. Using a quaternary ammonium compound in conjunction with, but not at the same time as, salt
baths can be useful, in both clearing the skin and gills of excess mucus
and debris as well as soothing damaged tissues.
Malachite and formalin is often affective, although my experience is
that the stronger dose needs to be used. It most cases, where the flukes
are a nuisance rather than a real danger the old M&F will probably
For all out war, the best treatments are organophosphates such as
Another promising treatment is high dose, bath treatments with chloramine-T. A few trials I have carried out show that in most cases chloramine-T is quite effective, but it is early days to draw any firm conclusions. The draw-back of bath treatments is that it does mean handling the fish and having a large treatment tank. Due to the complications of using chloramine-T in a typical pond, it is unlikely to be so successful as a pond treatment. Bath treatments also allow the life phases which exist in the ponds (eggs and embryos) to continue to survive. Your freshly "cleaned" individual fish will be returned to a pond with emerging stages in it!
Whilst, with enough determination, beating skin flukes is relatively
easy, however, the same can't always be said for gill flukes. Because of
gill hyperplasia and increased mucus, the gill-dwellers are often
afforded a high degree of protection from chemical treatments. I have
had several experiences of flukes surviving multiple treatments
safely ensconced in the gill. In such a situation the outlook for
the fish is poor, because in addition to the gill flukes there will
almost certainly be other gill damage and gill disease.
The most important step in preventing a serious fluke or indeed any
parasite problem is first to investigate abnormal behaviour - that is
incessant rubbing and flashing or lethargy. Take a scrape and see what's
going on. Don't just say "parasites" and dump some chemical
into the water. It is important to know what you're dealing with and how
severe the problem is. In severe cases it is important to do a follow up
at the end of the treatment to see how effective it has been. How severe
is severe? One or two flukes in a mucus sample is not abnormal. Any more
than that definitely warrants treatment.