Source: <http://ryoko.biosci.ohio-state.edu/~parasite/echinococcus.html>
Echinococcosis is a parasitic cestode of the phylum platyhelminthes, commonly referred to as tapeworms.  There are three forms that occur in humans: cystic (E. granulosus), alveolar (E. multilocularis), and polycystic (E. vogeli and E. oligarthrus) which is rare. The cystic strain is the most common and is known as Hydatid Disease, though all strains have similar life cycles and clinical presentations but affect different organs.


History of Discovery
The presence of hydatid cysts in both humans and animals was well known in ancient times, and was described in the works of Hippocrates in the 4th century AD and Arataeus and Galen in the 1st and 2nd centuries AD respectively.  However, it was not until the 17th century that the parasitic natures of these cysts were recognized by Francisco Redi.  In 1766 German Pierre Simon Pallas formulated the hypothesis that hydatid cysts were larval stages of tapeworms, and then later in 1853 Carl von Siebold demonstrated that cysts from sheep lead to adult tapeworms in dogs; however it was another decade later until Bernhard Naunyn recognized that the adult tapeworms directly develop from hydatid cysts.  Full understanding of the clinical features of this disease came about in the late 1800’s, which progressed to the development of immunological diagnostic tests by the early 1900’s.  Surgical techniques to remove cysts were first attempted in the 1600’s, which has proven to be an effective treatment that and has evolved with medical technology.

Epidemiology

Source: TMCR <http://tmcr.usuhs.mil/tmcr/chapter3/geographic.htm>

Global distribution of E. granulosa (black) and E. multilocularis (x

Hydatid disease unlike most parasitic diseases is more prevalent in the northern hemisphere.  Human infection is most common in sheep-raising countries such as Australia and New Zealand, throughout England and Europe, the Middle East, Russia, Northern China, and Japan.  In the Americas the disease is especially prevalent in the Southern Cone countries such as Argentina, Uruguay and Chile, and also occurs in Alaska and Canada.

Alveolar hydatid disease (E. multilocularis) is a less widespread disease that occurs in countries with larger distributions of the reservoir hosts dogs, wolves, foxes and cats.  The disease is mainly prevalent in northern and central Europe, Alaska, and parts of Canada.

The spread of Echinococcus infection depends on the presence of dogs and foxes as definitive hosts, susceptible intermediate hosts such as sheep, goats and swine, an environment that enables egg survival, and human populations living in close contact with domesticated animals.

Clinical Presentation

Hydatid disease is caused by the larval stage of the parasite and can range from an asymptomatic infection to serious disease that can be fatal.  Severity depends on the location and size of cysts. Symptoms occur when growing hydatid cysts cause pressure necrosis of the surrounding tissues:
 
Organ affected
Percentage of Cases
Symptoms
Liver
75%
abdominal pain, hepatic mass, bile duct obstruction
Lungs
22%
chest pain, cough, hemoptysis
abdominal/pelvic cavity, spleen, kidney, heart
6%
k

Infected liver with hydatid cysts

Source: Mich Dept. Natural Resources <http://www.michigan.gov/dnr/0,1607,7-153-10370_12150_12220-117400--,00.html>

 

Leakage of fluid from the cyst (hyatid sand) causes eosinophilia.  Rupture of a hydatid cysts as a consequence of trauma or surgery is very dangerous because it can cause anaphylactic shock, possible allergic reaction, and risk spreading hydatid sand which can give rise to new cysts throughout the body.
 

Source: CDC <http://www.dpd.cdc.gov/dpdx/HTML/ImageLibrary/Echinococcosis_il.htm>

Hydatid Sand


Morphology

The adult parasite is a small tapeworm that is 3- 6mm long, and lives in the small intestine of canines.  The segmented worm contains a scolex with suckers and hooks that enable attachment to the mucosal wall, since tapeworms do not have a digestive tract.  A short neck connects the head to three proglottids, the body segment of the worm which contains the eggs to be excreted in the feces.

Source: TMCR <http://tmcr.usuhs.mil/tmcr/chapter3/epidemiology2.htm>

 

Life Cycle
Source: CDC <http://www.dpd.cdc.gov/dpdx/html/Echinococcosis.htm>

(1) adult worms in bowels of definitive host. (2) eggs passed in feces, ingested by humans or intermediate host. (3) onchosphere penetrates intestinal wall, carried via blood vessels to lodge in organs. (4) hyatid cysts develop in liver, lungs, brain, heart. (5) protoscolices (hydatid sand) ingested by definitive host. (6) attach to small intestine and grow to adult worm.

