a deadly little parasite
Written by Frank Prince-Iles
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 )
|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
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.
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
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.
Article and pictures placed here with permission from the author,