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A common parasite
As with many fish parasites, small numbers of Costia (or more
correctly Ichthyobodo) are not uncommon and appear not be
detrimental to the fish's health. In small numbers these parasites seem
to live on cellular debris in a commensalistic relationship with their
fish host. Costia occasionally live on the skin and gills
of healthy fish and it is believed that the fish's defences keep the
parasite population under control.
Ichthyobodo (Costia
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click on pictures to enlarge them |
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Histological section of a gill with a heavy
infestation of Ichthyobodo (Costia) attach to the
gill epithelium. The parasites feed on the cell contents.
Note the pyriform shape, |
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photo:
Frank Prince-Iles |
It becomes a serious threat when, for various reasons, the parasite
becomes established in large numbers. As with all parasite infestations,
large numbers will affect fish health by causing serious tissue damage
to both skin and gills, as well as secondary effects such as hyperplasia
or secondary infections - particularly of the gill.
They can reproduce at a phenomenal rate -
under ideal conditions
The main danger from Costia is the rate at which it can
reproduce; quickly taking advantage of any shift in the balance of
health. It is not unusual to see very sick fish literally alive with
parasites (see the fish disease movies). However, the question that has
to be asked in such cases is: "is the fish
sick because of the heavy parasite infestation, or has it attracted
parasites because it is sick?". At higher temperatures
the generation doubling time can be as little as a few hours!
They do not reproduce sexually, they simply divide into two, a
reproductive process called binary fission. The conditions that
encourage this type of explosive population growth are those that we
would expect; that is, stressed or sick fish, poor water quality
and/or overcrowding. Under such (ideal!) conditions it reverts to
a parasitic existence, attacking living cells with disastrous
consequences.
Identification
Costia infestations cause a typical irritation response from
the fish. Heavy and laboured 'breathing' (judged by watching
operculum movements), flashing and rubbing, skin cloudiness caused by
excess mucus, focal redness, lethargy. At a later advanced stage (which
may be too late for treatment) fish often isolate themselves,
sometimes near the water surface or water return. They can also exhibit
extreme lethargy with long spells laying on the bottom with clamped
fins. I should also point out that these clinical signs are not
exclusive to parasite infestations and can be caused by several other
factors including adverse water quality.
For an accurate diagnosis a skin scrape and a gill biopsy should be
taken as it is not unusual for the skin to be 'clean' yet the gills
suffering from a severe parasite infestation - or vice versa, or indeed
the two areas to be heavily colonized by two different parasite
species!
Under the microscope you will probably need 400x magnification to see
these small parasites, as they are only 10 -20 µm long (1µm = 1/1000
millimetre). Because they are so small it sometimes helps to rack down
the microscope condenser and add a little more contrast. Free-swimming Costia
is identified by its characteristic flickering, caused as it turns its
crescent-shaped body. It is a fast moving parasite, constantly moving in
and out of focus.- Click on the Costia
movie to see it live.
When attached to the skin or gill it assumes a pyriform shape
and clusters of parasites can sometimes be seen on the edge of
gill epithelium in gill biopsies - as seen in the photomicrograph above.
One or two parasites per slide is not cause for concern. If numbers are
higher than this, treatment and a review of environmental conditions
should be the order of the day
Treatment
Treating mild to moderate outbreaks is fairly easy, usually requiring
just one treatment. Most proprietary parasite treatments will work. My
own preference is malachite and formalin, provided that fish are not
suffering from gill damage. Prolonged immersion with potassium
permanganate is another option but again this is not advisable if gill
damage is suspected. When gill damage is suspected salt offers the
safest route as either a bath treatment (20 - 25g/litre for 20 - 30
minutes) or as a long-term immersion at 3-5g/litre.
In advanced and severe cases, resolution may be very difficult
because of the numbers of parasites involved, excess mucus and
hyperplasia helping to protect the parasites from chemical treatments
and the poor health of the fish. In such circumstance tank treatments
with chloramine-T and long term salt support is the best option with
treatments being repeated until the fish shows signs of improvement. |
