Prevalence and types of bacteria associated with subclinical mastitis in Bloemfontein dairy herds

Bacteria associated with subclinical mastitis were isolated from machine-milked dairy herds in the Bloemfontein area supplying fresh milk during the period July to December 1980. The 151 quarter milk samples examined, were also subjected to somatic cell counts. Identification of the isolated bacterial strains showed that Staphylococcus aureus was the dominant mastitis-associated organism, constituting 66,4% of all bacteria isolated. Compared with other recent mastitis surveys a low prevalence of classical mastitis streptococci (0,7%) and of Gram-negative bacterial infections (6,3%) was encountered. The Gram-negative bacteria were almost invariably isolated from neglected herds in which the cows were generally in poor condition and the hygienic measures employed were totally inadequate. Other bacterial strains isolated included Corynebacterium bovis (6,3%) and the coagulase negative staphylococci (11,0%). The high somatic cell counts of the quarter milk samples yielding S. aureus, the mastitis streptococci and the Gram-negative bacteria suggested a major pathogenic role for these isolates. The frequent occurrence of C. bovis strains and coagulase negative staphylococci in samples with high somatic cell counts similarly suggested that these organisms were more pathogenic than is generally assumed.