Page 171 - The Veterinary Laboratory and Field Manual 3rd Edition

140 Susan C. Cork and Mani Lejeune

produce large numbers of eggs, which pass out cLInIcaL SIGnS
with the faeces. Given appropriate environmen- Clinical signs are rarely specific for a particular
tal conditions the eggs hatch out in the faecal parasite and in most cases there will be a mixed
mass to become first stage larvae. The larvae population of helminths in the gastrointestinal
moult and pass through a number of changes tract, which may include cestodes as well as
(L1–L2) before they become infective L3 (3rd nematodes and possibly trematodes as well. A
stage larva), which migrate onto the grass near heavily infected animal may become weak, lose
the faeces. Animals grazing on the contaminated weight, develop a scruffy coat and, in chronic
pasture ingest the L3, which develop further to cases, may become anaemic. Mild to heavy diar-
the L4 and L5 adult stages during migration rhoea is commonly, but not always, apparent.
through the host’s tissues (Figure 3.21). The Very heavy burdens of gastrointestinal parasites
entire life cycle takes from 3 to 8 weeks depend- may cause acute disease (diarrhoea, dehydra-
ing on the species of worm. See also Figures 3.22 tion, metabolic imbalance) and rapid death
and 3.23. whereas moderate burdens may result in chronic
weight loss eventually leading to cachexia with
death after 2 to 3 months unless the animal is
treated with anthelmintic or is placed on good
quality clean pasture and develops immunity.
Mild infections may cause few clinical signs and

Figure 3.22 Life cycle of a nematode (Capillaria
contorta) with an indirect life cycle. Capillaria sp.
are nematodes which infect mammals and birds.
Some species require an intermediate host to com-
plete the life cycle, for example, C. contorta. This
species infects birds and utilizes an earthworm as
the intermediate host. The adult parasite lives in
the oesophagus and crop of poultry and wild birds
(A), eggs are passed out in the faeces (B) and the
developing larvae are ingested by earthworms (C).
Birds are re-infected when they ingest earthworms
during feeding. Other species of Capillaria (C. obsig-
nata and C. caudinflata) also occur in birds. Both
of these species live in the small intestine but C.
obsignata has a direct life cycle and C. caudinflata has an indirect life cycle. The prepatent period for all
these species of avian Capillaria is 3–4 weeks; the clinical signs observed in infected birds depend on the
level of infection and the immune status of the bird. Control of Capillaria sp. infection requires identifica-
tion of the parasitic species involved. Prevention of infection is generally more difficult for species with an
indirect life cycle (C. contorta and C. caudinflata) than for those with a direct life cycle (C. obsignata) due to
the fact that it is usually not possible to prevent access of extensively reared birds to potentially infected
intermediate hosts. Other nematodes with indirect life cycles may use other invertebrates such as a free
living or parasitic mite as the intermediate host. All parasite life cycles have evolved to utilize the hosts’
normal feeding patterns so that transmission is ensured.

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