Neurological Disease in Lambs Associated with Exposure to High Environmental Temperature and Humidity

Background We detected a pattern of lambs presenting with hyperthermia and neurological signs during the summer. Objectives The main objectives of this study were to compare clinical findings and results of diagnostic testing and to identify a potential etiology. Animals Fifteen clinical cases of lambs less than 12 months of age presenting with neurological signs, tachypnea, and hyperthermia over 4 summers. Methods Retrospective case series. Medical records were searched for lambs less than 12 months of age that presented with neurological signs including the following: kyphosis, pelvic limb hyperextension, treading of feet, muscle tremors and recumbency, and hyperthermia of greater than 104°F. A grading system was established to describe severity of presenting neurological signs. Weather data were collected from weather stations near the farm of origin for 3 days prior to presentation. Results The lambs were from 7 flocks in central Texas. All cases occurred between July and September, with a median heat index of 90.5 for the 3 days before presentation. Complete blood count, serum chemistry, necropsy examination, rumen content, virology, brain MRI, liver copper, selenium, and vitamin E failed to identify a consistent etiology for the signs presented. The only common factor was high heat and humidity. Histopathological examination identified axonal degeneration and skeletal muscle necrosis in some lambs. Conclusions and Clinical Importance These clinical cases appeared similar to the Australian disease humpyback and indicate that lambs exposed to high environmental temperatures and humidity might be at risk of developing the described clinical presentation.