Prevalence of mastitis pathogens in South African pasture-based and total mixed ration-based dairies during 2008 and 2013

Recent years have seen a change in the relative prevalence of environmental and contagious intramammary pathogens, as well as a change in the relative number of total mixed ration (TMR)-based and pasture (PAS)-based dairies in South Africa. The objectives of the study were to determine and compare the prevalence of mastitis pathogens in TMR and PAS dairies in South Africa during 2008 and 2013; furthermore, the within-herd prevalence of Streptococcus uberis in Str. uberis-positive herds was determined and compared. The prevalence of each pathogen, as well as the within-herd prevalence of Str. uberis, were compared between the two years and the two management systems using bacterial culture results from routinely collected composite cow milk samples submitted to the Onderstepoort Milk Laboratory, Faculty of Veterinary Science, University of Pretoria. Coagulase-negative staphylococci had the highest prevalence in both TMR and PAS dairies for both 2008 (29.60% [95.00% CI: 28.80% - 30.40%] and 26.90% [95.00% CI: 25.50% - 28.30%], respectively) and 2013 (20.20% [95.00% CI: 19.30% - 21.10%] and 22.70% [95.00% CI: 22.20% - 23.10%], respectively), which decreased significantly from 2008 to 2013 in both TMR and PAS dairies (p < 0.001). Streptococcus uberis showed an increase in prevalence in both TMR (p = 0.002) and PAS dairies (p = 0.001) from 2008 (2.36% [95.00% CI: 2.10% - 2.65%] and 2.63% [95.00% CI: 2.16% - 3.16%], respectively) to 2013 (3.10% [95.00% CI: 2.72% - 3.51%] and 3.64% [95.00% CI: 3.45% - 3.83%], respectively). Staphylococcus aureusshowed a significant decrease in both TMR (p = 0.011) and PAS (p < 0.001) dairies from 2008 (4.71% [95.00% CI: 4.34% - 5.10%] and 5.62% [95.00% CI: 4.94% - 6.36%], respectively) to 2013 (3.95% [95.00% CI: 3.52% - 4.40%] and 1.71% [95.00% CI: 1.58% - 1.84%], respectively). The median within-herd prevalence of Str. uberis for the combined dairy systems showed a significant increase from 2008 (1.72% [IQR: 0.88% - 5.00%]) to 2013 (3.10% [IQR: 1.72% - 4.70%]) (p < 0.001). Statistically significant differences were found in the prevalence of most of the major contagious and environmental mastitis pathogens between 2008 and 2013 and between TMR and PAS dairies. The within-herd prevalence of Str. uberis increased from 2008 to 2013, with the highest within-herd prevalence in PAS dairies in 2013.