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Echinococcosis or hydatidosis, in humans is a zoonotic infection
caused by larval stages (metacestodes) of cestode species of the genus
Echinococcus.
Echinococcus granulosus causes cystic hydatid
disease.
E.
multilocularis causes alveolar hydatid disease.
E. vogeli or
E. oligarthrus is
the cause of polycystic hydatid disease , a disease with
features similar to those of alveolar hydatid disease.
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;
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;
Image Link3 (lung)
Areas of
high incidence:
East Africa
include East Africa, Spain, Greece, the Middle East, Iran, western
Australia, Chile, Argentina, and Uruguay.
E.
multilocularis causes alveolar hydatid disease that is seen mostly in
Alaska, Canada, the Soviet Union, and central Europe.
Environmental factors causing echinococcosis (Echinococcus
multilocularis):
Transmission
of the fox tapeworm Echinococcus multilocularis, the causative agent
of human alveolar echinococcosis, is known to depend on various
environmental factors which are subject to human influence.
Epidemiological data suggest that in most endemic regions
anthropogenic landscape changes (e.g. deforestation and agricultural
practices) have led to more favourable conditions for the parasite's
animal hosts, especially arvicolid rodents, thereby increasing the
risk for parasite transmission and human disease. Examples are
the conversion of forests or crop fields into meadows and pastures in
Europe, China and North America, and overgrazing of natural grassland
in central Asia.
Other
anthropogenic factors include interference with host population
densities by wildlife disease control, changing hunting pressure and
provision of new habitats, e.g. in urban areas.
Domestic dogs
may, under certain conditions, get involved in the otherwise largely
wildlife-based transmission, and thereby greatly increase the
infection pressure to humans.
The
introduction of neozootic host species may increase transmission, or
even initiate the parasite's life-cycle in previously non-endemic
regions.
Lastly, the
parasite itself may be accidentally introduced into non-endemic areas,
if suitable host populations are present.
Life Cycle:
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The definitive
hosts for the adult cestode are carnivores, commonly the domestic dog
for E. granulosus and the fox for E. multilocularis .
The adult
tapeworms which may number several thousands in the heavily infected
dog intestine, shed both eggs and gravid proglotids, which can be
found in the host's stool .
Human and
natural intermediate hosts become infected when they swallow these
immediately infective eggs.
In the
duodenum the larvae or onchospheres are freed and using their hooklets,
find their way through the intestinal mucosa into the lumen of blood
vessels.
They are then
carried by the blood until they lodge in capillaries at almost any
site.
In 60% of
cases, the larvae are retained in the sinusoids of the liver.
The remainder
pass through the hepatic circulation, and 20% are retained in the
lung, the others gain access to the systemic circulation.
The embryos of
E. granulosus that survive develop into hydatid cysts containing
numerous scolices provided with hooklets. These scolices represent the
future head of the adult tapeworm.
The cysts have
an outer laminated, elastic layer and an inner germinal layer. They
enlarge gradually for several months until they attain a diameter of
10 - 20cm.
Abundant clear
fluid is contained within the cysts. The germinal layer develops
numerous papillae, which becomes pedunculated vesicles (brood
capsules) containing scolices.
Cysts of
echinococcosis occur more frequently in the liver or in the lung.
Approximately 70% of primary echinococcal cysts are found in the liver
and most of these are in the right lobe.
Clinical
presentation:
It has
been estimated that approximately 25% of people infected with
Echinococcus go through life without any symptoms referable to the
tapeworm.
Symptoms may
take a long time to develop for the disease progresses slowly. The
cysts may become secondarily infected, suppurate, and produce the
clinical picture of hepatic abscess.
Some of the
cysts may collapse and undergo fibrosis and in some cases there is
calcification.
The hydatid
fluid, when liberated into the circulation, gives rise to pronounced
eosinophilia. There may be allergic manifestations such as urticaria
and angioneurotic edema.
Diffuse
implantaion in the peritoneal or pleural cavities may develop after
rupture of subpleural and subperitoneal cysts.
The disease is
fatal in most untreated patients and in a significant percentage of
those treated with surgery.
Features:
In E.
multilocularis, the daughter cysts, which arise from the germinal
membrane by budding , develop on the outside of the original (mother)
cyst. This results in invasion of surrounding parenchyma by the new
scolices , which are not contained by the laminated cuticular
membrane.
The pattern of
growth resembles that of malignant neoplastic lesions.
Diagnosis of
the disease:
The diagnosis
of hepatic hydatid disease is suggested by the presence of an
abdominal mass detected by palpation and confirmed by ultrasonography,
liver scan or CT scans. The diagnosis must then be confirmed
serologically. The various methods available include complement
fixation, enzyme-linked immunosorbent assay (ELISA) or indirect
hemagglutination.
Immunodiagnosis:
Accurate
immunodiagnosis of the infection requires highly specific and
sensitive antigens to be used in immunodiagnostic assays. The choice
of an appropriate source of antigenic material is a crucial point in
the improvement of the diagnostic features of tests, and must be based
on the developmental stage of the parasite and the host. The most
common antigenic sources used for the immunodiagnosis of echinococcal
disease are hydatid cyst fluid, somatic extracts and excretory-secretory
products from protoscoleces or adults of E. granulosus. Hydatid cyst
fluid is the antigenic source of reference for immunodiagnosis of
human hydatidosis, which is mainly based on the detection of antigens
B and 5. Somatic extracts have been widely used in the serodiagnosis
for E. granulosus infection in dogs and ruminant intermediate hosts,
although in the last few years the detection of excretory-secretory
products of the worm in faeces (coproantigens) have become the most
reliable method for the detection of the parasite in the definitive
host.
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