Page 149 - The Veterinary Laboratory and Field Manual 3rd Edition

118 Susan C. Cork and Mani Lejeune

a SIMPLE SEdIMEntatIon tEcHnIquE
Sedimentation relies on the principle that para-
site eggs will settle to the bottom of a suspension
after a period of time, this is best facilitated by
prior centrifugation (1500–2000 rpm, 3 min) of
the sample and then leaving the sediment to set-
tle for at least 10 min. A direct smear can then be
made (see above). There may be some difficulty
in identifying parasite eggs using this method if
there is a lot of debris present and centrifuga-
tion can distort the structure of some eggs, for
example, Fasciola sp. trematodes.

Quantitative techniques
For the purposes of survey work and diagnosis
it is usually necessary to determine the number
of parasite eggs per gram (EPG) of faeces, this
is because some parasites are only pathogenic
in high numbers and therefore low counts may
not be of any clinical significance (for example,
Moniezia sp. in cattle, cyathostomes in horses).
Most animals will carry a low number of a
range of helminth parasites without showing
any clinical signs. However, some parasites are
pathogenic even in low numbers (for example,
Fasciola hepatica in sheep) and some produce
more eggs than others (for example, Haemonchus
Figure 3.4 Larval identification is difficult but some contortus in sheep). For these reasons, it is
common species may be recognized by the charac- important to understand the life cycle and epi-
teristic length and shape of the tail as well as the demiology of parasite infections before making
number of cells in the intestinal tract. Free living decisions about the significance of the results.
larvae may be stained using iodine solution which Quantitative assessment is done using various
may help distinguish them from potentially patho- methodologies such as the differential centrifu-
genic species. Try to get specialist training before gal flotation technique, McMaster technique
attempting to identify helminth larvae. Oe= oesoph- (flotation based, see Figure 3.5) or Stoll count-
agus; int = intestine; sh = sheath; a = anus; t = tail ing method (sedimentation based). Another
of the larvae; sht = tail of the sheath. commonly used method is the Wisconsin double
centrifugation flotation technique. As indicated
can be prevented by keeping samples in air tight earlier, flotation relies on the principle that
container or refrigerated (< 4°C) or by adding a parasite eggs will float in a solution of higher
fixative (10% formal saline or 70% alcohol) to specific gravity (SpG). Some of the most com-
the sample. monly used techniques for the determination
See Plate 1 for nematode larval culture. of EPG in a sample are outlined below. More

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