Signalment:  

4-year-old intact male miniature Schnauzer dog (Canis familiars).The dog had experienced some weight loss in the past several months. Firm nodules were palpated on the testicles. The referring veterinarian submitted the testicles to rule out infection and neoplasia.

Gross Description:  

At castration surgery the testes and epididymis were thickened by white material. Both testicles were submitted for histopathology. At sectioning, multiple cystic nodules were present within the tunics.

Histopathologic Description:

Both testicles were equally affected. The testicular cords (between the pampiniform plexus and spermatic duct), tunica albuginea and the stroma of the epididymis were multifocally expanded by many cross sections of variably sized and variably degenerate acoelomate larvae within cystic spaces lined by either streams of fibrous connective tissue or a thick bands of inflammatory cells. Cestode larvae ranged from 1,000 x 600 μm to 1,600 to 900 μm and had thick (45 μm), convoluted hyaline cuticles overlying single layers of palisading epithelial cells. Within the loose mesenchymal parenchyma digestive tracts were not observed, and there were numerous, 50 x 40 μm, oval to pleomorphic, clear vesicles with widely scattered, small, weakly basophilic concretions (calcareous corpuscles). No scolices were observed. Inflammatory infiltrates flanking parasites were composed of moderate numbers of lymphocytes, plasma cells, epithelioid macrophages, neutrophils and eosinophils; granulocytes were predominantly within areas of degenerating larvae. Within the testicular parenchyma spermatogenesis was present; epididymal ducts were histologically within normal limits and contained some sperm.

Morphologic Diagnosis:  

Periorchitis, epididymitis and funiculitis, lymphohistiocytic and pyogranulomatous, severe, bilateral and multifocal, chronic with intralesional cestode larvae (most likely Mesocestoides sp.).

Lab Results:  

None.

Condition:  

Mesocestoides sp.

Contributor Comment:  

Mesocestoides spp. are tapeworm parasites with unusual 3 host life cycles.(7) The first intermediate hosts are purported to be coprophagous insects, which are ingested by a variety of small reptiles and rodents. Within the second intermediate host, the metacestode, called a tetrathyridium, penetrates the intestinal wall and undergoes asexual replication within the peritoneal cavity. Ingestion of the second host by the definitive host completes the life cycle; infection with adult Mesocestoides are largely asymptomatic. Proglottids from adult parasites are not directly infectious to definitive or secondary intermediate host species.(7) Definitive hosts are mainly wild and domestic canids, although multiple species, including humans(5), can become infected. Distribution of the parasites is worldwide. Veterinary interest in the species is largely related to rare migration of ingested tetrathyridia into the abdominal cavity of domestic dogs and occasionally cats.(12) The subsequent prolific asexual reproduction of metacestodes and the ensuing severe granulomatous peritonitis has been termed canine peritoneal larval cestodiasis (CPLC).(2) In rare cases, parasites extend into abdominal organs and into the thoracic cavity.(11) It is not known whether CPLC truly represents aberrant migration of tetrathyridia through the intestinal wall after ingestion of the second intermediate host, or if it represents aberrant use of the dog as a secondary intermediate host after accidental ingestion of the first intermediate host.(2)

Diagnosis and treatment of CPLC can be challenging. Clinical signs are vague and include lethargy, weight loss, vomiting and ascites.(4) Occasionally subclinical cases are identified during routine ovariohysterectomy or castration.(2) In two reported cases, scrotal swelling was among the initial presenting complaints.(10,13) Diagnostic procedures include radiology, showing changes indicative of diffuse peritonitis, and ultrasonography.(12) Cytologic examination of the ascites fluid or aspiration of affected organs often reveals intact tetrathyridia, acephalic forms or calcareous corpuscles.(3,9) Diagnosis of infection by Mesocestoides spp. can be confirmed by morphologic identification of tetrathyridia or by PCR.(2) Recommended treatment involves peritoneal lavage and long term treatment with fenbendazole(4) although other drugs, such as praziquantel, have also been used.(8) Prognosis after treatment depends upon the severity of infection at the time of diagnosis and upon how aggressively therapy is instituted.(2) At least some dogs experience recrudescence months after therapy is discontinued, although reinfection cannot be ruled out.(1) The dog in this report had a massive peritoneal infection and was treated aggressively with lavage and 30 days of fenbendazole therapy. Four months after initial diagnosis he presented with vomiting and radiographic evidence of intestinal obstruction. Partial necropsy after euthanasia revealed massive abdominal adhesions, but no samples for histopathology were collected; whether adhesions represented complications of healing peritonitis or recurrence of cestodiasis was not determined.

Although PCR to confirm Mesocestoides infection was not performed in this case, histologic lesions were consistent with previously reported cases. Characteristics of cestodes include a thick cuticle, acoelomate body and numerous calcareous corpuscles.(6) Although most of the metacestodes were degenerate, examination of multiple sections failed to reveal definitive scolices or suckers; it is likely, therefore that most of the organisms in this dog were the acephalic forms. Both tetrathyridia and acephalic forms have been described in cases of CPLC.(4)

JPC Diagnosis:  

Testes, vaginal tunic: Larval cestodes, multiple, with mild granulomatous periorchitis.

Conference Comment:  

With the poor preservation of the larval cestodes in most sections, conference participants struggled to assign a specific etiologic diagnosis to this case. The presence of calcareous corpuscles indicates that the parasites are cestodes, few species of which are present specifically in the vaginal tunics, an extension of the peritoneum. R. Spirometra spp. is a cestode which may encyst in the peritoneal cavity of carnivores. Its larval form, sparganum, is also solid-bodied and only differentiated from the tetrathyridium of Mesocestoides spp. by its lack of suckers or scolex.(6) Obtaining definitive evidence of suckers or a scolex requires serial sections through a well-preserved organism, thus we favored the diagnosis of proliferating larval cestodes in this example.

