Biochemistry of natural and amprolium-induced polioencephalomalacia in sheep

Polioencephalomalacia (PEM) induced in sheep was compared with the disease found in naturally occurring cases. Blood biochemical indicators measured were pyruvate, lactate, glucose, erythrocyte transketolase (TK) and stimulation of TK by addition of thiamine pyrophosphate (TPP effect). Faeces and rumen contents were assayed for thiaminase activity. The effect of treating affected sheep with thiamine was also noted. It was found that amprolium treatment could induce thrombocytopenia, but once the sheep became accustomed to amprolium in the diet they seemed to be resistant to this effect. In sheep receiving amprolium significant weight losses preceded the onset of clinical signs. Further weight loss continued throughout the recovery period despite removal of amprolium from the diet and treatment with thiamine. Blood glucose was variable, and was elevated only when marked clinical signs were present. Pyruvate and lactate levels showed marked variation throughout the trial. TK values were depressed and TPP effects increased well before the onset of clinical signs, although some naturally occurring cases had normal levels. Faecal thiaminase activity was negligible in all the sheep on the amprolium trial but most field cases had a high level. High faecal thiaminase was observed in about 5% of clinically normal animals from affected flocks. Depression of erythrocyte TK activity coupled with the presence of faecal thiaminase appeared to be the most reliable diagnostic biochemical parameters for PEM. Treatment of PEM affected sheep with thiamine rapidly brought the biochemical status of the animals to normal. However where advanced brain lesions were present the damage was permanent and such sheep treated with thiamine remained partially decorticate.