Echinococcus sp.
Classification
Phylum: Platyhelminthes
Class: Eucestoda Cestoda
Order: Cyclophyllidea
Genus/Species : Echinococcus granulosa/ Echinococcus multilocularis
Common Name: Tapeworm hydatid disease. E. granulosa is also known as the dog tapeworm.
Morphology
The adult Echinococcus sp. measures between 3 and 9 mm in length and is one of the smallest of the
tapeworms. It’s body consists of a scolex and 3 proglottids- immature, mature, and gravid. The scolex
has 4 suckers and a distinct rostellum with a double row of 30-36 hooks. The Echinococcus sp. egg is
round and about 30-35 micrometers across, making it one of the smaller eggs we have looked at. The
egg has a striated shell and is indistinguishable from the Taenia sp. egg.
Figure 1: Adult Echinococcus granulosa.
Figure 2: E. granulosa scolex.
Figure 4: E. granulosa eggs.
Figure 3: Hyatid cyst with protoscolices.
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Lifecycle
Echinococcus granulosa
The definitive host for this organism
is usually a dog, but can be another
canid. Gravid proglottids release eggs
that are passed in the feces. These eggs
can be ingested by almost any large
vertebrate mammal. The most common
intermediate hosts are sheep, cattle,
horses, and humans. The most
common way for humans to ingest the
eggs occurs through allowing a domestic
dog to lick its owner’s face after having
licked its anal area where expelled eggs
could be lurking.
Once ingested, the egg hatches in the
intestine and penetrates the intestinal
wall, migrating through the circulatory
system until reaching its final site where
it develops into a hyatid cyst. The most
common place for this to occur is the
liver,
but the cyst can also develop in the lungs
or brain. Once in place, the cyst
produces protoscolices by asexual
reproduction, each of which has the
capability to grow into an adult
tapeworm. It is not unusual for cysts to
reach golf ball size and in rare cases
have even grown to the size of
basketballs. Dogs become infected by
Figure 5: Echinococcus granulosa life cycle.
ingesting organs infected withthe cysts.
Once ingested, the protoscolices
evaginate and attach to the intestinal wall where they develop into the adult tapeworm.
Echinococcus multilocularis
The life cycle for E. multilocularis is the same as for E. granulosa except that the definitive host is
usually a fox rather than a dog, but can be a variety of canids. Within the definitive hosts, larvae may
proliferate indefinitely, resulting in invasion of the surrounding tissues. In addition, the intermediate
hosts are small rodents rather than large mammals.
Geographic Distribution
Echinococcus sp. has a worldwide distribution, but is most prevalent in sheep-raising areas and in
rural, grazing areas where dogs ingest organs from infected animals. Echinoccocus granulosus is
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common in North America, mostly in Alaska and northern Canada, in Europe, Asia, Middle East,
Japan and parts of Africa, South America and Australia. Echinococcus multilocularis is most
prevalent in the far north in Europe, Asia and North America. High-risk individuals include hunters
and fur traders.
Figure 6: Geographic distribution of Echinococcus sp.
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Pathology and Symptoms
Echinococcus granulosus
Larvae - In domesticated animals clinical signs appear to be uncommon, whilst in man they will vary
in their seriousness depending on where in the body the hydatid develops, and how large it grows.
Sometimes the infection is asymptomatic, the only time an infection is noticed is the presence of
calcified cysts on autopsy after death due to an unrelated cause. In other instances, in humans, the
liver and the lungs are common sites for the cyst to form. The cyst is a fluid-filled sac which can
become quite large (football-sized), and contains many developing scolices. The immature scolices
are called "hydatid sand." The cyst can rupture from physical activity causing contents of hydatid to
be released into the circulatory system. In this case cysts can form throughout the body. In addition
the hydatid cyst fluid is highly allergenic and cyst rupture may result in anaphylactic shock and rapid
death.
Adults – In the canine hosts, the tapeworm isn’t usually pathogenic but sometimes large infections can
cause intestinal inflammation.