The class of flatworms (Platyhelminthes) referred to as cestodes are more commonly known as tapeworms, and infect nearly every vertebrate species. Cestodes typically inhabit the gastrointestinal tract, to include the ducts of the liver and pancreas. They are generally of minor significance to the animal though they may reach spectacular size in some species (Consider the 60-foot long, Diphyllobothrium latum of man!). All tapeworms utilize at least two hosts to complete their lifecycle, and as the contributor concisely describes, some such as Mesocestoides spp. require more.

Adult cestodes are segmented into proglottids and both larvae and adults often contain suckers or hooks. All contain calcareous corpuscles which are of unknown function but most helpful in initially classifying the organism as a cestode. In this case, the corpuscles are shrunken and surrounded by a clear space which is likely a commonly observed artifact of fixations. Spirometra falls into the category with Diphyllobothrium of Pseudophyllideans. Mesocestoides are grouped with other common veterinary parasites such as Taenia, Hymenolepis, and Echinococcus in the Cyclophyllidean category characterized by four anterior suckers and muscles separating medullary and cortical regions.(6)

Since nearly all adult tapeworms are parasites of the digestive tract, those observed in tissue sections are usually larva. Along with the discussed sparganum and tetrathyridium larvae, most other larval forms have specific nomenclature. Pairing the appropriate larval and adult tapeworm species together can be an overwhelming challenge, though a focus on the following cestodes may cover those of greatest importance in veterinary pathology.

COMMON VETERINARY CESTODE SPECIES
ADULT	LARVA	SPECIES WITH ASSOCIATED PATHOLOGY
Spirometra spp.	Sparganum (Plerocercoid)	Dog, Cat
Mesocestoides spp.	Tetrathyridium	Dog, Cat, NHPs
Rodentolepis nana, R. microstoma, Hymenolepis diminuta	Cysticercus	Rodent
Taenia taeniaformis	Cysticercus fasciolaris	Rodent
Taenia pisiformis	Cysticercus pisiformis	Rabbit, Dog
Taenia hydatigena	Cysticercus tenuicollis	Ruminants, Swine
Taenia ovis	Cysticercus ovis	Sheep
Taenia saginata	Cysticercus bovis	Cattle
Taenia solium	Cysticercus cellulosae	Swine
Taenia multiceps	Coenurus cerebralis	Sheep, Goat
Moniezia spp., Thysaniezia giardi, Stilesia globipunctata	Cysticercus	Ruminant
Anoplocephala perfoliata	Cysticercus	Horse
Diphyllobothrium spp.	Cysticercus	Fish--�‐eating carnivore
Diplydium caninum	Cysticercus	Dog, Cat
Echinococcus granulosus	Hydatid cysts	Many
Echinococcus multilocularis	Hydatid cysts	Many

References:

1. Barsanti JA. Mesocestoides infections: Recurrence or reinfection? JAVMA. 1999;214:478.

2. Boyce W, Shender L, Shultz L, et al. Survival analysis of dogs diagnosed with canine peritoneal larval cestodiasis (Mesocestoides spp.) Vet Parasitol. 2011;180:256-261.

3. Caruso KJ, James MP, Paulson RL, et al. Cytologic diagnosis of peritoneal cestodiasis in dogs caused by Mesocestoides sp. Vet Clin Path. 2003;32:50-60.

4. Crosbie PR, Boyce WM, Platzer EG, et al. Diagnostic procedures and treatment of eleven dogs with peritoneal infections caused by Mesocestoides spp. JAVMA. 1998;213:1578-1583.

5. Fuentes MV, Galan-Puchades MT, Malone JB. A new case report of human Mesocestoides infection in the United States. Am J Trop Med Hyg. 2003;68:566-567.

6. Gardiner CH, Boynton SL. An Atlas of Metazoan Parasites in Animal Tissues. Armed Forces Institute of Pathology/ American Registry of Pathology. Washington, DC. 2006:50-55.

7. Padgett KA, Boyce WM. Life history studies in two molecular strains of Mesocestoides (Cestoda: Mesocestoidae): Identification of sylvatic hosts and infectivity of immature life stages. J Parasitol. 2004;90:108-113.

8. Papini R, Matteini A, Bandinelli P, et al. Effectiveness of praziquantel for treatment of peritoneal larval cestodiasis in dogs: A case report. Vet Parasitol. 2010;170:158-161.

9. Patten PK, Rich LF, Zaks K, et al. Cestode infection in 2 dogs: Cytologic findings in liver and a mesenteric lymph node. Vet Clin Path. 2013;42:103-108.

10. Rodriguez F, Herraez P, Espinosa de los Monteros A, et al. Testicular necrosis caused by Mesocestoides in a dog. Vet Rec. 2003;153:275-276.

11. Topli N, Yildiz K, Tunay R. Massive cystic tetrthyridiosis in a dog. J Small Animal Med. 2004;45:410-412.

12. Venco L, Kramer L, Pagliaro L, et al. Ultrasonographic features of peritoneal cestodiasis caused by Mesocestoides sp in a dog and in a cat. Vet Radiol & Ultrasound. 2005;46:417-422.

13. Zeman DH, Cheney JM, Waldrup KA. Scrotal cestodiasis in a dog. Cornell Vet. 1988;78:273-279.

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3-1. Testis

3-2. Testis

3-3. Testis

3-4. Testis

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