JPC SYSTEMIC PATHOLOGY
DIGESTIVE SYSTEM
October2021
D-P22

 

Signalment (JPC# 2317378):  Laboratory mouse

 

HISTORY:  None

 

HISTOPATHOLOGIC DESCRIPTION:  Liver:  Focally expanding, compressing, and replacing the hepatic parenchyma is a 5 mm diameter, thin-walled, parasitic cyst that contains multiple tangential sections of a larval cestode (strobilocercus).  The larval cestode is 1.5 mm in diameter with a segmented strobila that lacks a pseudocoelom and digestive tract, has a 10-20 um thick, homogenous, acellular tegument supported by a basement membrane, longitudinal subtegumental and transverse parenchymal muscle layers, and numerous 10 x 5 um diameter, oval, clear structures containing central basophilic to eosinophilic amorphous material (calcareous corpuscles).  Surrounding the cyst is a thick, up to 100 um wide, fibrous capsule that is multifocally infiltrated by plasma cells, lymphocytes, neutrophils, few macrophages, and rare eosinophils.  Scattered throughout the remaining parenchyma, there are aggregates of myeloid and erythroid precursors (extramedullary hematopoiesis). 

 

MORPHOLOGIC DIAGNOSIS:  Liver:  Strobilocercus, focally extensive, etiology consistent with Cysticercus fasciolaris, breed unspecified, murine.

 

ETIOLOGIC DIAGNOSIS: Hepatic cysticercosis/strobilocercosis

 

CAUSE:  Cysticercus fasciolaris

 

GENERAL DISCUSSION:

• Cysticercus fasciolaris is the metacestode (i.e. larval) form of the cat tapeworm, Taenia taeniaeformis
• Adults live in the small intestine of domestic and wild cats (i.e. definitive host) and occasionally in dogs and other carnivores, where they are usually clinically unapparent
• Larvae are found in the liver of mice, rats, and hamsters (i.e. intermediate host)
• In Wistar rats, cysticerci of Taenia are reported to induce hepatic fibrosarcomas; chronic inflammatory reaction to the cysticerci capsule is thought to play a role in this pathogenesis

 

PATHOGENESIS:

• Embryonated eggs are ingested by the intermediate host and migrate from the small intestine into the liver through the portal circulation
• Larvae enter the liver several days post-infection; portal triads are infiltrated by eosinophils, but there are no inflammatory cells around the parasite
• Larval migration results in necrotic migration tracts with eosinophilic inflammation and the parasite is surrounded by a narrow zone of proliferating fibroblasts
• At 2-3 weeks post infection, the capsule is infiltrated by mast cells, lymphocytes, plasma cells, and eosinophils
• The capsule undergoes progressive fibrosis; capsular and portal inflammation is minimal; portal fibrosis is common

 

LIFE CYCLE:

• Adult tapeworms (cestodes) in the small intestine of the cat (definitive host) pass gravid proglottids and eggs in feces > eggs can remain viable in the environment for at least 6 months > embryonated eggs are ingested by rodents (intermediate host) and hatch in the small intestine > larvae migrate through the intestinal mucosa and reach the liver via portal blood, forming an invaginated cysticercus > the scolex evaginates and attaches to the cyst wall and elongates forming a segmented strobila (strobilocercus) (note: most of the other Taeniid tapeworms do not begin to bud and form strobila until in the definitive host, most remain as cysticerci or coenuri in the intermediate host) > the definitive host is infected via ingestion of infected rodent liver > the strobila is digested, and the scolex attaches to the intestinal wall

 

TYPICAL CLINICAL FINDINGS:

• Infection is usually asymptomatic in the rodent and cat
• Heavy parasite burdens in the definitive host can cause diarrhea and weight loss
• The adult buries its scolex deep in the mucosa and occasionally may perforate the bowel, causing peritonitis

 

TYPICAL GROSS FINDINGS:

• Rodent liver: Usually 1-10, white to clear, thick-walled cysts, up to 4 mm in diameter, containing coiled strobila
• Enlarged hepatic and mesenteric lymph nodes or splenomegaly
• Hypertrophic gastritis reported in rats; unknown mechanism
• Ectopic locations for cysts include: lung, mesentery, pleura, abdominal wall, kidney

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Cestodes have a segmented body (strobila), a thick tegument with basement membrane, lack a pseudocoelom and digestive tract, have calcareous corpuscles, and two principal zones of non-striated (longitudinal subtegumental and transverse parenchymal) muscle fibers
• Adults are 15-60 cm long and 5 mm in diameter, have no neck, and have bell-shaped posterior segments
• Gravid proglottids contain an egg-filled uterus that has a central longitudinal stem with 16 to 18 lateral branches; the spherical eggs are 25 um in diameter, with a striated capsule
• Cysticerci have a thin wall with a single invaginated armed scolex
• The scolex bears an armed rostellum with 2 crowns of hooks
• Larvae attach to the cyst wall by a segmented stalk up to several centimeters in length
• The composition of cyst walls surrounding larvae are similar to that of granulation tissue, characteristic of tissues recovering after injury (Lee, J Vet Diagn Invest. 2016)

