Page 148 - The Veterinary Laboratory and Field Manual 3rd Edition

Parasitology 117

numbers are known as quantitative techniques. slip for microscopic examination. Alternatively,
Quantitative techniques are important for rou- a cover slip can be positioned over the menis-
tine screening of livestock and can also be used cus for the required period and then placed on a
to determine whether or not a parasite treatment microscope slide. Note that centrifugation steps
or control program is required, or to determine can help speed up the process and also increase
if an anthelmintic treatment has worked. These the sensitivity of flotation technique.
quantitative methods will be discussed later. Nematode eggs, most cestode eggs and
coccidial (protozoan) oocysts will float to the
surface of the meniscus and adhere to the cover
Qualitative techniques
slip. Heavy trematode eggs such as Fasciola sp.
dIrEct SMEar may not float in saturated salt (NaCl) solu-
For a quick qualitative assessment, a direct tion and cysts of Giardia sp. (protozoan) may
smear can be made by taking a few drops of a be distorted. Sheather’s sugar solution with
standard suspension of faeces and adding a few 1.32 SpG may float fluke eggs but will distort
drops of water to make a smear on a glass slide, the delicate egg wall. Zinc sulphate (ZnSO )
4
adding a cover slip and examining the prepa- with 1.18 SpG is preferred for detecting lung-
ration under the microscope. This is a simple worm larvae and Giardia sp. cysts. Strategically,
method but it is not very sensitive, that is, only a faecal sample can be processed simultaneously
a very small sub-sample of faeces is examined by both Sheather’s sugar and ZnSO methods
4
so many eggs may be missed. The method is to recover the whole range of parasite species
usually adequate to detect heavy infections. present in the sample. The appearance of hel-
minth eggs in faecal samples is illustrated in
a SIMPLE FLotatIon tEcHnIquE Plates 2a, 2b and 3. If the sample is not fresh
Life stages of parasite species possess specific (> 12 h old) some parasite eggs may have
buoyant densities. The flotation technique hatched and there will be larvae in the sample.
works on the principle that a flotation fluid It is possible to identify the larvae of different
with specific gravity greater than the buoyant helminth species but this is time consuming
density of a parasite stage will allow it to float and requires skill and experience (see Figure
in that solution. Based on this, it is possible to 3.4). As outlined earlier, larval development
selectively segregate parasite eggs (and larvae)
from coarse faecal matters. A basic flotation
solution includes a saturated (super concen-
trated) salt (NaCl) solution (1.2 specific gravity
[SpG]). This flotation fluid can be added to the
sediment from a standard faecal suspension. In
most cases a standard 15 or 20 ml test tube is
used but smaller volumes can be prepared, for
example, using 2.0 ml Eppendorf tubes (Figure
3.3). Add a small volume of flotation solution
and mix thoroughly. Top it up with enough solu-
tion to reach the top of the test tube to form a Figure 3.3 A temperature controlled centrifuge
positive meniscus. Allow it to stand for about with an Eppendorf rotor. Eppendorf tubes hold up
10–15 min and using a loop or a glass rod scoop to 2.0 ml of fluids and usually have a well-fitting lid
the fluid at the meniscus and place on a cover- which reduces the risk of contaminating the rotor.

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