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    Pathology of Muscle


    Paul Hanna Winter 2018

  • Disturbances of Growth and Postmortem Alterations

    Muscle Atrophy

    • reduction of muscle size and myofiber diameter, due to loss of myofibrils / organelles

    • reversible providing the source of injury is removed in relatively short time interval

    • histologically, see reduction in myofiber diameters with an unchanged amount of CT

    • types of muscle atrophy include:

    a) Denervation atrophy

    b) Disuse atrophy

    c) Atrophy of malnutrition / cachexia / senility

    d) Atrophy of endocrine disease

  • Equine Laryngeal Hemiplegia ("roarers”)

    FIG 6-52 & 53 Craniocaudal (left) and dorsal (right) view of the larynx of a horse with a left-side paralysis that resulted in severe denervation

    atrophy of all the intrinsic muscles of the larynx except for the cricothyroid. This is especially evident here in the cricoarytenoideus dorsalis

    muscle. Veterinary Neuroanatomy and Clinical Neurology, 4th Edition.

    Denervation Atrophy


    Fig 23-6B (Dyce) Muscles

    associated with shoulder and elbow

    joints; lateral view.

    7, supraspinatus muscle

    8, infraspinatus muscle

    Equine Suprascapular Neuropathy (“Sweeney”)

    Figure 23-43 (Dyce) Distribution of the

    nerves in the right forelimb; medial view.

    The axillary artery at the shoulder joint is


    2, suprascapular n.

    Denervation Atrophy

  • Great horned owl, unilateral atrophy of pectoral muscles.

    Illustration showing what the pectoral muscles in this bird would

    look like in cross-section; note “shrinkage” of the birds right

    pectoral muscle indicating atrophy

    Denervation Atrophy

  • Great horned owl, further dissection shows cause of the unilateral

    pectoral muscle atrophy; ie tearing / damage (avulsion) of right

    brachial plexus (see arrow) resulting in denervation atrophy.

    Denervation Atrophy

  • Great horned owl, atrophic

    muscle on left of screen,

    normal muscle on right

    Denervation Atrophy

  • Disuse Atrophy

    • innervation is intact but there is reduced movement, eg pain, bone fracture,, limb immobilization / cast, etc

    • lesions are localized to affected groups of muscles & is mostly type 2 myofibers

    Horse with severe muscular atrophy

    (white asterisk) of the left leg.

    Muscle atrophy was localized to this

    limb and was caused by lack of

    movement (disuse atrophy) as a

    result of a chronic joint injury resulting

    in partial ankylosis.

    Horse with atrophy of the muscles

    of the left upper hindlimb; could be

    due to nerve damage or disuse

    secondary to a local injury

  • Atrophy of malnutrition

    muscle becomes the source of nutrients

    atrophy can start within 24 hours following starvation

    type 2 fibers affected more than type 1

    Normal pectoral muscle bird (left; with some yellow fat in surface fascia) compared to marked bilateral pectoral muscle atrophy (right; ignore

    couple of knife cut artifacts) due to malnutrition / starvation, ie muscle proteins broken down and used for basic energy requirements

  • Carcass of dog showing severe muscle atrophy due to starvation (owner was

    charged by the police). Note the extensive atrophy of scapular muscles (arrow) and

    intercostal muscles (asterisks). Intercostal muscles are so atrophic that the lungs

    can be seen right through them.

    Atrophy of malnutrition

  • • cow with Johne’s disease which causes diarrhea & weight loss due to protein-losing enteropathy

    • other diseases with excessive protein loss (eg intestinal parasitism, protein-losing glomerulopathy) can also cause emaciation

    Atrophy due to excessive protein loss

  • Protein losing enteropathy in Johne’s disease is a result of segmental, granulomatous enteritis


    Note expansion of lamina propria with inflammatory cells

    Normal control





    ileum from

    affected cow

    Atrophy due to excessive protein loss

  • • muscle wasting / weight loss can occur during chronic disease; even with normal energy consumption.

