59. Phylum Platyhelminthes. Class Cestoda. Echinococcus granulosus.

Echinococcus granulosus (Dog Taenia)

General Characteristics:

• Also called the dog taenia
• A very small tapeworm, measuring only 2 to 7 millimeters in length
• Adult worm consists of three segments or proglottids: an immature segment, a hermaphroditic (mature) segment, and a gravid (egg-filled) segment

Host Information:

• Definitive host: Dogs (and other canids)
• Intermediate hosts: Sheep, cattle, goats, pigs, and occasionally humans
• Adult worms live in the small intestine of dogs
• In intermediate hosts, the larval form appears as a hydatid cyst

Hydatid Cyst Details:

• Hydatid cysts are the larval stage of the worm and are large, fluid-filled cysts
• These cysts most commonly develop in the liver and lungs but can also form in other organs
• The cyst has two walls: an inner germinal layer and an outer protective acellular layer
• The cyst may be surrounded by a collagen capsule produced by the host
• Inside the cyst are protoscolices and daughter cysts, which can each form new worms
• Rupture of a cyst inside a host can result in the formation of secondary cysts

Life Cycle Summary:

1. Embryonated eggs are passed in the feces of an infected dog
2. These eggs are infectious upon release and serve as the diagnostic stage
3. Intermediate hosts ingest the eggs, often through contaminated food or close contact with dogs
4. The eggs hatch in the small intestine, releasing six-hooked larvae (oncospheres)
5. Oncospheres travel through the bloodstream to organs like the liver and lungs
6. There, they develop into hydatid cysts containing protoscolices
7. Dogs become infected when they eat the organs of intermediate hosts that contain these cysts
8. Once ingested, the protoscolices attach to the dog’s intestinal lining and mature into adult worms

Human Infection (Hydatid Disease):

• Humans become infected by accidentally ingesting the eggs
• The infection is often asymptomatic at first and may go unnoticed for years
• Symptoms develop slowly as cysts enlarge in organs
• Hydatid disease is especially prevalent in communities with close contact between humans, sheep, and dogs
• The disease is known as cystic echinococcosis

Echinococcus multilocularis (Brief Mention)

• Causes a condition known as alveolar echinococcosis
• Definitive hosts include foxes, coyotes, jackals, and sometimes dogs
• Intermediate hosts are typically rodents
• Rarely infects humans
• Cysts from this species do not form a single large cavity but instead grow into many interconnected chambers within the liver and surrounding tissue