Transmission
  • Human accidental ingestion of feces containing eggs
  • Dogs ingest offal (farm feed of ground organs containing cysts)
Reservoirs
  • E. granulosa: dogs, coyotes, wolves, sheep, pigs, deer, wild herbivores
  • E. multiocularis: foxes, dogs, wolves
Vector
  • Bowflies, birds, arthropods as mechanical vectors of eggs
Incubation Period
  • E. granulosa: variable – months to years (depends on number of cysts and growth rate)
  • E. multiocularis: 5-15 years


Diagnostic Methods

Radiographic examination:

calcification of the cyst shows a sharp outline

 

 

 

Source: TMCR <http://tmcr.usuhs.mil/tmcr/chapter3/imaging6.htm>

Ultrasound:

multidimensional image of cyst in organs and soft tissue, identify hydatid sand and calcifications

 

 

Source: TMCR <http://tmcr.usuhs.mil/tmcr/chapter3/imaging8.htm>

CT:

identify hyatid cysts to assess effectiveness of therapy - rim calcification demonstrates parasite degeneration

 

Source: TMCR <http://tmcr.usuhs.mil/tmcr/chapter3/imaging9.htm>

 

Serologic Testing: detect antibody response, depends on location and progression of cyst

  • IHA (indirect hemagglutination test)
  • ELISA (enzyme-linked emmunosorbent assay)

Treatment
Surgical Removal of Hydatid Cysts
  • 90% effective but can be risky depending on location, size, and advancement of cyst
  • may need chemotherapy to prevent recurrance
Chemotherapy
  • Albendazole is preferred treatment because it penetrates into hyatid cysts.  Dosage: 10mg/kg body weight or 400mg 2x daily for 4 weeks, repeat cycles as necessary (up to 12)
  • Mebendazole Dosage: 40mg/kg body weight 3x daily for 3-6 months
PAIR Treatment
  • Puncture, aspiration, injection, respiration
  • Inject protoscolicidal substances into the cyst

 

Prevention and Control Strategies
Source: AMREF <http://www.amref.org/index.asp?PageID=63&ProjectID=87&PiaID=3&CountryID=1>

Human infection of hydatid disease can be prevented if people are made aware of the risks and the proper safety precautions are taken.  It is crucial to intercept the continuation of life cycle on farms and slaughterhouses by abstaining from feeding raw offal to dogs and enforcing thorough meat inspection procedures.  In addition, exercising good hygiene is essential, which includes hand washing before meals, thoroughly cooking food, and wearing protective clothing and gloves when necessary to avoid contact with infected fecal material.

Current control strategies are aimed at improving primary health care of the population, which includes increasing access to diagnostic treatment and drug therapy, education, access to safe water, and improving sanitation and meat inspection.

More specific programs are aimed at eliminating E. granulosa directly by de-worming dogs to kill tapeworms and controlling the stray dog population to prevent infection.  Vaccines for sheep are being developed in Australia and could provide a breakthrough intervention strategy for the future.



Links

CDC:  http://www.dpd.cdc.gov/dpdx/html/Echinococcosis.htm
WHO:  http://www.who.int/zoonoses/diseases/echinococcosis/en/
Hydatid Disease on Wikipedia: http://en.wikipedia.org/wiki/Hydatid_disease
OSU Parasitological Resources:http://ryoko.biosci.ohio-state.edu/~parasite/home.html


Contact Information

Stephanie Connolly, Junior, stephcon@stanford.edu
Stanford University
Parasites & Pestilence: Infectious Public Health Challenges
Prof. D. Scott Smith, ssmith@stanford.edu


References

African Medical and Research Foundation.  Hydatid Disease Control.  2006.  <http://www.amref.org/index.asp?PageID=63&ProjectID=87&PiaID=3&CountryID=1>

CDC.  Parasites and Health: Echinococcosis.  2004.  <http://www.dpd.cdc.gov/dpdx/html/Echinococcosis.htm>

Cox, F.E.G.  “History of Human Parasitology”. Clin Microbiol Rev. 2002 October; 15(4): 595–612.

Craig, P.S., Rogan, M.T., Campos-Ponce, M. “Echinococcosis: disease, detection, and transmission”.  Parasitology.  2003; 127: S5-S20.

Eckert, J., Gemmell, M.A., Meslin, F-X., and Pawlowski, Z.S.  “WHO/OIE Manual on Echinococcosis in Humans and Animals: a Public Health Problem of Global Concern”.  OIE, Paris, France, Jan 2002.

Grove, D. I.  “A History of Human Helminthology”. CAB International, Wallingford, United Kingdom, 1990; (12): 319-347.

“Hydatid Disease and Public Health”.  British Medical Journal. Nov 23, 1929; i:970.

John, David T., Petri Jr., William A.  “Markell and Voge’s Medical Parasitology, 9th Edition”.  St. Louis: Elsevier, 2006.

Parasties and Parasitological Resources.  OSU.  Echinococcus granulosus.  <http://ryoko.biosci.ohio-state.edu/~parasite/echinococcus.html>

“Some Problems of Hydatid Disease”.  British Medical Journal. Jan 24, 1931; ii:146.

Tropical Medicine Central Resource.  Hydatid Disease (Echinococcosis).  2005.  <http://tmcr.usuhs.mil/tmcr/chapter3/intro.htm>

WHO.  Cystic Echinococcosis and Multilocular Echinococcosis.  2006.  <http://www.who.int/zoonoses/diseases/echinococcosis/en/>