The safety of dimetridazole alone and in conjunction with oxytetracycline in Hereford crossbred steers.

Dimetridazole was given intraruminally alone, and in conjunction with oxytetracycline to healthy, 10-11 month-old Hereford cross-bred steers (n = 6). Intraruminal treatment with dimetridazole was given through a fistula at 75 mg kg-1 daily for 5 d, while the oxytetracycline was injected intramuscularly at 10 mg kg-1 on Days 1 and 3 of the dimetridazole treatment. The animals were observed at various intervals throughout the trial period for adverse reactions, including effects on ruminal activity and motility, changes in live-mass, venous acid-base balance, haematology and ruminal and serum ammonia concentrations. Dimetridazole, either when used alone or in conjunction with oxytetracycline, had a marked effect on ruminal function. Within 6 h of dosing, the ruminal pH fell to below 5, but then returned to pretreatment values over the next 24-48 h. This was followed by the eradication of the ruminal protozoal population in all animals tested and an increase in the methylene blue reduction time to more than 6 min. Ruminal motility remained unaffected throughout this period. During the week of treatment, the mean live-mass of the animals dropped by 20 +/- 9.9 kg in the dimetridazole treated group and by 13.3 +/- 2.8 kg in the animals treated with both dimetridazole and oxytetracycline. A mild to severe watery diarrhoea, which continued for 1 to 2 d, occurred in 4 animals after the first dimetridazole treatment. A compensated metabolic acidosis and an increase in haematocrit were observed. An initial transient rapid rise in rumen ammonia concentrations did not result in a concurrent rise in serum ammonia concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)