Pathophysiologic study of 3-methylindole-induced pulmonary toxicosis in immature cattle

In 5 Friesian calves given 3-methylindole (3-MI) (100 mg/kg once a week for 8 weeks, except calf 4, given a 50 mg/kg dose on weeks 3 to 8), pulmonary function (PF) values and arterial blood gas tensions (PaO2 and PaCO2) were measured 24 hours after dosing was done and were correlated with clinical, biochemical, and pathologic changes. Three of the calves (No. 1, 2, and 3) showed acute respiratory distress syndrome 24 hours after the first 3-MI treatment, with a large increase in respiratory frequency, minute viscous work, and PaCO2 and a large decrease in tidal volume, dynamic lung compliance, and PaCO2. They died 36, 38, and 84 hours after dosing. Pulmonary function changes were compatible with the severe pulmonary edema and alveolar damage observed at necropsy. The 2 other calves, after they were given the 1st dose, showed only subacute respiratory distress syndrome with less severe changes in PF values recorded at 24 hours. Furthermore, they became progressively more tolerant to the 2nd, 3rd, and 4th weekly treatments, and showed base-line PF values after the 5th weekly treatment. Pathologic changes were not observed in lung biopsy material from these 2 animals at 2 and at 12 weeks after the 8th (or last) 3-MI treatment.