Cysticercosis in Laboratory Rabbits JAMES R. OWINY, BVM, PHD There are no data on the current incidence of Taenia pisiformis in laboratory rabbits. Two cases of cysticercosis most likely due to T. pisiformis in laboratory rabbits (intermediate host) are presented. Both rabbits had no contact with dogs (final host); their caretakers did not work with dogs, and these caretakers changed into facility scrubs and wore gloves when working with the rabbits. Rabbit 1 may have been infected after being fed hay at our facility. In light of the life cycle of the parasite and the history of rabbit 2, it potentially could have been infected prior to arrival at our facility. There have been only three cases of tapeworm cysts in rabbits in our facility (average daily census, 250) during the last 10 years (incidence, , 1%). This report indicates that although cysticercosis is rare in laboratory rabbits, one should always be aware of such incidental findings. Although it may not produce overt illness in the rabbit, hepatic migration could adversely affect the outcome of some experimental procedures. Two adult, female, Dutch Belted rabbits (Oryctolagus cuniculi) used in a toxicology study approved by the institutional animal use and care committee were submitted by an investigator for necropsy. Both rabbits were purchased from a commercial breeder (Myrtle’s Rabbitry, Thompson Station, Tenn.) and had been in our facility for 3 months. Vendor health records indi- cated freedom from Pasteurella multocida, Treponema cuniculi, CAR bacillus, Clostridium piliforme, Encephalitozoon cuniculi, Eimeria stiedae, Psoroptes cuniculi, Cheyletiella parasitovorax, Listrophorus gibbus, Passalurus ambiguus, and Taenia pisiformis. The rabbits were housed conventionally in banks of stainless-steel cages with drop pans containing autoclaved aspen chips (Northeastern Products, Warrensburg, N. Y.) in an AAALAC-accredited facility. Drop pans were changed twice a week, and cages were changed every 2 weeks. Temperature was maintained at 18 to 208C, with approxi- mately 40% relative humidity and 10-12 air changes per hour. A 14:10 h (light: dark) cycle was used. Rabbits were fed a breeder diet (Prolab breeder rabbit, 5P29, PMI Nutrition International, Inc., Brentwood, Mo.). Water was supplied in bottles to allow monitoring of daily water intake. Rabbit 1 was 20 days pregnant when it was presented for Figure 1. Cysticercus of Taenia pisiformis retrieved from the peritoneal necropsy. The rabbit had gone off feed 2 days earlier but report- cavity of a rabbit. Arrow (➞) indicates location of scolex (white spot). edly was drinking normally and was well hydrated. The rabbit Bar = 5 mm. was given grass hay from a local farm supply store to stimulate its appetite. On the morning of the third day after going off feed, rump) in length, were present in the uterus. The heart, lungs, and she became depressed, with a temperature of 37.18C. A small kidneys were within normal limits. Several oval cysts (diameter, amount of reddish brown vaginal discharge was noticed. The approximately 1 cm) were found either attached to the omentum rabbit died shortly thereafter and was presented for necropsy. or floating free in the peritoneal cavity (Fig. 1). Each cyst con- Upon arrival at our facility 3 months earlier, she had developed tained clear fluid and a single white spot (diameter, approximately anorexia but recovered uneventfully after being fed hay. 1 to 3 mm). Selected tissues were fixed in 10% formalin (Fisher Rabbit 2 had a decreased appetite for several days and was Scientific, Fairlawn, N. J.), embedded in paraffin, sectioned at given hay and oral fluids plus nutritional supplement (Nutri- 5 mm, and stained with hematoxylin and eosin. Cal, Evsco Pharmaceuticals, Buena, N. J.). The rabbit delivered Microscopic examination of the mesentery revealed several fluid- a live pup at 29 days of gestation, but the adult was found dead filled cysts surrounded by fibrous connective tissue with abundant the next morning. heterophils and mononuclear inflammatory cells. The cysts had profiles of immature cestode larvae containing an invaginated Pathologic findings scolex armed with hooks and suckers but no ova (Fig. 2). Each larva had a thick cuticle, which covered a segmented At necropsy, rabbit 1 was in fairly good body condition. Approxi- parencymatous body containing numerous calcenous corpuscles mately 50 ml of straw-colored fluid was present in the peritoneal (Fig. 3)—features consistent with a cestode larva (1). The liver cavity. The liver was firm and contained scattered yellowish, had multifocal areas of fibrosis in portal triads (Fig. 4). These foci shrunken areas (diameter, 2 to 5 cm) interspersed with promi- contained numerous mononuclear inflammatory cells and, in some nent white streaks (width, 2 to 4 mm). There was very little ingesta cases, macrophages with brown pigment. There were few tracts in the intestine. Eight pups, each approximately 4 cm (crown-to- containing mononuclear inflammatory cells, with minimal fibro- sis and loss of hepatocytes adjacent to these well-delineated fibrotic