hydatid disease or hydatidosis

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http://ryoko.biosci.ohio-state.edu/~parasite/echinococcus.html

 

 

 

Echinococcus granulosus

(hydatid disease or hydatidosis)

 

 

The life cycle of Echinococcus granulosus includes dogs (and other

canines) as the definitive host, and a variety of species of warm

blooded vertebrates (sheep, cattle, goats, and humans) as the

intermediate host. The adult worms are very small, usually consisting

of only three proglottids (total length = 3-6 mm), and they live in

the dog's small intestine. Eggs are liberated in the host's feces,

and when these eggs are ingested by the intermediate host they hatch

in the host's small intestine. The larvae in the eggs penetrate the

gut wall and enter the circulatory system. The larvae can be

distributed throughout the intermediate host's body (although most end

up in the liver) and grow into a stage called a hydatid cyst (view

diagram of the life cycle).

 

Hydatid cysts have the ability to grow quite large; cysts the size of

golf balls are not uncommon, and cysts the size of basketballs are

reported on rare occasions. The pathology associated with hydatid

disease in the intermediate host depends on the size of the cyst and

its location. One or two small cysts in the liver of a host might go

unnoticed for years. However, a single large cyst in the liver could

prove fatal. Hydatid disease is far more serious when the cysts are

found in other locations, particularly the brain (see below).

 

The infection is transmitted to the definitive host when the hydatid

cyst is eaten. As one might suspect, this species of parasite is more

common in areas of the world where dogs are used to herd sheep (view

geographic distribution). Under most circumstances humans are a " dead

end " in the life cycle, but hydatid disease in humans remains a

serious problem because the disease can cause such serious pathology.

 

The interior of a hydatid cyst is filled with " protoscolices "

(singular = protoscolex), each of which has the ability to grow into

an adult worm when ingested by a canine host. A small cyst might

contain hundreds of protoscolices; a large cyst might contain tens of

thousands! This tremendous reproductive potential poses a problem in

the intermediate host (particularly in humans). If a hydatid cyst

breaks open, each protoscolex could grow into a new hydatid cyst. How

might this happen? A sharp blow to the abdomen might rupture a cyst

in the liver. A number of cases have been reported in which cysts

have been damaged during routine surgery, allowing the cyst's contents

to leak into the patient's abdominal cavity.

 

A hydatid cyst (*) in the cranium of a child (the ruler at the top

measures 6 inches long, and the child's brain is below the hydatid

cyst). This infection resulted in the child's death.

 

A section of a hydatid cyst at low power. The cyst consists of a

thick outer layer (*), several thinner internal layers, and many

protoscolices. The protoscolices are often called " hydatid sand. "

 

Higher magnification of the protoscolices.

 

A single protoscolex. Note the " hooks " that will form the hooks

associated with the adult worm's armed rostellum.

 

An adult Echinococcus granulosus; note that the tapeworm's body

(strobila) consists of only three proglottids and measures only about

5 mm in length.

 

An egg of Echinococcus granulosus; these eggs are virtually

indistinguishable from other, closely related species of tapeworms

such as Taenia.

 

 

Echinococcus granulosus (the small, white objects) in the small

intestine of a dog. Although these tapeworms are quite small, a

single dog can be infected with many of them. (Original image from F.

Rochette, 1999, Dog Parasites and Their Control, Janssen Animal

Health, B.V.B.A. and used with permission.)