Page 163 - The Veterinary Laboratory and Field Manual 3rd Edition

132 Susan C. Cork and Mani Lejeune

will vary depending on the number, location and 3.14). Common genera (groups) are listed in
and species of nematodes present. Clinical signs Table 3.4.
during the early stages of infection will depend The life cycle of most nematodes involves
on the route of migration taken by the larval 4–5 larval stages (L1–L5), three of which may
forms of the nematode, for example, Strongylus develop in the external environment (see
vulgaris infection in the horse may result in Figures 3.21 and 3.23). The L3 stage is often
inflammation and blockage of abdominal blood the infective stage and will go on to develop
vessels that results in colic and Parascaris equorum into the mature parasite within the host species
infection in foals may be associated with pneu- if ingested from pasture (direct life cycle). L2
monia as larvae migrate through the lungs before within egg is the infective stage for most asca-
maturing in the intestine. The identification of rid worm. In some cases the L3 develops in an
gastrointestinal nematodes may be possible by intermediate host, for example, invertebrates
the examination of eggs in faecal samples but such as snails, earthworms and flies, and the
in some cases, because some eggs (for example, final host becomes infected by either ingest-
the strongyles) are difficult to distinguish in ing the intermediate host (indirect life cycle,
closely related species, it is necessary to exam- Figure 3.22) or ingesting L3 shed by the inter-
ine the larval or adult stages (see Figures 3.4 mediate host or L3 transmitted through bite of

Figure 3.14 Features used to assist in the identification of adult nematodes. (A) shape and size; (B)–(E)
morphology of the head; (F)–(H) morphology of the tail and reproductive organs.

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