It is not uncommon to find small populations of the parasite Trichodina
on fish. At low levels they don't pose a threat to fish health.
Unlike many parasites it doesn't actually feed on fish; these
parasites only use the fish host as a home and means of transport!
However, in large numbers they are extremely irritating and it is likely
that some tissue damage will result from the sucking disc that
trichodinids use for attachment. Healthy fish can control the numbers of
parasites. Severe trichodinid infestations are usually associated with
overcrowding and poor water quality. Under such conditions these
parasites can rapidly multiply.
click on pictures to enlarge them
This shows the outer rim of cilia and the
central sucking disc with its hook-like denticles
Trichodinids in a wet mount (mucus) at
Trichodina on the edge of a gill
filament. Note the density, with some sitting on top of others.
They are easy to recognize in a wet mount. They always remind
me of flying saucers, hovering and skimming over the surface of the gill
or skin. They are top-hat shaped when viewed from the side. When viewed
from the top it is possible to see an outer ring of cilia and the
concentric rings and hook-like denticles of the sucking disc. (See the
top photo) Gill trichodinids are smaller than their skin-dwelling
cousins, usually less than 30 mm. They also
tend to move faster than skin trichodinids. Those found on the skin are
usually > 50 mm.
The fish react to Trichodina in much the same way as any other
parasite by rubbing and flashing. There may be focal areas of reddening.
At a later stage the fish will be lethargic, they may isolate
themselves, stop feeding and lay on the bottom with clamped fins.
While they are not as immediately life-threatening as most parasites,
heavy infestations do cause considerable stress to the fish and it is
not uncommon for other ectoparasites and bacteria to take advantage
leading to secondary infections.
My own experience is that this parasite can be tricky to treat.
Sometimes it responds to proprietary parasite treatments and other times
they have little or no effect. The same applies to malachite green and
formalin. The most effective treatment against Trichodina seems
to be potassium permanganate which seems to work virtually every time.
Usually only one treatment is needed to. Salt baths can be useful if
only dealing with just one or two fish.