Flat worms: rabbit as an intermediate host
Esther van Praag, Ph.D.
Warning: this file contains pictures that may be distressing for some persons.
The tapeworm Taenia pisiformis is a common parasite of carnivores like dogs, foxes, and sometimes of cats. It is found all over the world, predominantly in rural regions.
The development of the parasite occurs in two stages:
1. An adult stage, as an adult parasite in the definitive host, the dog. The parasite lives in the small intestine (duodenum jejunum and ileum) of the dog and may reach a length of 2 meters (about 79 inches). Mature segments of the tapeworm (protoglottid) containing mature eggs are shed along with the feces.
2. An intermediate stage in an herbivorous host where the larval stage is found (mesacestoide). Rabbit or other lagomorph species may ingest the eggs while grazing contaminated grass.
The intermediate stage (mesacestoide) is characterized by the presence of bladder like structure in the abdominal/peritoneal cavity and liver. This intermediate larval stage is called Cysticercus pisiformis. The development of the larva is blocked and they will survive in the bladder structure. Viable tapeworm related cysts reach a size up to 2-3 cm in diameter, rarely up to 8 cm large or more.
Development into the adult stage can only occur after ingestion of the viscera of an infested rabbit by a dog, fox or cat. The development of the larva will continue into adults.
Blister-like mature cysticerci (green arrows)containing a larva of the tapeworm Taenia pisiformis in the body cavity of a rabbit.
Detail of a cyst, showing the larval taperworm Cysticercus pisiformis surrounded by fluids.
The parasitic larvae use the hepatic portal vein to invade the liver of rabbits. The migration phase of the larva is usually accompanied by focal granulomatous hepatitis-like symptoms: inflammation of the liver, local hepatocellular necrosis, and hepatic scaring if the condition becomes chronic. A microscopic analysis of the tissue shows the presence of necrotic tissue, blood, degenerative granulocytes, giant cells as well as mononuclear cells. Fibroblastic transformation of the tissue may be present, and the affected tissue will be replaced by connective tissue. After 15 to 30 days, the larva will migrate to the hepatic parenchyma and form cysts.
A severe infection can cause chronic extreme weakness or sudden death.
Aberrant migration is possible. Some of these cysts have been found in the peritoneal fluids or in the lungs of a rabbit, filling almost all the cavity of the lung and causing respiratory distress. The classical symptoms for brain cysticercosis lead to seizures, increased intracranial pressure, and altered mental status. On X-ray, there are signs of hydrocephalus, aseptic meningitis and/or calcified cysts. CT scanning and MRI enable to see the cysts definitively, often accompanied by ring-enhancing lesions, or hydrocephalus. Since MRI does not show the calcified pocket clearly, a contrasting dye administrated IV, can be used to visualize the inflamed and destroyed regions better.
As long as there is no direct contact and ingestion of the cysts containing the larva (unlikely in pet rabbits), no contamination is possible.
Treatment can be attempted with praziquantel.
This parasitic worm is also referred to as Taenia serialis. As with Taenia pisiformis, the development of the parasite has two stages: an intermediate one in hares or wild rabbits, and an adult form in dogs and cats. Its incidence in house rabbits in rare. In rabbits, cysts of Multiceps serialis are essentially found in the subcutaneous tissues and muscle mass. The cysts are elongated and the larva can easily be seen.
The development stage is blocked at the cyst stage in rabbits. Maturation and development into the adult stage will only occur after ingestion of the viscera of an infested rabbit by a dog or fox.
The risk of human contamination is possible, when direct contact with the cysts and ingestion of the intermediate larva (e.g. hunted hares/rabbits and contact with the digestive tract). With house rabbits, the risk of contamination is close to zero.
My gratitude goes to Prof. Richard Hoop (Institut für Veterinärbakteriologie, University of Zurich) for the permission to use his picture related to Taenia cysts in rabbits
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