Adult woodchuck, Marmota monaxThe tissue is from a woodchuck that was killed and field dressed by a hunter in early summer in Maryland. An approximately 4 cm diameter mass was identified in the muscles of the shoulder. The hunter had seen similar masses in multiple woodchucks throughout the spring.
The excised nodule is approximately 4 cm in diameter and encased in thick, fibrous connective tissue. On cut section, numerous fluid-filled cysts ranging from 5 to 20 mm in diameter that are separated by a variably thick fibrous stroma. Cysts contain multiple, 0.5-1 mm diameter, white cysticerci that are variably attached to the inner cyst wall or are floating within the cyst fluid.
A large, multilocular mass effaces and distorts skeletal muscle. The mass is composed of cystic cavities that contain multiple cysticerci and are surrounded by thick bands of fibrous connective tissue and inflammatory infiltrates. In some areas, there is complete loss of the cyst wall (cyst rupture) with neutrophils, eosinophils, macrophages, multinucleated giant cells, and lymphocytes infiltrating degenerate cysticerci with loss of distinct architecture and variable parenchymal mineralization. Cysticerci measure up to 1 mm in diameter and have a thick tegument that surrounds a large bladder and an inverted neck. They lack a pseudocoleom, a digestive tract, and reproductive organs. The parenchyma of the neck contains numerous calcareous corpuscles, excretory ducts, and an inverted scolex covered by a thick tegument and associated with large muscular suckers. Rostellar hooks are present in few cross-sections. The surrounding mature fibrous connective tissue is infiltrated by many neutrophils, eosinophils, macrophages, and fewer lymphocytes and plasma cells admixed with necrotic cell debris. Surrounding skeletal muscle fibers are in varying stages of degeneration, ranging from swollen, hypereosinophilic myocytes to myocytes with fragmented, flocculent sarcoplasm that lack cross-striations. Few myocytes in the surrounding skeletal muscle are distended by intracytoplasmic apicomplexan cysts containing myriad bradyzoites that measure approximately 5 μm in diameter (Sarcocyst spp., presumptive). Postmortem bacterial overgrowth is prominent.
1. Skeletal muscle: Cysticercosis with eosinophilic, granulomatous myositis, fibrosis, and myodegeneration.
2. Skeletal muscle: Sarcocytosis
Cysticercosis has been reported several times in woodchucks, most commonly due to infection with Taenia crassiceps. The axillary subcutis is most commonly affected, though other subcutaneous regions may also be affected as well as the peritoneal cavity, thoracic cavity, nasal sinuses, liver, lung, and brain.(1,3,5)
Feral woodchucks may bear heavy cysticerci burdens that may be particularly prominent after emergence from hibernation.(2) Laboratory woodchucks may also develop cysticercosis, especially if the animals are of wild-caught origin.(1,5)
The species of tapeworm affecting this woodchuck is not definitively known. Previously reported cases of woodchuck cysticercosis were consistent with T. crassiceps larvae; the fox was considered the likely definitive host in these cases. (1-3,5) Morphologic features of this parasite are consistent with T. crassiceps and include an anterior scolex with four suckers, an apical rostellum that contains large and small rostellar hooks, and a posterior bladder.(1,4,5)
The inflammatory response in this case is relatively mild and composed of a mixed population of inflammatory cells. Inflammation is more intense in areas of cyst rupture. Previous reports of the inflammatory reaction woodchuck cysticercosis describe a similarly mild response with more prominent lymphocytic infiltrates and a similar variability in the degree of reactive fibrosis.(1)
Skeletal muscle: Multiple cysticerci with fibrosis and moderate histiocytic and atrophic rhabdomyositis.
The interesting aspect of this case is its unique presentation. As is consistent with previously reported cases highlighted by the contributor, the cysticerci were found in a focal aggregrate in the skeletal muscle of the shoulder.
There are two orders in the phylum Platyhelminthes which comprise tapeworms.(4) The order of pseudophyllideans grows into much larger adults, such as Diphyllobothrium spp., and never shed their proglottids. This is contrast to the order of cyclophyllideans, as observed in this case, which shed gravid proglottids each containing thousands of infectious eggs. The cyclophyllideans are more readily transmissible and as a result, are the most significant cause of CNS disease in people in South America.(6) When the eggs are ingested, the larvae migrate into various tissues resulting in significant pathology. Conference participants speculated on what drove these larvae to all migrate to the same location in this woodchuck.
We agree with the contributor that, although T. cracisseps is the most often reported and most likely species in this case, the presence of an anterior scolex with four suckers and a posterior bladder is only indicative of the type of larval cestodes known as a cysticercus and cannot be differentiated further. However, some references indicate the possibility of species identification based on the length of small and large hooks in the rostellum.(3) Other groups of larval cestodes include the cysticercoids which have a tiny bladder and a scolex surrounded by parenchymous arms, coenurus which has more than one scolex, and hydatid cysts with a bladder and numerous small protoscolices each with a scolex and suckers. Solid-bodied cestodes are plerocercoids (lack suckers) or tetrathyridium (has suckers).(4)
1. Anderson WI, Scott DW, Hornbuckle WE, King JM, Tennant BC: Taenia crassiceps infection in the woodchuck: a retrospective study of 13 cases. Vet Dermatol. 1: 85-92, 1990.
2. Beaudoin RL, Davis DE, Murrell KD: Antibodies to larval Taenia crassiceps in hibernating woodchucks, Marmota monax. Exp Parasitol. 24(1): 42-6, 1969.
3. Brojer CM, Peregrine AS, Barker IK, Carreno RA, Post C: Cerebral Cysticercosis in a Woodchuck (Marmota monax). Journal of Wildlife Diseases. 38(3): 621624, 2002.
4. Gardiner CH and Poynton SL: Morphologic characteristics of cestodes in tissue section. In: An atlas of metazoan parasites in animal tissues: American Registry of Pathology, Washington, D.C. 199, 50-55.
5. Shiga J, Aoymana H, Yamamoto K, Imai S, Saeki H, Sasaki N, Koshimizu K: A case report of cysticercosis caused by Cysticercus longicollis, a larval form of Taenia crassiceps in a woodchuck (Marmota monax). Jikken Dobutsu. 36(2): 213-7, 1987.
6. http://www.cdc.gov/parasites/cysticercosis. Center for Disease Control. April 16, 2014.