JPC SYSTEMIC PATHOLOGY

MUSCULOSKELETAL SYSTEM

April 2025

M-P03 (NP)

 

Signalment (JPC #2050449): Three-year-old steer

HISTORY: Cheek muscle taken at slaughter

HISTOPATHOLOGIC DESCRIPTION: Skeletal muscle: Focally separating skeletal muscle fibers is a 4.0mm x 1.5mm parasitic cyst with a thin, fibrous capsule containing a longitudinal section of an invaginated cestode larva (cysticercus) with a thin, 4 µm wide, eosinophilic tegument; lacy, fibrillar, eosinophilic parenchyma; and scattered, 5 µm, basophilic, calcareous corpuscles. The cyst capsule is infiltrated by numerous lymphocytes, eosinophils, and hemosiderin-laden macrophages. Inflammatory cells extend into the adjacent connective tissue and expand the endomysial and perimysial layers of muscle fibers. Scattered within the remaining muscle are small aggregates of lymphocytes, macrophages, and neutrophils. Multifocal myocytes are pale and swollen (degeneration), or intensely eosinophilic with loss of cross striations and nuclear pyknosis or karyolysis (necrosis).

MORPHOLOGIC DIAGNOSIS: Skeletal muscle: Cysticercus, focal, with mild lymphocytic and eosinophilic myositis, breed unspecified, bovine.

ETIOLOGIC DIAGNOSIS: Cysticercal myositis

CAUSE: Cysticercus bovis

CONDITION: Beef measles; measly beef

GENERAL DISCUSSION:

• Cysticercus bovis is a larval cestode that occurs in cattle and occasionally other herbivores worldwide; it preferentially infects heart (C-P04) and masticatory muscles, although cysts also occur in liver, lungs, and lymph nodes
• The adult stage, Taenia saginata, is the most common and longest tapeworm in humans; it has 4 suckers and lacks a rostellum and hooks
• A cysticercus is fluid-filled, thin-walled, muscular cyst with an invaginated scolex and neck

PATHOGENESIS:

• Likely similar to the pig tapeworm, Cysticercus cellulosae (adult: Taenia solium), which secretes antigen B protein that binds the first complement component and collagen, inhibiting initiation of classical complement pathway
• Larvae are eventually destroyed by the immune system, mineralized, and removed

LIFE CYCLE:

• Humans with adult intestinal tapeworm pass gravid proglottids with feces > proglottids rupture releasing eggs > eggs eaten by intermediate host > egg hatches in the duodenum > released hexacanth or oncosphere penetrates the mucosa and enters intestinal vasculature > circulates to muscle and other tissues > develops into encysted cysticercus form
• Humans are infected by eating insufficiently cooked beef or veal containing cysticerci
• Bile salts cause evagination of the scolex and neck and subsequent budding; the parasite attaches to intestinal mucosa and develops into an adult

TYPICAL CLINICAL FINDINGS:

• Usually asymptomatic in cattle
• Rare cases of massive infection result in pyrexia, weakness, anorexia, and death

TYPICAL GROSS FINDINGS:

• One or more white to gray cysts, <1 cm in diameter, projecting from the surface of the heart, skeletal muscle, or other tissue
• Cysticercus bovis larvae prefer the heart and masticatory muscles but may be found in many different muscles

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Myofibers displaced by cyst; surrounded by a fibrous capsule and minimal lymphohistiocytic and eosinophilic inflammation
• Cysticerci: The invaginated scolex and neck are composed of an eosinophilic tegument, parenchyma containing calcareous corpuscles and a web-like, lightly eosinophilic matrix, suckers, scolex with no hooklets, surrounded by a thin-walled cyst
• Glycoprotein-rich cyst wall provokes little host reaction when intact; rupture evokes granulomatous inflammation, fibrosis, and mineralization

ADDITIONAL DIAGNOSTIC TESTS:

