Page 161 - The Veterinary Laboratory and Field Manual 3rd Edition

130 Susan C. Cork and Mani Lejeune

and management, the recommendations for par-
The OIE Manual of Diagnostic Tests and asite control based on worm egg counts will vary
Vaccines for Terrestrial Animals is a com- from one situation to the next. The life cycle,
pilation of diagnostic procedures and a pathogenicity and fecundity of the parasite must
ready source of information for any veteri- also be considered. However, some general rules
nary diagnostic laboratory. The Terrestrial can be applied. In most cases, helminths of low
Manual has been designed for practical use pathogenicity such as the tapeworm Moniezia
in the laboratory setting and contains over sp. must be present in high numbers (that is,
110 chapters on infectious and parasitic dis- > 2000 EPG) before the count would be consid-
eases of mammals, birds and bees. http:// ered significant. In contrast, low numbers of the
www.oie.int/en/standard-setting/terrestrial- nematodes Ostertagia sp. (EPG < 200) can cause
manual/access-online/ clinical disease. The life cycle of some helminth
parasites can be complex, resulting in a long pre-
patent period (time between infection and the
Interpretation of parasitology results presence of eggs in the faeces), for example, the
liver fluke (Fasciola hepatica). In cases of fascio-
Table 3.3 provides a rough guide to the interpre- losis the animal usually develops clinical signs
tation (clinical significance) of some examples before eggs are found in the faeces so the pres-
of worm egg counts in cattle and sheep. It is ence of any F. hepatica eggs is always considered
generally accepted that a low parasite burden significant.
is ‘normal’ in grazing animals with low levels of
helminth parasites inducing natural resistance
in older livestock. Complete elimination of para- 3.3 Helminths of veterinary
sitic helminths may therefore not be beneficial. importance
However, at certain times of the year, especially
when young animals are born, or at other times The term helminth means ‘worm’ and the drugs
when additional nutritional or other ‘stresses’ used to control worm infestations are ‘anthel-
may occur, previously subclinical infections may mintics’. Helminth life cycles may be simple,
become clinical with resultant rapid weight loss, with direct infection of the final host by inges-
diarrhoea and other clinical signs. Helminth tion of infective eggs or larvae, or complex,
parasites may reproduce rapidly in non-immune involving one or more intermediate hosts and
younger stock and reach high levels very quickly; several developmental stages. The life cycles
this is especially common where rotational of helminths of particular significance will be
grazing is not practiced and young stock graze described in the relevant subsection (see index).
areas which may have previously been heavily
contaminated with worm eggs. The infective
larval stages of most helminths tend to develop nematodes (roundworms)
rapidly when the temperature and humidity are
at a suitable level for larval development in the Nematodes, for example, Nematodirus spp.,
environment, for example parasitic disease tends Ostertagia spp., Haemonchus sp. also known as
to be more prevalent following the rains and in roundworms, are a common cause of ill thrift
areas where the temperatures are mild. Owing to and diarrhoea in young animals. Roundworms
factors such as variation in land topography and vary in size from thread-like worms < 5.0 mm
climate, differences in herd/flock age structure long (Trichostrongyloids) to large worms

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