Taking a skin scrape
Taking and preparing a skin scrape involves using a blunt scraper,
such as a wooden spatula, to gently take a sample of mucus from either
immediately behind the gill cover, alongside the dorsal fin or the base
of the tail.
The scraper is held at approximately 45 degrees to the body and drawn
backwards towards the tail in a smooth movement, lifting off a small
amount of mucus from the sample site. The mucus sample is then smeared
onto a clean microscope slide along with a drop of pond water. Never use
tap water as any residual chlorine could kill any parasites that are
The sample is then covered with a cover-slip and examined under the
microscope, usually low-power, for the presence and number of parasites.
I would normally take at least two samples, from different sites, from
each fish being examined.
Taking a skin scrape and gill
click on pictures to enlarge them
Taking a skin scrape
using a wooden spatula to gently lift off a mucus sample
from just behind the operculum
The mucus sample is put
onto a glass microscope slide. A drop of pond water is added and
a cover slip put on top
Taking a small gill
biopsy using a fine pair of scissors
There are certain considerations if the examination is to yield
useful results and avoid causing damage to the fish being examined. The
most important concern is to avoid injury to the fish during what should
be a simple, safe procedure.
If the fish does have parasites they will be found in the mucous
layer (though some will also penetrate the epidermis, e.g. 'Ich'), so it
is the mucous layer we need to sample for the wet-slide preparation. It
is important to realise that damage caused to the epidermis while taking
a mucus sample will be detrimental to the fish, so only gentle pressure with a blunt scraper should be
applied - never use anything sharp that might damage the epidermis.
To sedate or not sedate - that is the question
Another consideration is whether the fish should be sedated while a
scrape is taken. This is a subject that most books and magazines seem to
avoid so I will put my neck on the block and give my thoughts on the
In the first instance we want the fish to be still enough to do the
scrape properly, while at the same time avoiding damage to the
epidermis. With two people, one holding the fish and the other taking
the scrape, it is possible to sample smaller fish and docile larger ones
without the need for sedation. However, if there is likely to be a lot
of flapping around, there is a real chance that mucus may be stripped
off by the constant handling, thus giving an inaccurate result or,
alternatively, a danger that the fish may be damaged.
On the other hand, if fish are routinely sedated prior to taking a
scrape, there is the slight added risk that the fish could die from an
overdose of anaesthetic. There is also the consideration as to whether
the anaesthetic will affect the parasites and give a false result-
although this has not been my experience if the scrape is taken and
prepared quickly. In short, it seems to be a question of experience in
deciding whether the procedure can be carried out effectively and safely
without anaesthetic or whether the additional risk involved in sedating
the fish is the lesser of two evils.
I must say that I have come across some rather inaccurate conclusions
when people have tried to take scrapes from large, unsedated, lively
fish! When dealing with larger koi on my own I invariably find MS222
anaesthetic a help. Your thoughts on this subject would be welcome!
If a parasitic infestation of the gill is suspected it is possible to
sample mucus from the gill. It goes without saying that this procedure
is potentially dangerous and must be carried out with extreme care and
only on sedated fish.
A mucus sample can be taken from the gills by gently inserting a
cotton-bud under the operculum and rolling it over the gill filaments.
Under no circumstances should any pressure be exerted that may damage
this vital and delicate structure. The mucus is then spread onto the
microscope slide. For accurate results it is important that the sample
is prepared and examined as quickly as possible.
With care it is also possible to take a small sample of gill using
fine scissors to take a small biopsy from the lamellae tips. I urge that
this procedure is not carried out unless one is totally confident about
the procedure as the tiniest slip could cause considerable damage to the
How many is too many?
Generally one or two observed parasites per slide should not be a
reason for concern, whereas ten or more per field of view would be.
Usually, when there is a serious infestation it is quite obvious as the
slide often seems alive with parasites!
There are two potential problems that may be encountered when
examining a skin or gill scrape. All of the smaller parasites are
virtually transparent and unless the slide is scanned slowly and
methodically there is always the possibility that some parasites may be
missed, especially if they are not moving. If there seems to be a
problem it is worth lowering the microscope condenser to improve
contrast or, better still, if the option exists, try viewing the slide
If the mucus scrape is particularly thick it may be necessary to view
at different levels, starting from the bottom and working progressively
to the top. Otherwise, there is the chance that smaller parasites may be
missed because they are hidden in the mucus. This becomes more likely at
Don't jump to conclusions
The second, more common, potential problem is simply jumping to
conclusions. It is just too easy to spot the most obvious parasites, for
example skin and gill flukes, and conclude that these are the problem.
Even when you have spotted parasites it is vital that the whole slide is
still methodically examined to make sure that nothing important is
missed and so you build up to the correct conclusion. My own record is:
five different species of parasite on one slide - but
it took a full 15 minutes to find them all!
If a fish is found to be heavily infested it is worth taking a scrape
from one or two others - to determine whether there is a general problem
in the pond or if this is an isolated instance. It is often the case
that rather than the whole pond requiring treatment, just the one fish
is ill and the parasite explosion has resulted as a secondary infection
for that fish, not the whole collection.