Page 169 - The Veterinary Laboratory and Field Manual 3rd Edition

138 Susan C. Cork and Mani Lejeune

cestodes intermediate host. The common flukes of veteri-
nary importance are discussed in pages 157–160.
Cestodes, or tapeworms, range in size from Control and prevention requires a good under-
small worms < 1 cm long (Echinococcus sp.) to standing of the life cycle of the trematode
long tape-like structures 1–2 cm wide and several involved and may require control of the inter-
metres long (Taenia sp.). The typical ‘tapeworm’ mediate host.
is the mature stage of a life cycle, which results
in patent infections in the final host. Eggs or
worm segments may be passed in the faeces of Effects of parasitic helminths on
the definitive host. In many cases the segments their hosts
are still mobile and result in irritation around
the anus (for example, Dipylidium caninum in There are many species of parasites that are rela-
dogs and cats). Most cestodes have an indirect tively harmless. However, there are also many
life cycle which involves a final host (predator) species that produce pathological changes lead-
and an intermediate host (prey). The intermedi- ing to ill health and/or the death of the host.
ate host may be another mammal, a bird, reptile, Parasites cause damage to the host in following
fish or an invertebrate. Cestodes rarely cause ways.
clinical disease in the final host but the inter-
mediate (cystic) stages of tapeworms may cause 1 Parasites can compete with the host for food
clinical signs related to the size and location of and nutrients in the intestinal tract resulting
the cyst. Some important examples are discussed in emaciation and poor condition, for exam-
further in other parts of the text and are outlined ple, Diphyllobothrium sp.
in Table 3.2. The life cycles of Taenia saginata and 2 By mechanical obstruction or compression of
Echinococcus sp. are outlined in Figures 3.27 and organs, for example, ascarids in the intestine
3.29. Control and prevention of tapeworm cysts of young animals, Syngamus trachea causing
in the intermediate host (for example, ‘gid’ in ‘gapes’ in birds or cystic stages of tapeworms,
ruminants) requires the use of anthelmintics to for example, Echinococcus sp. hydatid cysts.
kill the adult tapeworm in the final host and also 3 Parasites feed on the tissues of the host, for
the prevention of re-infection of the final host by example, worms such as Chabertia species
making sure that the cystic stage is not ingested cause irritation of the gut lining (enteritis).
(that is, do not feed raw offal to dogs). 4 They suck blood from the walls of the intes-
tinal tract and cause anaemia, for example,
wire worms (Haemonchus sp.) and hookworms
trematodes (flukes) (Ancylostoma sp.).
5 Migration of the larval stages of helminths
Trematodes, or flukes, generally have indirect life may cause damage to the lungs, liver and
cycles which involve an invertebrate intermedi- other organs (Ascaris suum). In some cases the
ate host (snails). Flukes range in size from small clinical signs are more severe in the prepatent
flat organisms 2–3 mm long to large specimens period than when the adult worms are pres-
of Fasciola gigantica which may reach 5–6 cm in ent in their final location. Due to the fact that
length. Trematodes can cause significant clini- these clinical signs occur before eggs appear
cal disease, for example, liver fluke (Fasciola in the faeces it may be difficult to confirm the
hepatica), but may only be common in particular diagnosis unless additional assays are avail-
regions due to the environmental needs of the able to detect antibody or antigen.

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