Antibiotic and sulpha sensibility tests performed on milk samples obtained from cows showing variable grades of udder inflamation are analized. Tests were conducted by means of plate antibiograms of pre-incubated samples, with readings after 24 hrs. cultures at 37° C. Drugs tes ted were seven antibiotic and three sulfonamides, without previous isolation of pathogen strains, since the main objective of this rutinary controls was aimed to the correct therapy of clinical mastitis. The results of tests were expressed in three cathegories; "Resistence", "Moderate Sensibility" and "Sensibility". Different diameters of the halo of non-growth around the disc was the base to distinguish between both levels of sensibility. The following resistence levels were recorded, expressed in percent of the total of controls for each drug: Penicilin 50,5 per cent; Streptomycin 34,8 per cent; Neomicin 11,7 per cent; Terramycin 45,3 per cent; Tetracicline 16,1 per cent; Aureomycin 24,1 per cent; Cloramphenicol 23,9 per cent; Triple Sulphas 69,5 per cent; Tetracicline-Cloramphenicol (associated) 3,2 per cent. The total of controls registered 3,6 per cent of cases of resistence to all drugs tested. Particular comparisons are made between the three Tetraciclines tested and between the association of two antibiotics and each drug separately. Emphasis is pressed on the permanent variability of the sensibility showed by mastitis causing strains, these being exhibited between cows of the same farm at a given control and between the same group of cows in succesive tests.

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1 Médico Veterinario, Cooperativa Lechera Bío-Bío Ltda., Los Angeles

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