The hydatidic disease is a zoonosis produced by the larval stage of the tapeworm Echinococcus granulosus.
There are other types of hydatid disease caused by E multilocularis,
E.
vogeli and E.
Oligarthrus but these are much less frequent.
It is a worldwide disease.
It`s considered an endemic disease in regions such as the Mediterranean countries,
Africa,
South America,
Australia and New Zealand.
Mainly affects rural areas.
Spain is one of the countries where the disease is endemic,
although the incidence of hydatid disease has decreased in recent years is still of the highest in the EU,
which means that the disease is far from being eradicated.
The life cycle of E granulosus requires two hosts to complete.
The definitive hos is usually the dog but can be any other carnivore. The adult tapeworm lives in them without causing damage.
The most common intermediate host is usually the sheep.
It eats grass contaminated with feces of parasitized dogs.
When the definitive host eats the viscera of the intermediate host is complete cycle.
Humans become intermediate hosts by contact with the definitive host usually the domestic dog or indirectly by ingestion of water or vegetables contaminated with feces from said host,
parasitized dogs or wild canids.
Once Liver acts as the first line of defense once the parasite passes through the intestinal wall into the portal circulation or lymphatic system.
This is the reason explains the liver is the organ most frequently affected.
The growth of hydatid cysts is usually slow and asymptomatic,
and clinical manifestations are caused by compression of the involved organs.
The disease may mimic benign or malignant primary tumors,
single or multiple metastases,
cysts,
abscesses,
empyema,
infarcts and other lesions (3).
Once in the human liver,
cysts grow to 1cm during the first 6 months and 2-3cm annually thereafter,
depending on host tissue resistance(1).
The hydatid cyst has three layers:
1.
- The outer: Pericyst,
composed of modified host cells (fibroblasts,
giant cells,
eosinophils,
...) which form a dense protective layer,
is the host response to parasite.
2.
- Middle layer: Membrane laminated (ectocyst) is a thin acellular layer of about 2 mm.
thickness and permits the passage of nutrients.
3.
- The inner layer: Germinal layer,
producing the scolices (the larval stage of the parasite) and the laminated membrane.
The middle and inner layers form true cyst wall,
called endocyst.
The cyst is filled with a clear liquid,
which contains sodium chloride,
protein,
glucose,
ions,
lipids and polysaccharides.
It may also contain scolices or parts of them (hooklets).
This fluid is antigenic and if the cyst ruptures into circulation this can cause anaphylactic reactions.When the vesicles rupture within the cyst,
scolices pass the cyst fluid forming hydatid sand.
The thickness of each layer depends on the tissue cyst which rests for example in the liver tends to be thicker than in muscle.
The diagnosis of hydatid disease is usually based on identification of a cystic image by imaging and detection of specific antibodies against the parasite by immunological tests.
But the negativity of serological not rule out the presence of a hydatid cyst.
Imaging tests used for diagnosis are: Chest radiography,
ultrasonography(US),
computed tomography(CT),
magnetic resonance imaging(MR),
and even urography.
The imaging method used will depend on the organ affected and the cyst stage.
Radiology and serology can usually establish the diagnosis but an unusual location or some atypical imaging findings may complicate the differential diagnosis.
Familiarity with atypical manifestations of hydatid disease can help make an early and accurate diagnosis.