PREVALENCE OF MASTITIS ORGANISMS IN PASTURE-BASED AND TMR SYSTEMS.

In recent years, there has been an increase in the prevalence of environmental pathogens as a cause of within udder infections in dairy cattle. Increases in prevalence could potentially be ascribed to improved control methods for contagious pathogens, difficulties in controlling pathogens from an environmental reservoir and the ability of Streptococcus uberis and Escherichia coli to persist in the udder. Streptococcus uberisshowed an increase in prevalence over an 11-year study period (1996–2007) in South African dairies. A further increase in prevalence of environmental within udder infection is evident in data collected from cow milk samples cultured between 2008 and 2012. During the same period, there was an overall increase in the number of pasture (PAS)-based dairy farms compared to dairy farms feeding total mixed rations (TMRs) in South Africa. In association, the spreading of dairy effluent (manure) to fertilise pastures has become more popular and this practice has not been investigated as a potential risk for the prevalence of Str. uberis. The objective of the study by the authors referred to below, was to use retrospective culture results from composite cow milk samples to estimate and compare the prevalence of mastitis pathogens in TMR and PAS systems between the years 2008 and 2013. Secondly, the within-herd prevalence for Str. uberis was estimated and compared between the two systems for 2008 and 2013, as it was deemed important to determine whether or not the number of herds that were regarded as Str. uberis - positive were on the rise.  

The prevalence of each pathogen and the within-herd prevalence of Str. uberis were compared between the two years (2008 vs 2013)and the two management systems (TMR vs PAS) using bacterial culture results from routinely collected composite cow milk samples submitted to the Onderstepoort Milk Laboratory.                                                                   

Coagulase-negative staphylococci had the highest prevalence in both TMR and PAS dairy farms for both 2008 (29.60% and 26.90% respectively) and 2013 (20.20% and 22.70% respectively), but decreased from 2008 to 2013 in both TMR and PAS dairies (p < 0.001). Streptococcus uberis in contrast, increased in prevalence in both TMR and PAS dairy farms from 2008 (2.36% and 2.63% respectively) to 2013 (3.10% and 3.64% respectively). Staphylococcus aureus decreased in both TMR and PAS dairy farms from 2008 (4.71% and 5.62% respectively) to 2013 (3.95% and 1.71%respectively). The median within-herd prevalence of Str. uberis for the combined dairy systems showed a significant increase from 2008 (1.72%) to 2013 (3.10%). Statistically significant differences were also found in the prevalence of most of the major contagious and environmental mastitis pathogens between 2008 and 2013 and between TMR and PAS dairy farms. The within-herd prevalence of Str. uberis increased from 2008 to 2013, with the highest within-herd prevalence in PAS dairy farms in 2013. 

In conclusion, differences in the prevalence of the mastitis pathogens were found between years and farming systems. Coagulase-negative staphylococci were the most prevalent pathogens in both TMR and PAS dairy farms during 2008 and 2013. Staphylococcus aureus decreased in prevalence between 2008 and 2013, whereas Streptococcus agalactiae showed a steep increase in prevalence during the same period, potentially because of localised outbreaks. Streptococcus uberis increased in prevalence, both overall and within affected herds, with the highest prevalence in PAS dairy farms during 2013. As a consequence, the results suggest treatment regimes also need to be adapted to maintain efficacy. 

Reference: 

Blignaut, D., Thompson, P. & Petzer, I-M., 2018, ‘Prevalence of mastitis pathogens in South African pasture-based and total mixed ration-based dairies during 2008 and 2013’, Onderstepoort Journal of Veterinary Research 85(1), a1482. https://doi.org/ 10.4102/ojvr.v85i1.1482