Seven hundred and fifty‐four bulk milk samples from a total of 9,066 tested by the Aberdeen and District Milk Marketing Board in 1984 had total bacterial counts greater than 45,000 per ml. Of the failed samples, 43.8% had more mastitis‐related bacteria present than non‐mastitis types. More than a quarter of the failed samples had almost pure cultures of Streptococcus agalactiae, Str. dysgalactiae, Str. uberis, Staphylococcus aureus or coagulase‐negative staphylococci. No relation was found between the failed samples resulting from subclinical mastitis infections and the somatic cell count of the bulk milk. Seventy‐five per cent of the financial penalties imposed on producers were attributable to mastitis bacteria in bulk milk samples.
In a two-year study of the incidence of subclinical mastitis in two beef suckler herds over 2400 quarter milk samples from 180 cows were examined. Somatic cell counts and total bacterial counts were carried out and infecting organisms were isolated on sheep blood agar. Results of these tests indicated that: (a) in spite of extremely dirty udders, fewer contaminants were found in the suckler cow milk samples than in a group of over 1700 samples from typical dairy herds; (b) 18 per cent of all quarter milk samples were infected; (c) 67 per cent of all infections were due to staphylococci and 20 per cent to streptococci; (d) 56 per cent of staphylococcal infections were associated with cell counts less than 500,000 per ml compared with 39 per cent of other infections; (e) 70 per cent of samples had somatic cell counts less than 500,000 per ml. (f) 20 per cent of samples had somatic cell counts over 1 million per ml; (g) only 36 per cent of samples with cell counts over 1 million per ml were associated with udder infections; (h) high cell counts and udder infections were more frequent in early than in mid-lactation. Due to a shortage of animals in the late lactation little evidence was available to support the contention that cell counts rise and the numbers of infected quarters increase towards the end of lactation.
Since the introduction by Milk Marketing Boards throughout the UK of automated systems which enable total bacterial counts to be carried out economically and at frequent intervals on bulk milk consignments, streptococci associated with subclinical mastitis are more readily detected. In many cases the presence of these organisms has resulted in heavy financial penalties being imposed. We describe an improved selective medium which could assist in the rapid detection of such cases and allow prompt remedial action to be taken.
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