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Flatworms: rabbit as an intermediate host

Esther van Praag, Ph.D.

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Warning: this file contains pictures that may be distressing for some persons.

Taenia pisiformis

Tapeworm Taenia pisiformis is a common parasite of carnivores such as dogs, foxes and, on occasion, cats. It is found all over the world, predominantly in rural regions.

The development of the parasite is a two-stage process:

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 (approximately 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.

www.powhatananimalhospital.com/disease/tapeworms2.jpg

The intermediate stage (mesacestoide) is marked by the presence of a bladder-like structure in the abdominal/peritoneal cavity and liver. This intermediate larval stage is known as Cysticercus pisiformis. The development of the larva is blocked, and they will survive in the bladder structure. Tapeworm-related cysts are viable and can reach a size of up to 2-3 cm in diameter, with rare cases reaching 8 cm or larger.

The progression to the adult stage is only possible following the ingestion of an infested rabbit's viscera by a dog, fox, or cat. The development of the larva will continue until it reaches adulthood.

Richard Hoop

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, which include inflammation of the liver, local hepatocellular necrosis, and hepatic scarring if the condition becomes chronic. A thorough microscopic analysis of the tissue reveals the presence of necrotic tissue, blood, degenerative granulocytes, giant cells, and 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 infestation can result in chronic extreme weakness or sudden death.

There is a possibility of aberrant migration. 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. Cysticercosis of the brain can cause seizures, increased intracranial pressure, and altered mental status. The X-rays indicate the presence of hydrocephalus, aseptic meningitis, and/or calcified cysts. CT scanning and MRI are the preferred diagnostic tools for definitively identifying cysts, often accompanied by ring-enhancing lesions or hydrocephalus. Given the fact that MRI cannot clearly show the calcified pocket, the use of a contrasting dye administered intravenously is recommended to better visualize the inflamed and destroyed regions.

As long as there is no direct contact or ingestion of the cysts containing the larva (unlikely in pet rabbits), contamination is not a concern.

Treatment can be attempted with praziquantel.

Multiceps serialis

This parasitic worm is also referred to as Taenia serialis. As with Taenia pisiformis, the development of the parasite follows a two-stage pattern: an intermediate stage in hares or wild rabbits, and an adult form in dogs and cats. The occurrence of this condition is uncommon among domestic rabbits. In rabbits, cysts of Multiceps serialis are found in the subcutaneous tissues and muscle mass. The cysts are characterized by their elongated form, and the larva is clearly visible.

www.unbc.ca/nlui/wildlife_diseases/taenia_multiceps.htm

The development stage is blocked at the cyst stage in rabbits. Maturation and development into the adult stage is only achieved through the consumption of an infested rabbit.

There is a potential risk of human contamination, which can occur when there is direct contact with the cysts or ingestion of the intermediate larva (e.g., from hunted hares or rabbits, or contact with the digestive tract). The risk of contamination is minimal when it comes to pet rabbits.

Acknowledgement

I would like to express my gratitude to Professor Richard Hoop (Institut für Veterinärbakteriologie, University of Zurich) for his permission to use his picture related to Taenia cysts in rabbits

Further information:

Manning et al. The biology of the laboratory rabbit. 2^nd ed. London, UK, 1994.

Pinto RM, et al. Helminths of rabbits (Lagomorpha, Leporidae) deposited in the Helminthological Collection of the Oswaldo Cruz Institute. Rev. Bras. Zool. 2004, v. 21, n. 3, pp. 599-604.

Soltysiak Z, Bednarski M, Piekarska J. Wagrzyca watroby królika. Medycyna Wet. 2007, 63:1255-1257.

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