Clinical course of early-onset muscle weakness syndrome in 24 Holstein calves in Japan

This study investigated the clinical course, clinical examination findings, and gross necropsy findings in 24 calves diagnosed with early-onset muscle weakness syndrome (MW) in Japan. Seven (29.2%) of the 24 calves remained unable to stand, whereas 17 (70.8%) were able to stand at least once either with or without assistance. One calf is still alive and under observation. Although some calves developed normally with resolved clinical signs, many were slow to stand and had an abnormal gait, suggesting reduced productivity. Neurological examinations revealed decreased patellar tendon reflex and flexion reflexes in both the forelimbs and hind limbs in calves with astasia. Necropsy revealed limb skeletal muscle atrophy in 53.8% calves with astasia and 33.3% calves without astasia. Pneumonia was also frequently observed in both groups. The clinical course varied among the 24 cases, but factors underlying this variability remain unclear. This study provides the first detailed description of MW in Holstein calves in Japan.