Four-month-old Hampshire ewe (Ovis aries).One of a number of young meat sheep showing neurological signs and hyperthermia. This sheep progressed through phases of straight legs, muscle fasciculations, and recumbency. A neurological examination revealed cranial nerves normal, mentation normal, recumbency, good muscle tone, head turn to left, normal to increased muscle tone. Localized to cervical spinal cord. This sheep was treated with flunixin and antibiotics with no response by owner.

Gross Description:  

Bilaterally the superficial pectoralis muscle is green-black, dry, and firm with surrounding red tissue in an oval area measuring 8 x 5 x 1cm on the right and 7 x 6 x 1cm on the left (Figs1,2.: severe, myonecrosis and hemorrhage). The left quadriceps and the left lateral head of the triceps are irregularly pale pink to gray (Fig3.: myodegeneration/myonecrosis).

Numerous Hemonchus and Trichostrongyles are in the abomasum.

Histopathologic Description:

Longitudinal and cross sections of triceps muscle are examined.  On cross section, round, large sometimes hyalinized, hypereosinophilic or flocculant fibers are seen with central, large nuclei.  Some fibers remain as aggregates of activated satellite cells in a shrunken endomysium.  On longitudinal section, swollen, hyalinized, broken, flocculent fibers with myocyte nuclear loss and contraction bands are prominent.  Satellite cells are activated and proliferating with a large vesiculate chromatin and blue-grey cytoplasm.  Macrophages and small clusters of degenerate neutrophils enter pockets of broken fibers.  Capillaries are lined by endothelial cells with hypertrophied nuclei.

Morphologic Diagnosis:  

Subacute, monophasic myopathy with severe myofiber necrosis and satellite cell activation and early regeneration.

Lab Results:  

No monensin or lasalocid in rumen contents. Feed analysis: 3.4% lasalocid (this 10,000 times the legal limit of 30g/ton, 33mg/kg, 33ppm).


Ionophore toxicosis

Contributor Comment:  

The section shows a nice monophasic toxic myopathy.  Ionophores are added to feeds for growth promotion and as a coccidiostat.6,9   However, if toxic doses are achieved, they permit free cationic movement, especially calcium into myocytes and necrosis.  Monogastrics such as the horse and dog are more susceptible.

This breeder had been having a chronic, sporadic, neurologic problem in the flock of show sheep.  It affected sheep taken off pastured and “pushed” nutritionally for the show season.  Upon opening this ewe, it was obvious that a myopathy was the problem, and given the history, we analyzed the rumen content (negative for ionophores additives), and then, the feed from the bag used to fed these ewes was tested (toxic levels of lasalocid).  The levels of lasalocid the feed were extremely high, and it is presumed a pocket of unmixed, lasalocid salt was included in the test sample. In some reports2 and in this contributor’s experience, once the ionophore is over a certain con-centration, these additives are unpalatable, and animals refuse the feed rather than consume myotoxic levels.  Unfortunately, toxicity studies are based on bolus feedings.1,4

In spontaneous, ionophore (usually monensin) intoxications of sheep2,7,8,9,10  display an acute onset of signs of anorexia, dyspnea, muscle weakness, ataxia, a stiff gait and deaths follow feed changes.  Firm or atrophic rear limb muscles are sometimes reported.  A variety of autopsy lesions are reported including: cavitary effusions, pale-streaked hearts, diarrhea and pale-streaked skeletal muscles especially semi-membranosus and semitendinosus.  While some acute gastrointestinal lesions are reported, consistent macroscopic and/or histologic lesions are in cardiac and skeletal muscles.  In ovine monensin toxicity studies1,4, it was commented that acute myopathies were not visible in H&E-stained sections, but that lesions were demonstrable using electron microscopy.  Early ultra-structure changes include mitochondrial swelling and myofibrillar disarray.  Chronic histologic lesions include atrophy, fibrosis, and calcification.  The severity of the present case is impressive, and the minimal cardiac lesion is unexplained.  Is it possible that hypoxia from hemonchosis-exacerbated lesions?

Finally, many ionophore intoxications present as CNS disease.  Although in light of the muscle lesions, the signs could be explained as muscular pain and weakness, we should remember that an ionophore neuropathy is observed in some toxicity trials.5 Our ewe had no lesions in the sciatic or femoral nerves, but some conduction problems may be occurring.

JPC Diagnosis:  

Diagnosis: Skeletal muscle: Degeneration and necrosis, diffuse, severe, Hampshire ewe, Aries ovis.

Conference Comment:  

As mentioned by the contributor, this case is an excellent example of extensive monophasic skeletal muscle necrosis. Classification of muscle degeneration and necrosis is based on the distribution of the lesion and the duration of the insult. Characterization of the type of skeletal muscle necrosis gives insight on the potential cause and helps narrow down the list of differential diagnoses. As a result, a classification scheme breaks muscle necrosis into four broad categories, including focal monophasic, multifocal monophasic, focal polyphasic, and multifocal polyphasic. Focal monophasic is the result of a single mechanical injury, such as intramuscular injection or focal trauma. This case is representative of the multifocal monophasic pattern, which is caused by a single massive exposure of myotoxic drugs, such as ionophores (monensin and lasalocid), or metabolic disorder.11 Exertional capture myopathy also produces widespread monophasic skeletal muscle necrosis with a similar histologic appearance to this case. Polyphasic reactions are the result of repeated and ongoing skeletal muscle insult occurring over a prolonged period as a result of repeated trauma (focal) or nutritional deficiencies, inflammatory myopathies, or genetic disorders (multifocal). Skeletal muscle regeneration, mineralization, and deposition of fibrous connective tissue, all key features of polyphasic necrosis, are not prominent in this case.3,11 Nice examples of multinucleated satellite cells are scattered throughout this tissue section and represent only the beginning stages of regeneration.