 

ADDITIONAL DIAGNOSTIC TESTS:

• Fecal flotation for eggs or gravid proglottids
• PCR

 

DIFFERENTIAL DIAGNOSIS:

• Echinococcosis granulosus (D-P28B): Hydatid disease
  • Hydatid cysts are formed by members of the Echinococcus genera as uni or multilocular structures in which brood capsules develop along the inner germinal membrane
• Definitive host: Dogs and wild carnivores
• Intermediate host: Sheep, cattle, horses, cervids, swine, and humans
• Larval stages form unilocular hydatid cysts that localize in the lung, liver and other organs
• Thick outer membrane, encasing a germinal membrane that contains brood capsules (hydatid sand) that are free or connected to the membrane by a stalk; brood capsules with many ovoid, hooklet-laden scolices
• Hydatid cysts are usually surrounded by mononuclear cells, eosinophils, giant cells and fibrosis; in humans most cysts are located in the liver
• Cysts may calcify or rupture resulting in a severe inflammatory reaction or death

 

COMPARATIVE PATHOLOGY:

ADULT TAPEWORM	DEFINITIVE HOST	LARVAL FORM	INTERMEDIATE HOST	SITE - IH
Taenia saginata	man	Cysticercus bovis (M-P03)	cattle	muscle
Taenia solium	man	Cysticercus cellulosae	pig, man, dog	Muscle, CNS (N-P12A)
Taenia(Multiceps) multiceps	dog	Coenurus cerebralis	sheep, cattle	CNS (N-P12B)
Taenia hydatigena	dog	Cysticercus tenuicollis	sheep, cattle, pig	peritoneum
Taenia ovis	dog	Cysticercus ovis	sheep	muscle
Taenia pisiformis	dog	Cysticercus pisiformis	rabbit	peritoneum
Taenia serialis	dog	Coenurus serialis	rabbit	connective tissue
Taenia taeniaeformis	cat	Cysticercus fasciolaris (strobilocercus)	mouse, rat, hamster	liver
Taenia krabbei	dog	Cysticercus tarandi	reindeer	muscle
Taenia mustelae	wild felids		rodents	liver
Diphyllobothrium latum	bear, man	sparganum	fish	muscle
Diphyllobothrium pacificum	seal, sea lion	sparganum	marine birds	muscle
Spirometra sp	dogs, cats, lynx, racoons	Plerocercoid “sparganum”	tadpoles, snakes, rodents	connective tissue

 

Parasites associated with neoplasia (mnemonic SOCS-T):

• Spirocerca lupi: Esophageal sarcomas in dogs
• Opisthorchis , Clonorchis (Opisthorchis) sinensis (liver flukes): Cholangiocarcinoma in cats, NHPs, humans
• Cysticercus fasciolaris (Taenia taeniaeformis): Hepatic sarcoma in rats
• Schistosoma : S. mansoni associated with hepatocellular carcinoma in humans; S. hematobium associated with urinary bladder transitional cell carcinoma in humans
• Trichosomoides crassicauda: Papillomas of the urothelium in rats

 

REFERENCES:

1. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA: John Wiley & Sons; 2016: 85,153,190,302.
2. Bowman DD. Georgi's Parasitology for Veterinarians. 10th ed. St. Louis, MO: Elsevier Saunders; 2014:137-148,412-417.
3. Brown DL, Van Wettere AJV, Cullen JM. Hepatobiliary system and exocrine pancreas. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2017:446.
4. Gardiner CH, Poynton SL. An Atlas of Metazoan Parasites in Animal Tissues. Washington, DC: American Registry of Pathology; 1998:50-55.
5. Lee BW, Jeon BK, Kim HS, Yoon BI. Cysticercus fasciolaris infection in wild rats (Rattus norvegicus) in Korea and formation of cysts by remodeling of collagen fibers. J Vet Diagn Invest. 2016:28(3):263-270.
6. Strait K, Else JG, Eberhard ML. Parasitic diseases of Nonhuman primates. In: Abee CR, Mansfield K, Tardif S, Morris T, eds. Nonhuman Primates in Biomedical Research: Diseases. 2^nd San Diego, CA: Academic Press; 2012: 254-257.
7. Uzal FA, Plattner BL, Hostetter JM. Alimentary system. In: Maxie MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier; 2016:223-225.

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