Congenital Pseudomyotonia in Chianina Cattle

This report describes the clinical findings of a congenital pseudo-myotonia observed in ten genetically related Chianina cattle. The most important clinical finding was difficulty in performing muscular activities more intensive than a simple walk at a slow pace. In fact, when stimulated to move faster, muscles stiffened hindering physiological motion. Depending on the intensity and duration of the exercise, the animals either presented a stiff gait, the hind leg “froze up” for a moment, or a “bunny hop” type of gait began. If forced to continue, complete leg rigidity hindered them from advancing and the animals fell over with a rigor-mortis appearance. The hind legs were more affected, but all muscles were strongly stiffened. After a few seconds, the animals regained their ability to jump up and move, as if nothing had happened. Musculature contracture had cleared up. The same situation occurred once again if the animals were subjected to repeated exercise. The animals did not pass out or lose consciousness as in fainting but, instead, remained awake and alert and were not in pain. The animals also experienced general muscular stiffening when startled. Retraction of the bulbi and prominence of the third eyelid were common findings during this kind of crisis. At rest, clinical examination did not disclose any remarkable findings; no muscular abnormalities nor any disturbance of the muscle tropism and growth were observed. Percussion myotonia was absent as well. The clinical aspect of the disease could be considered to be of the same severity in all patients. Routine hematology (carried out in four cases) revealed no abnormalities. Biochemistry (four cases) showed slightly increased levels of creatine kinase, lactate dehydrogenase and aspartate aminotransferase. L-Lactate was normal at rest and after a free-crisis at a slow pace walk, but increased notably after a stronger exercise accompanied by the occurrence of crisis of stiffness. Standard needle electromyography (carried out in two subjects) showed normal spontaneous, insertion and “voluntary” activities and no electrical activity. In particular, there were no myotonic discharges in the muscles after exercise. Motor and sensory nerve conduction velocities were normal. The lifelong history of the disease never showed an increase in exercise-induced disturbances and all animals could be slaughtered. Performances were, however, less satisfactory than usual for the Chianina breed. Genealogical evaluation revealed common ancestors which might suggest a genetic etiology. The above-mentioned clinical findings resemble those occurring in Brody’s disease in humans, a rare autosomal-recessive disorder of muscle function characterized by painless muscle cramping and exercise-induced impairment of muscle relaxation due to a defect of calcium reuptake. To the best of the authors’ knowledge, this is the first report of such a pseudo-myotonia in cattle. Research on the pathogenesis and the eventual mode of inheritance is in progress.