Conference participants briefly reviewed the stages of skeletal muscle necrosis, re-generation, and repair. Myofibers are long and multinucleated and thus often undergo segmental necrosis rather than necrosis of the entire muscle fiber; however, extreme pressure, trauma, or ischemia can produce global myofiber necrosis. Necrosis is commonly triggered by increased intracellular calcium concentration, often released from high levels stored in the sarcoplasmic reticulum. Initially, segmental changes are represented by myofiber hyper-contraction, and cross sections appear large and dark with hyalinization and loss of cross striations. Further insult results in sarcoplasmic fragmentation that can lead to dystrophic myofiber mineralization, often seen in chronic myopathies.3,11

Skeletal myofibers are classified as permanent cells and are not capable of cell division. As a result, skeletal muscle regeneration depends on the activation of satellite cells, normally resting between the sarcolemma and the basement membrane. These cells are highly resistant to injury and are activated by necrosis of adjacent myofibers.3,11 Satellite cells begin proliferation and differentiation into myoblasts in the early stages of skeletal muscle regeneration. Concurrently, macro-phages migrate from the peripheral blood and phagocytose necrotic debris leaving a potential space within the damaged muscle. The initial infiltrating macrophages are of the M1 inflammatory phenotype but later switch to the M2 anti-inflammatory phenotype.11 If the basement membrane is intact, the potential space is filled by a scaffold, called the sarcolemmal tube, which prevents the local migration of fibroblasts and instead acts as a guide for proliferating myoblasts. Within the sarcolemmal tubes, satellite cells, known as activated myoblasts at this stage, can be observed undergoing mitoses. Within hours, sarcolemmal tubes fuse end-to-end and form myotubes that eventually mature into skeletal myofibers over the course of a few days. Initial infiltrating M1 macrophages are tough to stimulate proliferation of the myoblasts, while M2 macrophages promote the formation of the myotubes.3,11

In contrast, if large enough numbers of satellite cells are killed and if the basement membrane is destroyed, the sarcolemmal tube is not formed, and there is no proliferation of myoblasts. This allows the influx of fibroblasts into the areas of necrosis resulting in healing by fibrosis rather than regeneration. Additionally, in cases where there is disruption of the basement membrane but satellite cells are still viable, regeneration is disorganized and ineffective due to disorganization of proliferating myotubes. This is typified by the presence of muscle giant cells (large, pleomorphic multinucleated giant myoblastic cells) and fibrous connective tissue.3,11


1.      Anderson TD, Van Alstine WG, Ficken MD, Miskimins DW, Carson TL, Osweiler GD. Acute monensin toxocosis in sheep: Light and electronmicroscopic changes. Am J Vet Res. 1984; 45:1142-1147.

2.      Bourque JG, Smart M, Wobeser G. Monensin toxicity in lambs. Can Vet J. 1986; 397-399.

3.      Cooper BJ, Valentine BA. Muscle and tendon. In: Maxie MG ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. Philadelphia, PA: Elsevier; 2016:180-185.

4.      Confer AW, Reavis DU, Panciera RJ. Light and electron microscopic changes in cardiac and skeletal muscle of sheep with experimental monensin toxicosis.  Vet Pathol. 1983; 20:590-602.

5.      Gregory DG, Vanhooser SL, Stair EL. Light and electron microscopic lesions of broiler cickens due to roxarsone and lasalocid toxicoses.  Avian Dis. 1995; 39:408-416.

6.      Horton GMJ, Stockdale PHG. Lasalocid and monensin in finishing diets for early weaned lambs with naturally occurring coccidiosis. Am J Vet Res. 1981; 42:433-436.

7.      Jones A. Monensin toxicosis in 2 sheep flocks. Can Vet J. 2001; 42:135-136.

8.      Mendes O, Mohamed F, Gull T, de la Concha Bermejello. Monensin poisoning in a sheep flock. Sheep and Goat Res J. 2003; 18: 109-113.

9.      Novilla MN. The veterinary importance of the toxic syndrome induced by ionophores. Vet and Hum Toxicol. 1992; 34: 66-70.

10.  Nation PN, Crowe SP, Harries WN. Clinical signs and pathology of accidental monensin poisoning in sheep. Can Vet J. 1982; 23: 323-326.

11.  Valentine BA. Skeletal muscle. In: McGavin MD,ed. Pathologic basis of Veterinary Disease. 6th ed.  St. Louis, MO: Elsevier Mosby; 2017:922-926.

Click the slide to view.

1-1. Skeletal muscle, sheep.

1-2. Skeletal muscle, sheep.

1-3. Abomasum, sheep:

1-4. Skeletal muscle, sheep.

1-5. Skeletal muscle, sheep.

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