Epidemiology
Hydatidosis is a worldwide zoonosis caused by the larval stage of Echinococcus tapeworm,
there being four species known to be pathogenic in humans.
The two main types of Echinococcus infections are E.
granulosis,
which is the most common one,
and E.
multilocularis,
that is more invasive,
mimicking a malignancy.
Hydatid Disease (HD) is still endemic in Middle East,
Australia,
New Zealand,
Africa,
South America and Mediterranean Europe,
including Portugal.
[1-4]
Life cycle of Echinococcus granulosis
Dogs are usually the definitive hosts,
whereas ruminants like sheep are intermediate hosts.
Dogs’ faeces with the eggs of the parasite may contaminate food or water that might incidentally be ingested by humans (intermediate hosts) that become secondarily infected.
Therefore,
after the ingestion of the eggs’ capsule,
the larvae (oncosphere) penetrate the intestinal mucosa and migrate to the visceral organs where they form cysts.
The cycle is completed when the definitive host eats the viscera of the intermediate host.
[1-4]
Hydatid Cysts
Hydatid Cysts (HC) consists of three layered structures.
The outer pericyst is composed of modified host cells that form a peripheral protective layer of reactive fibrous tissue,
which is the first layer becoming calcified during the natural course of the cyst,
before complete calcification occurs.
Note that only complete calcified lesions might be assumed to be death.
The middle laminated membrane,
or ectocyst,
is acellular and permits the passage of nutrients,
but is impervious to bacteria.
Its disruption occurs easily and predisposes to infection.
The inner germinative layer is where the larval stage of the parasite (scolices) is produced.
The intermediate and the germinative layers together form the endocyst,
the true wall of the cyst.
Daughter cysts,
formed from rests of the germinative layer and being initially attached to it,
contain protoscolices.
Hydatid fluid is a pale,
neutral and antigenic fluid,
that may contains hydatid sand settled in the dependent part of the cyst,
which is a sediment with freed scolices,
released from ruptured vesicles.
HC’ growth rate depends on host tissue resistance,
often reaching 1 cm during the first six months and ranging from 2 to 3 cm annually,
thereafter.
[1-4]
Human HD
Any part of human body may be involved by haematogenous dissemination,
the liver being the most affected organ,
involved in 75% of cases,
as it acts as the first line of defence once the parasite reaches the portal venous system after having passed through the intestinal wall.
The second organ involved the most is the lung,
in 15%,
the other different locations occurring in 10%,
including the peritoneum,
the spleen,
the kidneys,
the bone and the brain,
in decreasing order.
[1-3,
5-7]
The symptoms of hydatidosis are usually secondary to cyst enlargement with compression of adjacent structures,
resulting in abdominal pain or palpable masses.
Fever,
pruritus and anaphylaxis might be respectively related to complications as infection,
bile duct compression or communication,
and rupture.
[8]
Laboratory findings may be eosinophilia and positive serologic tests (enzyme-linked immunosorbent assay) for Echinococcus (> 0,9 U/mL),
which higher estimated sensitivity and specificity was about 90% [10],
in contrast to 25% reported in the previous ones [5].
Eventually,
the analysis of cystic fluid obtained by percutaneous needle aspiration can be helpful if not contraindicated [11].
As the clinical presentation alone is nonspecific,
the diagnosis predominantly relies on serologic,
imaging and pathological findings,
the clinical and epidemiological data leading to the clinical suspicion.
Besides their diagnostic role,
imaging procedures are also essential in evaluation of the extent of the disease and in the surgical planning,
when indicated.