Laboratory Animal Resources, Painter Center, Colorado State University, Fort Collins, Colo- foci. The liver lesions were attributed to migration of cestode lar- rado 80523-2007 Address correspondence to: James R. Owiny, Johns Hopkins University School of Medi- vae. There was no necrosis or fatty change in the liver. None of the cine, 459 Ross Research Building, 720 Rutland Avenue, Baltimore, Maryland 21205 liver lesions were thought severe enough to cause death. The pla- Volume 40, No. 2 / March 2001 CONTEMPORARY TOPICS © 2001 by the American Association for Laboratory Animal Science 45 Analysis of the peritoneal fluid revealed a hazy, yellow-colored transudate with 2.3 g/dl protein and 100 cells/ml. The cytocentrifuge preparation contained 2% heterophils, 78% lym- phocytes, and 20% large mononuclear cells. Lymphocytes were predominantly small, with only a few intermediate-sized ones. A few lymphoid cells were plasmacytoid in appearance, and the large mononuclear cells were vacuolated. No microorganisms were present, and aerobic culture of the fluid yielded no growth. The underlying cause of the fluid effusion was not apparent. Rabbit 2 was in very good body condition. She had a bloody vaginal discharge and a smearing of blood on the front paws and muzzle. The liver was pale, soft, and friable, with a promi- nent reticular pattern. The stomach contained a large hairball. Most of the gastrointestinal tract was empty except for a few fe- cal pellets. The bladder also was empty. The uterus contained seven full-term pups. The lungs were dark and oozed blood from the cut surface. Aerobic culture of the lungs yielded no growth. Figure 2. Section through a cysticercus cyst showing scolex (➞) and cyst Five ovoid cysts (diameter, approximately 1 cm) containing a wall (➤). Bar = 1 mm. prominent white spot were attached to the abdominal fat. One cyst was floating free in the abdominal cavity. Cysts were filled with a clear fluid and resembled those found in rabbit 1. Histologically, the cysts from rabbit 2 resembled those from rabbit 1. There was marked midzonal fatty change in the liver, moderate congestion and multifocal edema of the lungs, marked lymphoid atrophy of the spleen accompanied by mildly increased heterophils, and multifocal subacute inflammation of the kid- ney. The uterus and placenta had large areas of necrosis and marked congestion without inflammation. Diagnoses of mesen- teric cestodiasis in a probable intermediate host, marked fatty liver, pulmonary congestion, and splenic atrophy with mild acute splenitis were made. The cause of death most likely was preg- nancy toxemia, as suggested by presence of a fatty liver. Discussion Both animals were diagnosed as having mesenteric cestodia- sis consistent with the larval stage (cysticercus) of T. pisiformis, in light of the appearance and location of the cysts in the rabbits. Figure 3. Scolex and part of the cyst wall (➤) containing mixed inflam- The cysts appear as transparent spheres about 10 mm in diam- matory cells. A thick segmented cuticle (➞), hooks (➤), and eter that contain a characteristic scolex, which is seen as a white parenchymatous body (asterisk) are evident. Bar = 100 mm. spot. In addition, white raised 1- to 3-mm foci or 2- to 3-mm linear lesions as well as fibrin deposits may be seen on the he- patic surface (2-4) and are usually attributed to parasite migration. The foci or linear lesions are solitary and coalescing granulomas with caseous cores containing monocytes, giant cells, and eosinophils surrounded by a layer of mixed inflammatory cells and fibroblasts. Some granulomas may contain parasites. In chronic cases, the lesions undergo fibrosis and mineraliza- tion (2,3). The liver lesions we saw were mild but still consistent with larval migratory tracks. T. serialis (Mulitceps serialis) forms a coenurus in the inter- muscular connective tissue, commonly in the flank and occasionally in the abdomen or brain. The coenurus is a fluid- filled bladderworm (diameter, 4 to 5 cm) with many scolices, each of which is on a separate stalk that invaginates into a com- mon bladder. Echinococcus granulosus forms a similar coenurus (hydatid cyst) and can occur in the abdominal cavity of the rabbit. Both of these organisms are easy to differentiate from a cysticercus. E. cuniculi can cause granulomatous and fibrosing hepatitis; however, it does not produce cysts, nor does it histo- Figure 4. Section of a liver showing fibrosis (➤), loss of hepatocytes, and logically resemble a cysticercus. mixed cellular infiltrate. Bar = 100 mm. Serologic and skin tests to detect rabbit cysticercosis have been developed (5,6) but are not commercially available. Because of centa appeared normal, and although there was some autolysis, the difficulty of ante-mortem diagnosis, treatment usually is not there was no evidence of necrosis or inflammation. There were no attempted; however, mebendazole at 1 g/ kg of feed (approxi- lesions in the kidneys. A diagnosis of mesenteric cestodiasis in a mately 50 mg/kg body weight daily) for 14 days has been reported probable intermediate host was made.