    • certain neoplasms and chronic inflammatory diseases; due to cytokines (eg TNF) from tumor cells or Mø’s

    Figure 6.23 Cachexia secondary to thoracic neoplasia. Knottenbelt and Pascoe's Color Atlas of Diseases and Disorders of the Horse, 2nd Edition

    Fig 5-1 Dog with lymphoma and secondary severe cachexia. Cancer cachexia can

    be a common paraneoplastic syndrome (PNS) in dogs and cats. The weight loss noted in

    cases of paraneoplastic cancer cachexia occurs despite adequate nutritional intake. The

    metabolic alterations associated with cancer cachexia usually occur before clinical signs

    of the inciting malignancy appear and unfortunately may continue after the patient is

    successfully treated for the tumor. Withrow and MacEwen's Small Animal Clinical Oncology, 5th Edition

    Atrophy of cachexia


    Gross Pathology:

    • muscle pallor; note, can be subtle in mild cases or inapparent in peracute cases

    • in severe cases, muscles are pale with chalky white areas or streaks

    Nutritional myopathy, skeletal muscle, bovine. Several areas of pallor within the muscles

  • WMD lamb. Pallor and white streaking in skeletal muscle of a lamb

    with white muscle disease. The extensive calcification of

    degenerate/necrotic myofibers that often occurs with WMD accounts

    for this pronounced white discoloration (as compared to the more

    subtle pallor seen with myodegeneration without calcification)


  • Bovine (calf) Heart - white muscle disease


  • Ionophore toxicosis

    Case 7-2. Pale discoloration of the skeletal muscle (ie

    necrosis). (from AFIP Wednesday Slide Conference - No. 7; October 14, 1998)

    Case 7-2. Within the pericardial sac there is 30-50ml of

    serosanguinous fluid. The epicadium is pale toward the heart

    base and contains dull reddish-pink paintbrush like

    hemorrhages apically.

  • Ionophore toxicosis

    Diffuse pallor, (a) Femoral quadriceps muscle in sheep with monensin poisoning and (b) Diffuse myocardium

    pallor, more intense in the left ventricle (arrow), sagittal section, heart in sheep with monensin poisoning. Clinical & Pathological Changes in Sheep During a Monensin Toxicity Outbreak in Brazil. AJAVA 2016;11(1): 73-78

  • Skeletal muscle, degenerative myopathy, subacute, moderate to severe, Quarter Horse, equine. Skeletal muscle. Degenerate / necrotic muscle fibers are

    hyalinized, fragmented, and have a loss of cross striations, and are infiltrated and separated by neutrophils, macrophages, serocellular debris.

    (from AFIP Wednesday Slide Conference - No. 7 14 October 1998)

    Ionophore toxicosis

  • Masticatory myositis of dogs

    Fig 7.7. Masticatory myositis in a German

    Shepherd. There is marked swelling of the

    masticatory muscles and bilateral exophthalmos,

    which is more noticeable in the left eye. (Dr Ingo Walde, FVM, Vienna)

    in the acute phase is muscle swelling

  • Acute eosinophilic myositis. Note, myofiber loss and expansion of endomysium with eosinophils.


    • acutely see edema & extensive infiltration of eosinophils

    Masticatory myositis of dogs

  • Masticatory myositis of dogs

    in the chronic stage there is severe atrophy

  • Histopathology: chronically see lymphocytes / plasma cells and myofibers become atrophic

    Chronic lymphoplasmacytic myositis.

    Above - Fig 3-85 (Maxie) Dense infiltrates of lymphocytes, including many plasma cells, separating myofibers in

    temporal muscle from a dog with masticatory myositis. This is a particularly florid lesion. H&E stain

    Right - In the chronic form,note the extensive loss of myofibers and inflammatory infiltrates; predominantly

    lymphocytes and plasma cells (latter marked with arrows). There has been some attempts at repair as shown by

    the presence of multinucleated myoblasts (asterisk).

    Masticatory myositis of dogs

  • Blackleg

    • note, animals often found dead without any clinical signs

    • when animals ill, can see lameness, crepitation and swelling

    Myonecrosis and hemorrhage in a steer

    which died from C. chauvoei infection

  • Blackle

    Note, emphysema, necrosis and hemorrhage in the

    muscle of bovine with blackleg


  • Myocardial necrosis and hemorrhage


  • Blackleg. muscle showing necrosis, and emphysema

    Blackleg. Skeletal muscle with emphysema will float

    when placed in water.


  • Necrosis, loss of nuclei and presence of large bacilli


  • Gram Positive bacilli in skeletal muscle


  • Fluorescent Antibody Test (FA test) Note the positive

    fluorescence for Clostridium chauvoei. Remember to

    submit fresh (non-fixed) tissues for this confirmatory test

    • Use FAT or anaerobic culture to confirm Cl. chauvoei

    Figure 16.3 Direct fluorescent antibody technique showing C. chauvoei in

    muscle tissue from a case of blackleg in a heifer. (×400). Clinical Veterinary Microbiology, 2nd ed.


