JPC SYSTEMIC PATHOLOGY
Signalment (JPC #75-10): Neonatal Chester white piglet
HISTORY: Six littermates of this piglet were stillborn and 6 more died within a few hours of birth. Remaining piglets, including this one, were unable to stand or adduct their hind legs, which extended forward and laterally (“splay legged”). Foreleg involvement was also seen in some piglets. Gross lesions were not evident at necropsy.
MICROSCOPIC DESCRIPTION: Skeletal muscle: Diffusely myofibers are irregularly sized and contain decreased numbers of myofibrils that are aggregated centrally or eccentrically and surrounded by clear space. Diffusely, myocytes are pale, lack cross-striations and frequently have large nuclei with vesiculate chromatin.
MORPHOLOGIC DIAGNOSIS: Skeletal muscle: Myofibrillar hypoplasia, diffuse, moderate, Chester white, porcine.
CONDITION: Myofibrillar hypoplasia
SYNONYMS: Splayleg, spraddle-leg
- Congenital disorder of neonatal piglets, resulting in lateral splaying of legs (abduction)
- Transient effect on locomotion with survivors normalizing within two weeks
- Incidence fluctuates, but frequently appears as a farm or regional outbreak
- Disease incidence within litters is variable, and consecutive litters may have the disease
- Unknown; proposed, but unproven etiologies: Genetic predisposition, infectious agents; nutritional causes, mycotoxins and potassium-sodium imbalances
- Most prevalent in neonatal Landrace and other large white breed piglets
- Males predisposed
- Usually affects the semitendinosus, longissimus dorsi, and triceps muscles
- In nomal piglets:
- Intrauterine skeletal muscle development occurs by a “stem line-template” method
- A large type 1 muscle fiber (slow twitch, rich in myoglobin) centrally located within a sub-lobule divides longitudinally to create a smaller daughter fiber with type 2 myofiber characteristics (fast twitch, rich in glycogen) or serves as a template for myoblasts to evolve new fibers
- The central template fiber continues to spawn new fibers and push previously formed fibers to the periphery forming a sublobule of 10-20 fibers
- About the time of birth, the new type 2 fibers change to a type 1 fibers and the original central fiber ceases dividing
- In splayleg piglets:
- About the time of birth, the transition of type 2 fibers to type 1 fibers is delayed (suggests delayed maturation of muscle in utero)
- Postnatal maturation continues, and the fibers complete the transition by day 7-8, there is rapid increase in muscle fiber size, and clinical signs disappear
- As piglets recover, their muscles appear similar to those of normal neonates
TYPICAL CLINICAL FINDINGS:
- Signs, characterized primarily by muscle weakness, are present at birth or by 2-3 hours after birth
- Hindlegs or all four legs are laterally extended
- Pigs are unable to adduct their limbs and propel by pushing against ground using pelvic limbs
- Animals are mentally alert and otherwise normal
- Spontaneously resolves in 1-2 weeks if animal survives
TYPICAL GROSS FINDINGS:
- Affected muscles are flabby, soft and pale
- All four legs extend out
- Flattened sternum due to propelling against ground
TYPICAL MICROSCOPIC FINDINGS:
- Myofibers have a reduced diameter
- Individual muscle fibers with abnormally small mass of myofibrils resulting in pale staining with eosin
- The remainder of the muscle fiber sheath has large clear spaces (glycogen) and often contains eosinophilic proteinaceous fluid
- PAS staining reveals cytoplasmic glycogen staining
- Occasional fibers may exhibit segmental myolysis
- These changes may occur in some piglets without clinical splayleg
- The muscles typically involved include the longissimus, semitendinosus and triceps groups
- Z-line streaming: Segmental Z-line distortion with dispersal of electron-dense material
- Loss of register and alignment of sarcomeres and thinning of myofibrils
- Myofilament bundles fill only part of the cytoplasm in the muscle cell, are occasionally split, and have irregular cross striations
- The remaining space is filled with granular material consisting of glycogen, ribosomes and lysosomes
- Splayleg conditions (without myofibrillar hypoplasia)
- Congenital spinal abnormalities: Dysraphism, arthrogryposis, hydromyelia, syringomyelia, and other abnormalities of the dorsal, central, or ventral gray matter and ventral median fissure (reduced myelination of the lumbar spinal may play a role)
- Iron injection myopathy
- Cattle: Myofibrillar hypoplasia reported in a 6 week old calf with similar lesions to those in pigs
- Skeletal muscle hypoplasia represents the only significant lesion in peripheral organs of ruminants infected with Schmallenberg virus during gestation
- Hypoplastic muscular changes are a characteristic finding in lambs and calves after natural or experimental infections with Orthobunyaviruses, as well as in hamsters and chicken embryos infected experimentally with Akabane virus
- Similary syndrome described in puppies (“swimmer pups”) but often resolves with specific clinical therapy
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- Olson EJ, Carlson CS. Bones, Joints, Tendons, and Ligaments. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2017:945.
- Radostits OM, Gay CC, Blood DC, Hinchcliff KW. Veterinary Medicine, a Textbook of the Diseases of Cattle, Sheep, Pigs, Goats and Horses. 9th ed. Philadelphia, PA: W B Saunders; 2000:1800-1801.
- Seehusen F, Hahn K, Herder V, Weigand M, Habierski A, Gerhauser I, Wohlsein P, Peters M, Varela M, Palmarini M, Baumgärtner W. Skeletal muscle hypoplasia represents the only significant lesion in peripheral organs of ruminants infected with Schmallenberg virus during gestation. J Comp Pathol. 2014; 151(2-3):148-52.