Page 186 - The Veterinary Laboratory and Field Manual 3rd Edition

Parasitology 155

carnivores. The adult tapeworm consists of a LIFE cycLE
scolex (head) and three to six segments. The The adult tapeworm is found in the small intes-
parasite is distributed worldwide and occurs in tine of dogs (Figure 3.29). The eggs are voided
the small intestine of the dog and many wild in faeces which may be dispersed over a wide
canids. The intermediate stage is called the area in dust and contaminated herbage and
‘hydatid cyst’ and is found in a wide range of pasture. This provides a means of infection for
species including domestic ruminants, pigs, herbivores. The embryonated eggs are capable of
horses and humans. An adult E. granulosus has survival on the ground for about 2 years. After
a life span of up to 20 months and can produce ingestion by the intermediate host, activated
6000–12,000 eggs/month. A dog may host a oncospheres (embryos) pass through the intes-
hundred or even thousands of these worms. tinal wall and are distributed to other organs via
Hydatids are transmissible to humans and so are the blood. The liver filters out most of them but
of public health importance. Children who have others can lodge in the lungs, kidney, bones and
close contact with infected dogs are exposed to brain.
the greatest risk of infection. Human infection, Oncospheres (Figure 3.26) that are not
in this case, does not occur through ingestion of destroyed in the tissues develop into cysts
cysts in meat but can occur through ingestion (hydatid cyst) which can attain a diameter of
of soil, water or any other vegetable material 1 cm in 5 months. At this stage the cyst con-
contaminated by dog faeces. tains a number of secondary cysts or brood
capsules each containing about 40 protoscolices
or tapeworm heads. Brood capsules may detach
themselves from the cyst wall to float free in

Figure 3.29 Life cycle of the hydatid tapeworm
(Echinococcus granulosus). (A) The adult tapeworm
lives in the small intestine of the dog and many wild
canids, which are the definitive hosts. The mature
worm is very small, about 6 mm long. (B) The eggs
are passed in the faeces of the dog to contaminate
food and water supplies. Eggs are accidentally
ingested by grazing animals and humans. (C) The
hydatid cysts develop in the liver, lungs and brain of
the intermediate hosts. (D) In humans, the hydatid
cysts may reach a very large size. The definitive
host (dog) becomes infected by ingesting the cyst
stage in the tissues of the intermediate host. This
commonly occurs where dogs and wild canids have
access to the raw visceral organs of animals which
die and are not buried. Infection can be controlled
by (1) treating infected dogs for the adult tapeworm
and (2) preventing access of dogs and wild canids to
dead livestock by burying carcasses or by cooking
any meat and offal fed to dogs.

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