  • Clostridial gas gangrene due to C novyi myositis in a Holstein cow.

    Local treatment includes surgical incision of skin and fascia to allow

    drainage (Courtesy of Dr. Henry Stämpfli)

    Malignant oedema is typically seen in rams during late summer/early

    autumn when head butting is a common behaviour. The most obvious

    clinical sign is marked swelling of the head (“bighead”) particularly

    surrounding the eyes which forces the eyelids closed

    Malignant edema and gas gangrene

  • Larval stages of Taenia tapeworms

    • cysticercus consists of a single bladder with one scolex

    • coenurus consists of a single bladder with many scolices

    • strobilocercus is a cysticercus that has begun to elongate / segment while still in the intermediate host

    Larval stages of Echinococcus tapeworms

    • hydatid cysts are spherical, turgid & fluid-filled; up to 5-10 cm in domestic animals (can be larger in humans)

  • • Taenia saginata (mostly humans) → Cysticercus bovis mostly in muscle / heart of cattle


    Cysticercus bovis is the larval stage of Taenia saginata which is the

    most common cestode parasite in humans. People become infected

    by eating under cooked beef containing these larvae. Cysticerci

    commonly found in heart and skeletal muscle (eg tongue, above)

    (Cysticercus bovis) within muscle of a beef cow.

  • • Taenia solium (mostly humans) → C. cellulosae mostly in muscle of domestic or wild pigs


    (Cysticercus cellulosae) found mainly in the heart and skeletal muscles of domestic or wild pigs

  • • Taenia ovis (many carnivores) → C. ovis mostly in muscles of sheep and goats


    C. ovis is the intermediate stage of Taenia ovis. The adult stage

    is only found in large canids (dog/wolf) that prey on sheep or

    consume offal from the slaughter of sheep

    C. ovis in sheep muscle

  • • Taenia serialis (canids) → Coenurus serialis in subQ & intramuscular tissues of mostly rabbits

    → rarely causes cerebral coenurosis in cats & humans


    Coenurus in fascia/muscle of hind limb of rabbit

  • Sarcocystosis

  • • generally too small to be visible grossly and generally no inflammatory reaction to the cysts

    Histologic section of muscle from a bovine shows cross-section of

    myofiber containing one Sarcocystis cyst filled with hundreds of


    Note no evidence of an inflammatory response.

    Fig 8-32 (Georgis) Sarcocyst of Sarcocystis muris in longitudinal

    section of skeletal muscle of a mouse Georgis' Parasitology for Veterinarians, 10th ed.


  • Sarcocystosis

    • exception #1 - there are some large sarcocysts of birds & mammals

    Large grossly visible sarcocysts are often seen in waterfowl. They can be few (left) or many (right)

  • Sarcocytis gigantea in esophagus and diaphragm of a sheep; about the size of a grain of rice

    Int J Mol Cell Med 2014; 3 (9) 1-6


    • exception #1 - there are some large sarcocysts of birds & mammals

  • Sarcocystis aucheniae cysts in llama meat. Note several macroscopic cysts (arrows). Merck Vet Manual - Courtesy of Dr. Gaston Moré.


    • exception #1 - there are some large sarcocysts of birds & mammals

  • • exception #2 – occasionally there is an inflammatory reaction to the cysts

    • cysts rupture in small % of cattle & swine → focal necrosis & eosinophilic granuloma

    • at slaughter see yellow-green foci, a few mm in diameter in skeletal & heart muscle

    Note multifocal eosinophilic myositis seen in bovine muscle

    at slaughter. Note, microscopically (inset) that the

    inflammatory response is centered on a degenerating

    sarcocytis cyst within a myofiber.


  • • exception #2 – occasionally there is an inflammatory reaction to the cysts

    • also, small % of cattle at slaughter have locally extensive green discoloration of muscles

    • microscopically see severe eosinophilic myositis; (HS reaction to sarcocysts?)

    Note locally extensive grey-green discoloration of muscle from beef cow found at

    slaugter (no clinical signs); this is characteristic of eosinophilic myositis of cattle Microscopically see expansion of the interstitium and effacement of myofibers

    with a massive infiltrate of predominately eosinophils.