Agriculture Reference
In-Depth Information
In the liver and lungs, the hydrated cyst is 7-20 cm in
diameter but in the abdominal cavity where unrestricted
growth is possible they may be much larger and may con-
tain several litres of fluid. Brood capsules may be formed.
In animals, the majority of infections are only evident at
the abattoir, in contrast to man where the hydatid, in its
pulmonary or hepatic site, is often of pathogenic signifi-
cance. Control is based on the regular treatment of dogs and
prevention of them eating material containing hydatids.
Fasciola hepatica
: Liver fluke
Adult fluke in the bile duct lay eggs into the bile which
travel to the intestine. Eggs that are passed in the faeces
develop and hatch, releasing motile ciliated miracidia,
and if certain conditions are correct will find their way
to an intermediate host, for example a snail of the genus
Lymnaea
, most commonly
L. truncatula
. In infected
snails, development proceeds through the sporocyst
and radial stages to the final stage in the intermediate
host, the cercaria. These are shed from the snail as a
motile form and attach themselves to grass blades and
encyst there to form the infective metacercariae. It
takes a minimum of 6-7 weeks up to several months
for development from miracidium to metacercariae.
These metacercariae when ingested by the final host
excyst in the small intestine, migrate through the gut
wall, cross the peritoneum and penetrate the liver cap-
sule. The young flukes tunnel through the liver paren-
chyma for 6-8 weeks, enter the small bile ducts and
then the larger ducts and sometimes the gall bladder
and reach sexual maturity. The pre-patent period is
10-12 weeks. The minimum period for completion of
life cycle is 17-18 weeks.
Fasciola hepatica
can survive
in untreated sheep for years. In cattle, it is usually less
than 1 year.
Damage to the liver in wet seasons in the United
Kingdom can be extensive and render the liver aestheti-
cally unsuitable for food (Fig. 9.23).
Taenia hydatigena
(known as
Cysticercus
tenuicollis
in larval stage)
This is the largest of the intestinal dog tapeworms with the
cysticerci found in the abdominal cavity and liver in inter-
mediate hosts (sheep, cattle, deer, pig, horse), the mature
Cysticercus tenuicollis
being about 5-8 cm in diameter when
they emerge as 'bladder worms' on the peritoneum. Dogs
and cats are infected by consuming the cysticercus in the
tissues of the intermediate host. If untreated, these may sur-
vive for several months up to a year or more. The interme-
diate host is infected through the ingestion of tapeworm
eggs in infected faeces on pasture. The ova hatch in the
intestine, and the oncospheres, infective to sheep, cattle and
pigs, are carried via the blood to the liver in which they
migrate for about 4 weeks before they emerge on the surface
and attach onto the peritoneum. These appear in the liver as
serpentine haemorrhagic tracts especially near the thin
edge. At first, these tracts are dark red in colour but soon
become brown or green and finally whitish due to fibrosis.
Paramphistomiasis
Paramphistomiasis, rumen fluke, has become a common
finding adhering to the wall of the rumen of cattle and
sheep, as an incidental finding at post-mortem, in the
United Kingdom in recent years (see Fig. 9.24). The life
cycle is similar to
Fasciola
, in that snails act as an inter-
mediate host with miracidium, sporocyst, rediae and
cercariae stages.
Taenia ovis
(previously known as
Cysticercus ovis
)
Taenia ovisis
is a tapeworm of the dog which measures
1-2 cm and forms its cystic stage in sheep. Cysticerci,
3-9.5 cm in size, are found in the skeletal muscles, par-
ticularly the heart, diaphragm and masseter muscles 3
months after ingestion (Fig. 9.22).
Sarcocystis
There are large numbers of
Sarcocystis
species. The life
cycle has the sexual reproductive stage occurring in the
predator or scavenger host which come infected by con-
suming meat containing infected sarcocysts. These are
shed as sporocysts or sporulated oocysts in the faeces after
about 1 week. The intermediate host (prey) is infected by
ingestion of sporocysts in contaminated food and water.
These are released in the intestine, penetrate the blood
and lymphatic systems and after several generations
encyst within the muscle, heart, liver, lung and neuronal
tissues. In muscle, the cysts lie within and between indi-
vidual muscle fibres and have a characteristic cigar shape
(4.5 × 0.35 µm). Grossly, infected tissue generally has a
greenish, eosinophilic myositis appearance. The final host
Figure 9.22
C. ovis in a lamb, courtesy of Ian Robinson, RMHI.
