Page 112 - JSOM Fall 2025

An Ongoing Series

Hydatid Disease

Jason Jarvis, MSc*

ABSTRACT
Hydatid disease is a zoonosis in which humans become the acci- and can settle into virtually any part of the body (most of-
dental host to the larval stage of Echinococcus granulosus, also ten within the lungs) and begin formation of the hydatid cyst.
known as the dog tapeworm. Occurring worldwide, the natural Within the sanctuary of the slowly developing hydatid cyst
lifecycle of E. granulosus tapeworms consists of a canid definitive grow a multitude of Echinococcus larvae, known as protosco-
host for the adult tapeworm stage, and an herbivorous animal lices. Subsequent ingestion of the hydatid cyst by a dog (the
(sheep, in most cases) intermediate host for the tissue- invasive definitive host) completes the lifecycle, as the protoscolices
and cyst-forming larval stage. Dogs and other canids harbor- mature into adult tapeworms within the canine gastrointesti-
ing gut-resident tapeworms readily transmit the tapeworm eggs nal tract. After 5–7 weeks of ingestion of even a single protos-
(and subsequent formation of larval cysts) to humans in close colex, the mature hermaphroditic tapeworm will begin passing
contact with them. It is this author’s experience that adoption its own eggs into the environment. Tapeworms within the dog
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of indigenous dogs as “camp mascots” is a common tradition in may survive for up to 2–3 years, and dogs may be reinfected.
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the military deployed setting. This practice puts troops in dan- As is typical in two-host parasitic lifecycles (such as malaria
ger of acquiring hydatid disease. The U.S. Veterans Health Ad- and schistosomiasis), the definitive host is the host in which
ministration 2025 report on echinococcosis in Veterans states sexual replication takes place, while the intermediate host is
there were 1,059 diagnoses of the disease in their system from the one in which asexual replication takes place. 2,4
2000 to 2024. In addition, deployed healthcare practitioners
should include hydatid disease in the differential diagnosis of Humans become accidental intermediate hosts following in-
host-country nationals presenting with space- occupying lesions gestion of Echinococcus tapeworm eggs. Ingestion typically
and/or anaphylaxis of uncertain etiology. occurs as a result of petting dogs upon whose fur the excreted
eggs are attached, followed by the touching of food or put-
Keywords: parasite; hydatid disease; hydatidosis; cystic ting fingers into one’s mouth. Dog fur may become contam-
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echinococcosis; Echinococcus granulosus; dog tapeworm; inated with the eggs as a result of the dog rolling around on
juvenile tapeworm; tapeworm larva; cestode; Platyhelminthes; the ground, or in the course of the dog grooming itself or other
flatworm; helminth dogs. Unwashed vegetables or water contaminated with Echi-
nococcus eggs may also serve as routes of infection. 2

The juvenile stages of different Echinococcus species tape-
Background and Lifecycle
worms cause different disease manifestations, as follows:
Adult Echinococcus genus tapeworms inhabit the small intes- E. granulosus (hydatid disease or cystic echinococcosis),
tine of infected dogs. The distal egg-bearing segment (gravid E. multilocularis (alveolar echinococcosis), E. vogeli (polycys-
proglottid) of the tapeworm breaks free from the proximal tic neotropical echinococcosis), and E. oligarthrus (unicystic
segments of the worm upon maturation and is passed into the neotropical echinococcosis). The vast majority (95%) of hu-
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environment with the feces. The eggs are hardy and can sur- man echinococcosis is caused by E. granulosus. Echinococco-
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vive for months in the outdoors. To continue the tapeworm’s sis in humans caused by this species is identified by the ICD-10
lifecycle, pasture-grazing animals must ingest the deposited code B67. Other Echinococcus species tapeworms use differ-
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eggs. While sheep are the most common intermediate hosts ent definitive and secondary hosts, such as with the boreal
for Echinococcus granulosus, other intermediate hosts include E. multilocularis (foxes and mice). 2
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goats, swine, cattle, camels and yaks. Within the gastrointes-
tinal tract of the grazing animal, the eggs hatch into onco- Epidemiology
spheres, which are then carried into the hepatic portal system.
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The majority of the oncospheres become lodged in the liver , Roughly 1 million people worldwide are infected with hydatid
where the formation of the hydatid cyst occurs. Roughly 25% disease, the disease being endemic to all continents except
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of the oncospheres continue hematageneously beyond the liver, Antarctica. The 6th edition of the Parasitic Diseases textbook
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*Correspondence to jjarvis18d@gmail.com
SSG Jason Jarvis is a teaching faculty member at the College of Remote and Offshore Medicine, Birzebbuza, Malta and a PhD student in Health
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Professions Education at Widener University, Chester, PA.
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