Echinococcus multilocularis
Larvae - The multilocular cyst is highly pathogenic due to its fast growth rate and invasive nature, in
extreme cases completely replacing liver tissue. As the cyst lacks the tough laminated layer seen in E.
granulosus, and by its nature grows by budding, metastases of grow may also be seen, colonising other
body organs. Due this aspect of the parasite it may also be transferred by transplantation. This parasite
must be considered one of the most pathogenic of the parasitic helminths.
Adults - As with E. granulolsus the adult tapeworm is usually non-pathogenic to its canine hosts.
Diagnosis
It is diagnosed by the presence of the hyatid cysts. Tests which can find these cysts include chest Xray, thoracic CT scan or ultrasound, an abdominal X-ray, an abdominal CT scan or ultrasound test for
antibodies to echinococcus, liver function tests may also be elevated.
Treatment
Surgery is the most common form of treatment
especially if the cysts are in troublesome
locations. This can be a complicated type of
surgery and removal of the parasitic mass is not
always 100% effective. Drugs may be needed
after surgery to prevent the cyst from coming
back.
For the treatment of Echinococcus granulosus the
most common drug used is albendazole.
Echinococcus multilocularis is usually treated
with albendazole or mebendazole. Praziquantel
may be used in combination with the other two.
Figure 7: Removal of a hyatid cyst.
A newer, although somewhat experimental treatment includes an extended course of benzimidazole
therapy and puncture, aspiration, injection, and re-aspiration of the cyst. This procedure is now
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somewhat widely used and compares favorably with conventional surgery in terms of rates of cure,
complications, recurrence and recovery time.
Public Health/Strategies for Eradication
Complete eradication of Echinococcus sp. is difficult because of its ability to infect so many different
vertebrates. However, Echinococcus sp. can be controlled through preventive measures that break the
cycle between the definitive and the intermediate host. This includes dosing dogs, inspecting meat and
educating the public on the risk to humans. Public education should include the importance of not
feeding uncooked food to dogs as well as providing an understanding of the life cycle of Echinococcus
sp.
References
Cam.University Schistosome Research Group February 6, 2005.
http://www.path.cam.ac.uk/~schisto/Tapes/Echino.html#Emult
Caramello, Pietro. May 2003. “Liver and Biliary Tree Parasites: Echinococcus multilocularis”. Atlas
of Medical Parasitology.
<http://www.cdfound.to.it/html/echi_mul.htm#geographic>
“Graphic Images of Parasites.” Ohio State University. February 5, 2005.
<http://www.biosci.ohio-state.edu/~parasite/echinococcus.html>
“Laboratory Identification of Parasites of Public Health Concern.” Center for Disease
Control. February 5, 2005.
<http://www.dpd.cdc.gov/dpdx/HTML/ImageLibrary/Echinococcosis_il.asp?body=AF/Echinococcosis/body_Echinococcosis_il2.htm>
Levy, Daniel. 19 January 2005. “Echinococcus”. Medline Plus.
<http://www.nlm.nih.gov/medlineplus/ency/article/000676.htm
Olsen, Christopher W., 30 December 2004. “Larva Migrans and Echinococcus
Hydatidosis” Zoonotic Diseases Tutorial: School of Veterinary Medicine, University of
Wisconsin-Madison. <http://www.vetmed.wisc.edu/pbs/zoonoses/larvamigrans/echino.html>
Permin, A. and Hansen, J.W. “Review of Echinococcus/hydatidosis: a zoonotic parasitic disease”
FAO Animal Production and Health Paper.
<http://www.fao.org/ag/againfo/resources/documents/WAR/war/T1300B/
t1300b0m.htm>
Sadjjadi, SM. Dr Sadjjadi's Online Atlas of Parasitology. Shiraz Medical School.
February 5, 2005. <medschool.sums.ac.ir/.../ Cestoda/page_01.htm>
University of Pennsylvania 2004. February 6, 2005.
<http://cal.vet.upenn.edu/dxendopar/parasitepages/cestodes/e_granulosus.html#diagnostic>
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Upton, SJ. Animal Parasitology. Kansas State University. February 5, 2005
<www.ksu.edu/.../ 625tutorials/Tapeworm03.html>
Kay Magnuson
Kevin Louder
Erin Hoffman
Spring 2005
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