History
A 5-year-old spayed female domestic rabbit (Oryctolagus cuniculus) was evaluated because of sudden-onset gastrointestinal stasis. The rabbit was previously healthy and had been fed a diet that included free-choice timothy hay, leafy vegetables, and limited timothy hay-based pellets.
Clinical and Gross Findings
At the initial evaluation, abdominal palpation revealed and radiography confirmed a mass in the right aspect of the midabdomen. Computed tomography after contrast medium administration revealed a cystic soft-tissue mass with a fine, poorly vascularized membrane that was surrounded by fatty tissue. Contents of the mass were not highlighted by contrast medium. Exploratory laparotomy was performed, and several white, fluid-filled cysts were identified and removed (Figure 1). The cysts varied in diameter from 1.0 to 4.5 cm and contained multiple 1-mm-diameter white nodular structures on their inner surfaces. The cysts were fixed in formalin for histologic evaluation. Although the liver appeared grossly normal at the time of surgery, a wedge biopsy specimen was obtained for histologic examination. With supportive care including IV administration of fluids, nutritional support, and analgesic measures, the rabbit recovered uneventfully from anesthesia and surgery. Clinical signs did not recur.
Photographs of a large, white, cystic structure removed during exploratory laparotomy from the abdomen of a 5-year-old female domestic rabbit that was evaluated because of sudden-onset gastrointestinal stasis. A—Multiple cysts were present in the abdomen, and each contained an acellular, straw-colored serous fluid. B—Following removal from the abdominal cavity and fixation in formalin, the cysts were seen to contain multiple white nodular structures, approximately 1 mm in diameter, adhering to the interior surface of the cyst and arranged in clusters and rows.
Citation: Journal of the American Veterinary Medical Association 250, 4; 10.2460/javma.250.4.393
Formulate differential diagnoses from the history, clinical findings, and Figure 1—then turn the page→
Histopathologic Findings
Five formalin-fixed translucent cystic structures were submitted for histologic evaluation. Each cyst consisted of a bladder wall with multiple invaginated scolices covered by a thick eosinophilic tegument (with small fibrillar projections) surrounding a loose parenchyma (Figure 2). Each scolex possessed muscular suckers and a rostellum with chitinized hooks (Figure 3). Multiple clear to pale basophilic oval structures (calcareous corpuscles) measuring 20 × 30 μm were present within the parenchyma and especially concentrated in scolices. The liver biopsy specimen had mild diffuse hepatocyte swelling with glycogen-type vacuolar change, which was interpreted as a normal physiologic state.
Photomicrograph of a section of a coenurus with multiple scolices removed from the rabbit in Figure 1. The exterior surface is comprised of a thick bladder wall (tegument [large arrow]) surrounding parenchyma. Notice that scolices are invaginations of the structure; thus, within scolices (S) the tegument appears to be interior to the parenchymal tissue (small arrow). H&E stain; bar = 1 mm.
Citation: Journal of the American Veterinary Medical Association 250, 4; 10.2460/javma.250.4.393
Photomicrograph of a scolex of a coenurus removed from the rabbit in Figure 1. The scolex has muscular suckers (long black arrow) and a rostellum with chitinized hooks (short black arrow). Clear to pale basophilic ovoid structures throughout the parenchyma (red arrows) are calcareous corpuscles, the function of which is not completely understood. H&E stain; bar = 100 μm.
Citation: Journal of the American Veterinary Medical Association 250, 4; 10.2460/javma.250.4.393
Morphologic Diagnosis and Case Summary
Morphologic diagnosis and case summary: intraperitoneal coenurosis consistent with Coenurus serialis (metacestode form of Taenia serialis) infection in a rabbit.
Comments
Coenurus serialis cysts (termed coenuri) are the metacestode form of the T serialis tapeworm. The convention of naming the metacestode stage (C serialis) as an apparently distinct genus from the adult parasite (T serialis) is a relic from initial investigations that failed to identify the 2 forms as life stages of the same organism.1
Taenia serialis typically propagates in a lagomorph-canid lifecycle. Rabbits and hares become infected after ingestion of embyonated eggs or mature proglottids shed by the definitive canine host. Within the gastrointestinal tract of the lagomorph intermediate host, oncospheres hatch from eggs, penetrate the intestines, and enter the systemic circulation.2 Development of the metacestode coenurus stage typically occurs in subcutaneous or intramuscular connective tissues. The definitive dog or fox host becomes infected after ingestion of coenuri in the flesh of the intermediate host, whereupon the adult parasite develops in the canine small intestine. Notably, the rabbit of the present report had no history of exposure to dogs or dog feces. It had been adopted at a very young age and was kept only indoors. It is postulated that contaminated hay was the source of the infection. No explanation is available for the atypical intraperitoneal localization of cysts in this rabbit.
Coenuri of T serialis have a distinctive morphology. Cysts are white with multiple grossly visible protoscolices that are located on the inner wall of the cyst and protrude into the lumen. Classically, the protoscolices have been described as radiating in rows from a central point, earning the species its specific epithet, but this morphology is not consistently demonstrable.3 Cysts are variable in size but have been reported to contain up to 500 mL of fluid.3 Histologic features in the case described in the present report are typical.4 In tissues, cysts are often surrounded by a mixed inflammatory infiltrate that includes an eosinophilic component.
The case of T serialis coenurosis described in the present report was atypical in that cysts of this parasitic species typically develop in subcutaneous or intramuscular connective tissues, rather than in the abdomen.5–10 Coenurosis is less common in pet rabbits than in wild lagomorphs but should be included as a differential diagnosis for cystic masses in pet rabbits. This is particularly true for soft, fluctuant intramuscular or subcutaneous masses. Lesions are often clinically unimportant but may cause disease depending on the site of localization. Case reports3,10 have described the formation of coenuri in extraocular soft tissues causing exophthalmos and in the retroperitoneal space resulting in death in rabbits. Taenia serialis coenuri may also develop in a variety of aberrant intermediate hosts, including chinchillas, opossums, and primates.11–13 Taenia serialis coenuri appear to be an uncommon cause of neural coenurosis in cats and humans.14–16 A PCR technique to analyze aspirated fluid cyst contents is available for presurgical diagnosis. The disease can be diagnosed definitively by histologic examination and immunohistochemical analysis of cysts after surgical excision.7 Surgery is curative if the cyst can be successfully removed. Medical management with benzimidazoles or praziquantel has been suggested as an alternative to surgery, particularly in human cases of coenurosis, but concurrent steroid or tetracycline treatment may be advisable to control the resulting immune response.7 Taenia serialis is considered a zoonotic agent, having been implicated in human cases of coenurosis that typically involve CNS, subcutaneous, or muscular tissues.14
Infection with Cysticercus pisiformis, the metacestode stage of the Taenia pisiformis tapeworm, should also be considered among the differential diagnosis for cystic abdominal masses in rabbits. This species tends to localize in the peritoneal cavity of the rabbit or rodent intermediate host, forming grape-like clusters of cysticerci that resemble coenuri but contain only a single inverted scolex.6
Another coenurus of veterinary importance is Coenurus cerebralis, the metacestode stage of the Taenia (Multiceps) multiceps tapeworm, which typically propagates in a sheep-dog lifecycle. This species forms a coenurus morphologically similar to that of T serialis, most commonly in the brains or spinal cords of sheep and causes a neurologic disease known as gid or staggers.17
References
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