The pathogenesis of acute bovine pulmonary oedema and
emphysema (ABPE) is related to the ruminal formation of
3-methylindole (3MI) from
L-tryptophan (TRP), a naturally occurring
amino acid and constituent of forage. The objectives of the present study were to determine whether
monensin and
lasalocid, both
polyether antibiotics, were effective in reducing ruminal conversion of TRP to 3MI in vivo and to confirm that reduction in ruminal conversion of TRP to 3MI prevented
tryptophan induced ABPE. Sixteen mature Hereford cows were assigned to one of four groups and given TRP to induce ABPE. Group 1 was given 100 mg
monensin orally twice daily starting one day before and ending four days after TRP dosing. Group 2 was given 200 mg
monensin once daily and group 3 was given 100 mg
lasalocid twice daily. Group 4, the control, was given only TRP without further treatment. All control cows developed clinical signs of respiratory disease and lesions of ABPE; one control cow died of ABPE. Mean ruminal 3MI concentrations in control cows reached a peak of 36.4 micrograms per ml. Clinical signs of
pulmonary disease appeared in two cows treated with
lasalocid and one died. Mean ruminal 3MI in these animals peaked at 38.8 micrograms per ml. No clinical signs of respiratory disease were observed in any of the
monensin treated cows and at necropsy there were no pulmonary lesions of ABPE. Mean ruminal 3MI concentrations in
monensin treated cows did not exceed 8.9 micrograms per ml. In all groups plasma 3MI concentrations generally reflected ruminal 3MI concentrations but at lower concentrations. The results of this experiment demonstrate that reduction in ruminal 3MI formation by
monensin prevents
tryptophan induced ABPE.