Parasitic worms of the digestive tract

Generalities

Esther van Praag, Ph.D.

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

Dozens of different parasites can be found in the digestive tracts of rabbits, hares, and cottontails. Some are very rare, some are pathogenic only under exceptional circumstances; others have barely been studied. We will describe those most commonly found in house rabbits.

In contrast to wild rabbits, healthy house/pet rabbits living in clean conditions are rarely affected by gastrointestinal parasites. The main reason for this relates to the life cycles of the various parasites, many of which are naturally disrupted when the appropriate conditions for development of the larval stages (L₁, L₂ and sometimes L₃) are not met indoors.

Many parasites are heteroxenous (multihost): in order to reproduce, they must live in several host intermediates before affecting the final host. For instance, the different juvenile forms of the liver flake (Fasciola hepatica) are found in certain snails in the grass of marshy areas before entering the rabbit digestive tract. The little fluke larvae (Dicrocoelium lanceolatum) need to live in snails and ants before becoming infective in rabbits. Those worm parasites are rarely found in house rabbits, as their life cycle is broken due to the absence of intermediate hosts in the house environment. Other parasites are monoxenous (single-host). Their larval stages are generally found in wet grassland, stagnant water, or hay before being ingested by the unique and final host, the rabbit. Such worms are more commonly found in house rabbits.

In order to prevent worm invasion in a house rabbit, it is essential that:

· forage not be gathered in areas where there are numerous dogs, cats, rodents,

· fresh greens and vegetables be washed carefully with running water,

· hay and straw used as litter be changed daily,

· hay that cannot be changed daily be given on a feed rack, where it cannot be soiled with feces or urine,

· the litter boxes themselves be cleaned daily with hot running water, acetic acid, or chlorine.

If a worm infestation appears in a house rabbit, it can easily be eliminated with the appropriate medication. A veterinarian should always be consulted prior to treatment, in order to determine which kind of worms affect the rabbit. The prescribed timing for the medication must be strictly applied.

Nematode parasitic worms

The word "nematode" originates from the Greek words "nematos" (thread) and "eidos" (form), referring to their filariform shape. Nematode worms belong to the most abundant multicellullar animals on earth.

An adult worm consists of about one thousand cells, of which hundreds are involved in reproduction. Other cells have specialized digestive, nervous, or excretory functions. These parasitic worms have neither circulatory nor respiratory systems. Their structure is simple, consisting basically of "a tube within a tube," with an alimentary canal extending from the poorly developed buccal part to the posterior anus.  Parasitic nematodes (pinworms and oxyurid) are commensal, little- or non-pathogenic, bacteria-feeding roundworms that generally live in the lower portion of the intestine. Although they are common among vertebrate hosts, they are usually host-specific, infecting mainly cattle and rabbits. Cases of cross-transmission have, however, been reported.

Nematodes are more commonly found in lagomorphs with a decreased immune resistance. The population of worms varies according to the age and the sex of the host, but also according to the intestinal bacterial flora of the host. Male rabbits are more frequently infested with pinworms than females are. A high-fiber diet is said to reduce the worm population. Pinworms are found worldwide, although some species occur more frequently in some regions. The population of worms fluctuates according to its life-cycle in the host.

Samuel Boucher and Loïc Nouaille

Left: Cecum and intestine of a healthy rabbit, devoid of parasitic nematode parasites

Right: Damaged cecum of a rabbit with a severe infestation of Passalurus sp.

At this point, the worms will be shed and can be observed in the dropping of the rabbit. Occasionally, they can be in such high number, resulting in a blockage of the intestine.

Simone van der Meij

Left: Fresh droppings with hair (fine white filament) and worms (thick translucent threads)

Right: Fresh dropping with hay fiber (short yellowish stalks) and live roundworm.

Once expelled, the surviving worms start a new cycle of growth in the cecum, shedding eggs that may be re-ingested, re-infecting the rabbit.

Nematode parasitic worms of the domestic rabbit

Parasitic nematodes found most frequently in the gastrointestinal system of rabbits are:

Oxyuridae	Passalurus ambiguus
	Dermatoxys veligera
Trichostrongylidae	Trichostrongylus calcaratus Trichostrongylus sp.
	Obeliscoides cuniculi
	Nematodirus leporus
	Graphidium strigosum
	Strongyloïdes sp.
Trichuridae	Trichuris leporis
Filarioidideae	Dirofilaria scapiceps
	Dirofilaria uniformis
	Brugia lepori
Metastrongyloidea	Protostrongylus boughtoni
Ascaroidea	Bayliascaris procyonis
	Bayliascaris columnaris
	Toxocara canis

Symptoms and diagnosis

The presence of thick brown threads of mucus in the droppings can be an indication for the presence of intestinal worms. When reaching a peak number, they cause pain and the mass of live and dead adult worms may lead to cecal impaction, accompanied by stasis, severe pain and gas formation. This is independent from the type of worm (pinworm or tapeworm) in rabbits.

http://www.powhatananimalhospital.com/disease/roundworminfect.jpg

Small intestine infested by roundworms

More rarely, sudden death occurs, without prior clinical symptoms of disease, no signs of parasitic worms during earlier fecal flotation tests. At necropsy, the cecum was found to be heavily infected with pinworms, the site where the worms were located was inflammatory and presented dystrophic modifications.

The presence of intestinal parasites is determined by a fecal flotation test. In rare cases, the fecal flotation test result of heavily infested rabbit can return negative. When left untreated, the presence of worms will be observed in the droppings.

Treatment

The presence of parasitic intestinal nematodes must be treated by administration of benzimidazoles, more specifically fenbendazole or thiabendazole. Piperazine is reported efficient, while ivermectin does not kill all the roundworm worms (e.g. Passalurus sp.). Praziquantel is used to treat tapeworm infestation in rabbits.

See the texts related to the particular worms, for details of treatment.

Acknowledgement

Thanks are due to thank Simone van der Meij, and her rabbit Jasper, for the pictures or worm-infested droppings, and to Arie van Praag, for the picture of Flora.

Further information

1. http://www.ianr.unl.edu/ianr/plntpath/nematode/nemata.htm

2. G. M. Urquhart, J. Armour, J. L. Duncan, F. W. Jennings, A. M. Dunn (1996) Veterinary Parasitology, Blackwell Science Inc; p. 307.

3. J.E. Harkness, J.E. Wagner (1995) The Biology and Medicine of Rabbits and Rodents, Williams and Wilkins. p. 372.

4. S. Boucher, L. Nouaille (2002) Maladies des Lapins. Editions France Agricole, p. 122-135

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