Hydatid disease is the larval cystic stage of Echinococcus granulosus infection,
that can cause illness in intermediate hosts; generally herbivorous animals and humans who are infected accidentally.
Clinical features of cystic echinococcosis are highly variable.
The spectrum of symptoms depends on the involved organs,
the size of cysts,
the interaction between the expanding cysts and the adjacent organs and finally the disseminated spread of the disease.
The classical findings in hydatid disease are well known; however,
findings related to disease complications and unusual anatomic locations are less frequently described in the literature.
Life Cycle of E granulosus Fig. 1
Hydatid disease is a common clinical pathology in many parts of the world.
There are two clinical forms of this disease: cystic hydatidosis caused by Echinococcus granulosus and alveolar hydatidosis caused by Echinococcus multilocularis.
The main species pathogenic for humans in Mediterranean and Southern European countries is Echinococcus granulosus.
In the normal life cycle of Echinococcus species,
adult tapeworms (3-6 mm long) inhabit the small intestine of carnivorous definitive hosts,
such as dogs or wolves,
and echinococcal cyst stages occur in herbivorous intermediate hosts,
such as sheep,
cattle,
and goats.
The adult worm of the parasite lives in the proximal small bowel of the definitive host,
attached by hooklets to the mucosa.
The tapeworm eggs are passed in the feces of an infected dog and may subsequently be ingested by grazing sheep; they hatch into embryos in the intestine,
penetrate the intestinal lining,
and are then picked up and carried by blood throughout the body to major filtering organs (mainly liver and/or lungs).
After the developing embryos localize in a specific organ or site,
they transform and develop into larval echinococcal cysts in which numerous tiny tapeworm heads (called protoscoleces) are produced via asexual reproduction.
These protoscoleces are infective to dogs that may ingest viscera containing echinococcal cysts (with protoscoleces inside),
mainly because of the habit in endemic countries of feeding dogs viscera of home-slaughtered sheep or other livestock.
Protoscoleces attach to the dog's intestinal lining and,
in approximately 40-50 days,
grow and develop into mature adult tapeworms,
once again capable of producing infective eggs to be passed to the outside environment with the dog's feces.
Because humans play the same role of intermediate hosts in the tapeworm life cycle as sheep,
humans also become infected by ingesting tapeworm eggs passed from an infected carnivore.
This occurs most frequently when individuals handle or contact infected dogs or other infected carnivores or inadvertently ingest food or drink contaminated with fecal material containing tapeworm eggs.
Hydatid cyst structure Fig. 2
The larva of E.
granulosus in intermediate host is known as hydatid cyst.
The cysts are more or less spherical in shape.
It ranges from 1 to 20 cm,
even reaching 40 cm in diameter,
consisted of cyst wall and inclusion content (hydatid fluid,
hydatid sand). Cyst fluid is clear or pale yellow and antigenic.
The cyst wall consists of three layers:
(a) the outer pericyst,
composed of modified host cells that form a dense and fibrous protective zone;
(b),
the middle laminated layer is a cream-white,
1-4mm thick,
noncellular structure which allows the passage of nutrients;
(c) The inner germinal layer is a 22-25μm thick structure,
possesses germinal cells from which masses of brood capsules and protoscoleces are differentiate and grow into the cyst cavity.
The middle laminated membrane and the germinal layer are known as the endocys.
The brood capsule wall is similar with germinal layer.
Each brood capsule may contains 1 to 70 protoscoleces.
Sometime,
the brood capsule forms external laminated layer to form similar structure with that of maternal cyst,
called as daughter cyst,
that at gross examination resemble a bunch of grapes.
The daughter cysts are replicas in miniature of the complete hydatid cyst; these in turn produce brood capsules,
which may contain protoscoleces.
In this way,
granddaughter cyst may be seen.
The free protoscoleces,
brood capsules,
daughter cysts,
and amorphous material found in the cyst are known as “hydatid sand”.
Cyst fluid is clear or pale yellow and antigenic.
Diagnosis
In humans,
most cases are infected during childhood period,
but they do not present with symptoms until adulthood.
Hydatid disease can involve almost every organ of the body; the liver (75%) and lungs (15%) being the predominant locations.
The diagnostic certainty is given us by the radiological and serological studies.
Analytical parameters can be altered while not specific; eosinophilia is only present in less than 15%of cases.
Plain radiography detects only calcified cysts in 20-30% of cases,
so it is not technique of choice.
MRI,
CT and ultrasound show the cyst and its features.
Ultrasound is commonly the first technique used as is the easiest and cheapest to perform,
with a sensitivity of 90-95%.
Then it is recommended to perform a CT or MRI scan that with a sensitivity of 95-100%,
will give you the number,
size and location of the cysts,
including extrahepatic cysts detected more accurately than ultrasound.
More specific findings include the presence of "daughter vesicles" within the larger cysts and cyst wall calcification.
They are also considered better than ultrasound for detecting cyst complications such as infections and intrabiliary spread.
Regarding the serological analysis,
the test more sensitive,
detecting IgG ELISA has a sensitivity near 90%; however the specificity of the test ranges from 98-100%.
Definitive diagnosis could be made by examining aspirated fluids looking scoleces hooks; however,
the diagnosis is not recommended as suction conventional method because of the risk of liquid leakage by the spread of the infection or the possible occurrence of anaphylactic reactions.