Antibacterial drugs used against mastitis in cattle by the systemic route

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Abstract
The veterinarian in clinical practice is often confronted with cases of mastitis that require systemic antibacterial treatment in addition to local treatment. This paper reviews the suitability of drugs available in New Zealand, taking into account their anti-staphylococcal activity, routes of administration, and their ability to attain and maintain therapeutic levels in the udder following systemic administration. The drugs considered include the more common penicillins, aminoglycosides and macrolides; oxytetracyline, chloramphenicol, trimethoprim, and several sulphonamides. The success of systemic therapy against mastitis depends to a large extent on the concentration of antibacterial drug achieved at foci of infection. Passage of drugs across the blood-milk barrier takes place by passive diffusion, and the factors influencing this diffusion are discussed. Whe mastitis is associated with sensitive organisms, penicillin is recommended, although, as with all other antibacterials discussed, the dose used must be higher than normal. For penicillin, doses of the order of 16,500 iu/kg are recommended. The intramuscular injection of oxytetracycline will not produce therapeutic levels in milk but, after intravenous injection of high doses (10 mg/kg), this antibiotic is likely to maintain therapeutic levels in milk over a 24-hour period. As a first choice for the systemic treatment of mastitis, either tylosin or erythromycin is recommended. At a dose rate of 12.5 mg/kg, either will maintain for 24 hours milk levels in excess of the average MICs for staphylococci. Of the sulphonamides, sulphanilamide and sulphadimidine produce the highest milk levels. After intravenous administration at a dose rate of 200 mg/kg, sulphadimidine will maintain therapeutic levels in milk for 12 hours. Although trimethoprim has a very short half-life in cattle which limits its usefulness, it readily enters the milk and a combination of trimethoprim (as a suspension) with sulphadiazine, at a dose rate of 48 mg/kg, might be expected to maintain therapeutic levels in the milk for 12 hours. It is concluded that dihydrostreptomycin, neomycin, chloramphenicol, and the combination sulphadoxine/trimethoprim, are not suitable for the systemic treatment of bovine mastitis.