Page 170 - The Veterinary Laboratory and Field Manual 3rd Edition

Parasitology 139

6 Meat quality may be impaired by the presence LIFE cycLE
of cystic stages of tapeworms for example, The life cycles for most of the common gastroin-
‘pork measles’ associated with Taenia solium, testinal nematodes in livestock are fairly similar.
this also has implications for public health. In most cases, infection is by ingestion of the
infective larval stage (free living L3 or L2/3 in
the egg) from the pasture, except for species
diseases caused by nematodes such as Ancylostoma sp. (canine hookworm) and
(roundworms) Bunostomum sp. (cattle hookworm) in which
L3 may enter the host through the skin and
Gastrointestinal nematodes
some (for example, various Capillaria sp.) which
These are probably the most economically involve a transport host. The adult worms live
important of the internal parasites of livestock in the stomach and the large or small intestine
and commonly occur throughout the world. of the definitive host where the female worms

Figure 3.21 Basic gastrointestinal nematode life cycle. Nematode adult worms are either male or female,
the females are usually larger and require plenty of nutrients. After mating (usually in the intestine of the
host) female worms lay eggs or larvae which are passed out in the faeces of the host. During development
the nematode moults at intervals shedding its cuticle (outer coat) at each stage. In the complete life cycle
there are usually four moults, the successive larval stages being designated L1, L2, L3, L4 and finally L5,
which is the immature adult. Some development occurs in the faecal pat of the host (L1 to L3) if the life
cycle is direct or in an intermediate host (for example, a free-living invertebrate) in an indirect life cycle. The
L3 is usually the ‘infective stage’ and can be ingested by the final host while grazing. The L4 and L5 larvae
develop within the definitive host. The time period between ingestion of the L3 and the appearance of eggs
in the faeces is known as the ‘prepatent’ period. Within the animal the larvae often migrate through the
liver and/or lungs on their way to the intestinal tract where they mature to the adult form and mate. When
eggs appear in the faeces the infection has become ‘patent’. In some cases the migrating larvae may cause
clinical signs in the animal before patency and so the absence of parasite eggs in faecal samples may not
necessarily preclude the possibility that nematode infestation is the cause of the problem.

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