A common but lethal disease
The ciliated parasite Ichthyophthirius, more commonly known as
white spot or Ich, is a very common fish disease capable of
affecting virtually all fish species. Ich has a fairly complex
life cycle that has a major bearing on treatment methods. The white spot
trophont (photo below) forms a nodule under the skin or gill
The trophont constantly turns and moves under the skin, feeding on
destroyed cells and body fluids. The parasite feeds on body cells until
mature and then 'punches' its way out of the skin. It then attaches
itself to a plant or some other object and forms a capsules around
itself. Inside the capsule, the tomont, as it is now called, repeatedly
divides, producing up to 1000 tomites that finally 'hatch' from the
capsule and swim to find a fish host. These small tomites are the
infective agent. They burrow into the fish's skin and the cycle
starts all over again. Clearly, with each turn of the cycle the number
of parasites increases dramatically.
click on pictures to enlarge them
White spot trophonts seen in at low power wet
mount of a skin scrape. Note the light horseshoe shaped nucleus
and the variation in size
Histological preparation of gill tissue with
embedded trophonts. Note the distortion to the gill tissue and
In large numbers the parasite can cause considerable tissue damage
from both its feeding activity while under the epithelium and
during the entry and exit from its host. The tissue damage caused,
particularly to gill tissue, is a major threat to fish health and can be
the site for secondary bacterial or fungal infections. A prime example
is seen in the microphotograph above. The trophonts can be clearly seen
in the gill tissue. Their irritating activity has caused severe
hyperplasia (abnormal cell growth). This particular fish ended up with
severe gill tissue erosion and bacterial infection. Altogether, around
30 koi in this pond died in this outbreak.
White spot cysts, each containing an active trophont, appear as small
white nodules on the skin, gills and fins, giving the fish the
appearance of having been dusted with salt. In a confirmatory skin
scrape the trophonts appear as dark round objects slowly rolling around.
The trophonts vary in size, up to 1mm and are considerably bigger than
most fish parasites. Other features sometimes seen are a lighter,
horseshoe shaped nucleus (seen in the top photo) and short cilia
covering the entire body (seen in the movie clips).
In the early stages of the disease, fish are likely to flash and rub
against objects because of the irritation. At a later, advanced stage
they will become lethargic and spend most of their time sitting on the
My own experience is that Ich commonly affects koi - but in the early
stages the characteristic white spots are very difficult to spot. They
are probably there but not very noticeable. This is one reason why it is
best to do a skin scrape when parasites are suspected - so that you know
exactly what you are dealing with! Even finding one trophont warrants
It is only the free-swimming stage of the parasite that is
susceptible to treatment; neither the trophonts under the epithelium or
the tomont cysts can be killed. So any treatment plan has to be carried
out over a period of time in order to kill the emerging parasites. This
in turn depends on temperature. At 7oC the life cycle will
take six weeks, whereas at 25oC it will be complete in a week.
Water should be monitored during the treatment
course in case there is any loss of filter activity.
It is also believed that fish that survive an
attack of Ich have an increased immunity against future attacks
Lurking in the background!
Ich is most often brought into the tank or pond on new fish or plants
(not if they are quarantined!). However, it is also believed that
some survivors of an Ich infection can become latent carriers, with the
parasites forming a latent stage at protected sites such as the base of
fins or the gills.
Subsequent stress or poor conditions can awaken white spot to
re-infect either its host or other fish. This certainly seems to be the
case with koi, when often small numbers of trophonts are often found
alongside severe fluke, Trichodina or Costia infestations,
even in ponds which have not had any new introductions.