nicrobiological aspect

1. ECHINOCOCCUS
GRANULOSUS
Man Bahadur Rana,BPH,ACAS,Nepal
Dog tapeworm

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4. Morphology
Adult worm:
- It is a small tapeworm, measuring 3-6mm in length.
- It comprises of Scolex (head), neck and Strobila
consisting of 3 segments .
1st: Immature
2nd : Mature
3rd : Gravid

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- The terminal segment is biggest, measuring 2-3 mm in
length and 0.6mm width.
- The Scolex bears four suckers and a protrusible
rostellum.
Neck :
The neck is short and thick.

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8. Egg
- It is spherical in shape and measures 32-
36umx 25-32um.
- It contains hexacanth embryo with 3 pairs
of hooks.
- Eggs are indistinguishable from those of
other Taenia sp.

9. Larval form
- This is found with in the hydatid cyst and represent
the scolex of the future adult worm.
- It remains invaginated with in a vesicular body.
- When it enters the definitive host, the scolex with
four suckers and hooklet, becomes evaginated and
develop into an adult worm.

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12. Life Cycle

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 The worm passes its life cycle in two host.
Definitive host : Dog, wolf, fox and jackal.
Intermediate host : sheep, goat, man.
 The eggs are discharged with the faeces of the definitive
host(dogs and allied animals).
 These eggs are swallowed by the intermediate host
sheep and other domestic animals while grazing in the
field and also by man.

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 The hexacanth embryo hatched in the
duodenum.
 After 8 hours after ingestion, the embryo
penetrate the gut wall and is carried to the
liver through the portal vein.
 About 70% of the onchosphere are lodged
(arrested) in the liver itself (liver acts as a first
filter).

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 Some of the embroys may pass through hepatic
capillaries, enter the pulmonary circulation and
filter out in the lungs (lungs act as a second filter).
 Few of the embroys enter the blood stream and
lodge in various organs.
 Wherever the embryo (oncosphere) settles, it
forms a hydatid cyst.

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 From the inner side of the cyst, brood capsules
with a number of scolices are developed.
 Upon ingestion of carcasses of intermediate host
by the definitive host, each scolices grows into an
adult worm in about 2 months.
 The cycle is repeated except with human host.

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 As the dogs have no access to the hydatid cyst
developed in the viscera of man, the life cycle of
parasite comes to a dead end.
 Life span of adult worm is 6 months and life
span of larval worm may continue to develop
for many years.

18. Pathogenecity
- The adult worms of E. granulosus in dogs do not
cause much inconvenience.
- They are found in large number (by 100 or even
1000) in small intestine of infected dogs where
they lie embedded in the mucous membrane and
on postmortem appears as small white specks.
- The larval worm of E. granulosus in man
causes unilocular hydatid disease
(hydatidosis).

19. Mode of Infection
- Infection is acquired by the ingestion of eggs in the dogs
faeces. This occurs in following ways.
i) by direct contact (handling) with infected dogs.
ii) by allowing the dog to feed from the same dish.
iii) by taking vegetables contaminated with infected dog
faeces.

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- Infection through contaminated water is not common as
the eggs being heavier sink to the bottom.
Infecting agent : eggs in dog faeces.
Portal of entry : Alimentary tract.
Site of localisation : Viscera ( liver, lungs
and other organs)

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Hydatid cyst :
The cyst wall consist of two layers:
- Outer layer (ectocyst).
- Inner or germinal layer ( endocyst).
Ectocyst:
. It is a laminated hyaline membrane, thickness of 1mm
. To the naked eye, the ectocyst has the appearance of the
white of hard boiled eggs.

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. Elastic in nature and when incised or ruptured curls itself
back, exposing its inner surface.
Inner or germinal layer (endocyst) :
. It is a cellular layer and it give rise to ectocyst on outer side
and brood capsule and scolices on inner side.
. It is very thin and about 25um in thickness.
. It is a vital layer of cyst and secrete the specific hydatid fluid.

24. Characteristics of hydatid fluid
 It is a clear colourless fluid (may be pale yellow
in colour).
 It has a low specific gravity (1.005-1.010).
 It is slightly acidic (pH 6.7) in nature.
 It contains sodium chloride, sodium sulphate,
sodium phosphate, and calcium salts of succinic
acids.
 Antigenic – being used for immunological test.
 It is highly toxic, when absorbed give rise to
anaphylactic shock.

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 Acephalocyst: Sometimes brood capsules are not
formed and if formed are without scolices. These types of cyst
are sterile and are called Acephalocyst. These cyst when
ingested by definitive host do not lead to infection.
 Endogenous daughter cyst formation: After a long
period of growth, daughter cyst are formed within the mother
cyst and may arise from the detached fragment of the germinal
layer. The endogenous daughter cyst also have both ectocyst
and endocyst with brood capsule and scolices.

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Exogenous cyst formation: In case of hydatid
disease of bone, herniation or rupture of germinal and
laminated layer may occur through some weaker part of
bone and result in formation of exogenous cyst.
Distribution of Hydatid cyst:
Liver (75%), lungs (8%), muscle (5%), brain (2%),
kidney (2%), heart (2%), bone (2%) and other parts of
body (4%).

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Rate of growth of hydatid cyst:
- The development of hydatid cyst in man is very slow.
- At the end of year, it is approximately 4cm in diameter.
Clinical features :
- The chief clinical manifestation are dependent upon
local signs and if cyst is situated superficially it may cause
visible swelling.
- The disease remain symptomless for many years and
its presence is only detected at autopsy.

29. Laboratory diagnosis
Parasitological : Detection of scolices in cyst fluid or
centrifuged deposit (hydatid cyst) is 100% confirmatory.
Casoni test:
- It is an immediate hypersensitivity (type-I) skin test
introduced by Casoni in 1911.
- Intradermal injection of 0.2ml of fresh sterile hydatid fluid
into the volar space of forearm.
- Rxn is seen after 30 min, a large wheal (5cm in diameter)
multiple pseudopodia is seen.

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Detection of antibodies:
Enzyme Linked Immunosorbent Assay (ELISA),
Complement Fixation Test (CFT), Indirect
haemagglutination Assay (IHA).
Detection of Antigen :
ELISA can be used for detection of antigen in the
serum.
Blood Examination:
Eosinophilia (20-25)% may be present.

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Radiological : This is often helpful in the diagnosis of
hydatid cyst of lungs and liver.
- USG of whole abdomen is useful in locating the site of
hydatid cyst of the abdominal organ.
- CT-scan is more helpful.
Histological examination: Surgically, removed
cyst may be examined to reveal different layers of the
hydatid cyst i.e ectocyst and endocyst.

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33. TREATMENT
- Surgical removal of the hydatid cyst can be performed
but there may be recurrences in 2-25% cases of surgery.
- Hence postoperative chemotherapy may be given for at
least two years after surgery.
- Praziquantel and Albendazole are the
chemotherapeutic agent used for the treatment of hydatid
cyst.

34. Prophylaxis
- Personal hygiene including avoidance
of close contact with dogs.
- Control in movement of dogs.
- General health Education.
- Not allowing the dogs to eat carcasses
of slaughtered animals in endemic
areas.