echinococcosis (hydatidosis, or hydatid disease) is caused by the larval
stages of cestodes (tapeworms) of the genus Echinococcus.
Echinococcus granulosus causes cystic echinococcosis, the form most
frequently encountered; E. multilocularis causes alveolar
echinococcosis; E. vogeli causes polycystic echinococcosis; and
E. oligarthrus is an extremely rare cause of human echinococcosis.
Echinococcus granulosus (3 to 6 mm long)
in the small bowel of the definitive hosts, dogs or other canids.
Gravid proglottids release eggs
are passed in the feces. After ingestion by a suitable intermediate
host (under natural conditions: sheep, goat, swine, cattle, horses,
camel), the egg hatches in the small bowel and releases an oncosphere
penetrates the intestinal wall and migrates through the circulatory
system into various organs, especially the liver and lungs. In these
organs, the oncosphere develops into a cyst
that enlarges gradually, producing protoscolices and daughter cysts that
fill the cyst interior. The definitive host becomes infected by
ingesting the cyst-containing organs of the infected intermediate host.
After ingestion, the protoscolices
attach to the intestinal mucosa
and develop into adult stages
32 to 80 days. The same life cycle occurs with E. multilocularis
(1.2 to 3.7 mm), with the following differences: the definitive hosts
are foxes, and to a lesser extent dogs, cats, coyotes and wolves; the
intermediate host are small rodents; and larval growth (in the liver)
remains indefinitely in the proliferative stage, resulting in invasion
of the surrounding tissues. With E. vogeli (up to 5.6 mm long),
the definitive hosts are bush dogs and dogs; the intermediate hosts are
rodents; and the larval stage (in the liver, lungs and other organs)
develops both externally and internally, resulting in multiple
vesicles. E. oligarthrus (up to 2.9 mm long) has a life cycle
that involves wild felids as definitive hosts and rodents as
intermediate hosts. Humans become infected by ingesting eggs
with resulting release of oncospheres
the intestine and the development of cysts
in various organs.
occurs practically worldwide, and more frequently in rural, grazing
areas where dogs ingest organs from infected animals. E.
multilocularis occurs in the northern hemisphere, including central
Europe and the northern parts of Europe, Asia, and North America. E.
vogeli and E. oligarthrus occur in Central and South America.
infections remain silent for years before the enlarging cysts cause
symptoms in the affected organs. Hepatic involvement can result in
abdominal pain, a mass in the hepatic area, and biliary duct
obstruction. Pulmonary involvement can produce chest pain, cough, and
hemoptysis. Rupture of the cysts can produce fever, urticaria,
eosinophilia, and anaphylactic shock, as well as cyst dissemination. In
addition to the liver and lungs, other organs (brain, bone, heart) can
also be involved, with resulting symptoms. Echinococcus
multilocularis affects the liver as a slow growing, destructive
tumor, with abdominal pain, biliary obstruction, and occasionally
metastatic lesions into the lungs and brain. Echinococcus vogeli
affects mainly the liver, where it acts as a slow growing tumor;
secondary cystic development is common.
The diagnosis of
echinococcosis relies mainly on findings by ultrasonography and/or other
imaging techniques supported by positive serologic tests. In
seronegative patients with hepatic image findings compatible with
echinococcosis, ultrasound guided fine needle biopsy may be useful for
confirmation of diagnosis; during such procedures precautions must be
taken to control allergic reactions or prevent secondary recurrence in
the event of leakage of hydatid fluid or protoscolices.
Surgery is the
most common form of treatment for echinococcosis, although removal of
the parasite mass is not usually 100% effective. After surgery,
medication may be necessary to keep the cyst from recurring. The drug
of choice for treatment echinococcosis is albendazole (Echinococcus
granulosus). Some reports have suggested the use of albendazole or
mebendazole for Echinococcus multilocularis infections.