Esther van Praag, Ph.D.
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This parasite is also known under the name Oxyuris ambigua. It is a common, cosmopolitan parasite that infests wild or pet rabbits cottontails and hares. In the USA, the presence of P. nonanulatus has also sometimes been observed in rabbits. Auto-infection is common through ingestion of the eggs with the food. The juvenile stages of Passalurus sp. are rather found in the mucosa of the small intestine and the cecum, while the adult worms are located in the anterior part of the cecum and the large intestine of rabbits.
Passalurus ambiguus is specific to lagomorphs and does not represent a public health danger.
The life cycle of Passalurus sp. is direct: the eggs are ingested by the animal. The eggs have typically a flat side and measure about 100*43 mm. During their development, resistance against dehydration increases. The larvae will emerge from the eggs and develop in the mucous layer of the small intestine and the cecum, where they will develop into mature adults. Two molts are reported, the first after 24 hours, and the second on the 3rd day.
The adult worms have different sizes, the males being smaller (± 5 mm) than the females (± 10 mm). The females, characterized by a long and narrow tale, are marked with about 40 circular, cuticular striations. It seems that the female worms deposit the eggs around the anus. The worms live about 106 days.
Passalurus sp. parasites are non-pathologic and their presence remains asymptomatic, even when the infestation is severe. A rabbit may be infested with over 1000 parasitic worms. Eggs and occasionally live adult worms can be observed in freshly excreted feces. Once out of the rabbit body, the worms will dry quickly and cannot be seen anymore after 5 minutes. It is interesting to note that the female worms emerging from the anus of rabbits contain eggs in the gastrula stage that are able to develop into an infective stage in the environment.
The presence of mucus in the droppings, frequently as threads, is an indication that a rabbit may be infested by intestinal worms. This should be differentiated from mucoid enteritis.
An overpopulation of the nematode worms in the digestive tract can lead to stasis and cecal impaction, pain and gas production. If not treated, the stasis can become chronic, with a bout every 4 to 6 weeks.
At necropsy, Passalurus sp. worms have been found in the lumen of the cecum, as well as in the crypts and mucosa of the colon. The site where the worms were located was inflammatory and presented dystrophic modifications. The most profound inflammatory and dystrophic changes were found in the cecum. Signs of vascular dystrophy were furthermore observed in the hepatic and renal parenchyma.
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 can often be observed in the droppings
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