• PCR
• Identification of eggs/proglottids

DIFFERENTIAL DIAGNOSIS:

• T. solium is the pork tapeworm; metacestode is C. cellulosae
  • Unlike saginata in which humans can only be definitive hosts, humans can be definitive and intermediate hosts for T. solium (the cause of neurocysticercosis in humans, the leading cause of seizures and epilepsy in the developing world)
  • Cysticerci often develop in cerebral or ocular locations and can be fatal
  • Adults have a rostellum with hooks, and the metacestode has hooks
• Echinococcosis granulosus (D-P28B): Hydatid disease
  • Definitive host: Dogs and wild carnivores
  • Intermediate host: Sheep, cattle, horses, cervids, swine, apes, 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 connected to the membrane by a stalk; brood capsules containing 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:

Tapeworm	Definitive Host	Metacestode	Intermediate Host	Anatomic location
T. solium	Man	Cysticercus cellulosae	Pig, man, Old world monkeys	Muscle
T. hydatigena	Dogs, carnivores	Cysticerus tenuicollis	Sheep, other ruminants, pigs, Old world monkeys	Omentum, mesentery
T. ovis	Dogs, carnivores	Cysticercus ovis	Sheep, goats	Muscle
T. krabbei	Wild carnivores	Cysticercus tarandi	Reindeer, other ruminants	Muscle
T. pisiformis	Dogs, carnivores	Cysticercus pisiformis	Rabbits, hares	Serosa
T. taeniformis	Cats, other felids	Cysticercus fasciolaris	Rodents	Liver
T. crassiceps	Dogs, carnivores	Cysticercus longicollis	Rodents, Old world monkeys	Peritoneal cavity
T. multiceps	Dogs, carnivores	Coenurus cerebralis	Sheep, goats	Brain, spinal cord
T. serialis	Dog, fox	Coenurus serialis	Lagomorphs, Old world monkeys	Subcutis and intramuscular

• Pigs: Majority of larvae (Cysticercus cellulosae) are found in the heart, masseter, tongue, and shoulder muscle
• Rats: Cysticercus fasciolaris (D-P22) associated hepatic fibrosis and granulomatous inflammation may progress to fibrosarcoma; additional lesions include gastric and intestinal hyperplasia
• Sheep: Infection of Coenurus cerebralis (N-P12B) in sheep is known as “gid”
• Red-ruffed lemur: Report of cysticercal pneumonitis and pleuritis
• Black lemur: Report of unilateral pulmonary cysticercosis with atelectasis, edema, and a mixed inflammatory infiltrate due to Cysticercus longicollis, the larval form of the canine tapeworm Taenia crassiceps

REFERENCES:

1. Abee CR, Mansfield K, Tardif S, Morris T. Nonhuman Primates in Biomedical Research: Volume 2: Diseases. 2nd ed. San Diego, CA: Elsevier; 2012: 252-256, 454.
2. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA: Blackwell Publishing; 2016:153, 302.
3. Bowman DD. In: Bowman DD, ed. Georgis’ Parasitology for Veterinarians. 10th ed. St. Louis, MO: Saunders Elsevier; 2013:145-147.
4. Cooper BJ, Valentine BA. Muscle and tendon. In: Maxie MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier; 2016:239-240.
5. Gardiner CH, Poyton SL. An Atlas of Metazoan Parasites in Animal Tissues. Washington, DC: American Registry of Pathology; 2006:50-55.
6. Lowenstine LJ, McManamon R, Terio KA. Apes. In: Terio K, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals, San Diego, CA: Elsevier 2018: 400.
7. Matz-Rensing K, Lowenstine LJ. New World and Old World Monkeys. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. London, UK: Academic Press; 2018:365-367.
8. McAloose D, Stalis IH. Prosimians. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. London, UK: Academic Press; 2018:337-338.
9. Valentine BA. Skeletal muscle. In: McGavin MD, Zachary JF, eds. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022: 10

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