Discipline: mastitis; Key words: Staphylococcus aureus, subclinical mastitis, antibiotis resistance, producer attitude.

The dairy industry is a major consumer of antibiotics globally and mastitis is the most treated disease of dairy cows. In South Africa, producers have unrestricted access to 12 of 22 registered within-udder antibiotic products without prescription, whereas the remaining 10 registered products are restricted for veterinary use. It is conceivable that the antibiotics available without prescription may be used incorrectly and may contribute to the emergence and/or persistence of antibiotic-resistant strains. 

The proactive udder health program of the Milk Laboratory of the Faculty of Veterinary Science, University of Pretoria, was initiated to reduce the development of resistance. The program is comprehensive and includes routine microbiological and cytological examinations for the whole herd. This allows the early identification of Staphylococcus aureus within-udder infections to facilitate eradication of this organism from a herd. In this study, the authors cited below report on the effectiveness of this program in 20 herds managed consistently over a 10-year period. 

The objective of their study was to investigate trends of resistance of S. aureus to antibiotics in the herds and to identify possible contributing factors. The resistance of S. aureus strains to eight commonly used antibiotics in South Africa from 2001 to 2010 was evaluated. A total of 2532 Staphylococcus aureus isolates were selected from cows with subclinical mastitis from the 20 herds. The isolates were selected from milk samples that had somatic cell counts (SCC) of more than 400 000 cells per mL and were tested for antibiotic resistance using a standard Kirby–Bauer test with published clinical breakpoints. The prevalence of antibiotic resistance was evaluated as a percentage of S. aureus isolates susceptible out of the total numbers for each antibiotic selected per year.

Staphylococcus aureus showed a significant increase in percentage of susceptible isolates over time for all antibiotics tested, except for ampicillin. The overall prevalence of mastitis did not change during the study period. However, the prevalence of mastitis caused by S. aureus (mostly subclinical cases) in the selected herds decreased numerically but not significantly. Reduction in the incidence of antibiotic resistance shown by S. aureus was presumed to be a result of the application of the proactive udder health management program. The fact that the overall prevalence of mastitis was kept stable was possibly because of the influence of the management program in conjunction with the return of infections caused by non-resistant strains.

Conclusions from the study: The results suggest that proactive dairy herd management for mastitis control can have a major beneficial influence on the population of bacteria. This type of information will assist in the development of government and industry policies on access and use of antibiotics and inform users on some general aspects and trends of effective antibiotics. The results also suggest that currently used antibiotics can still be effective, but the attitudes of producers and their staff will play an important role. Whereas it is believed that these results indicate that change in management practices is a valuable tool, these types of interventions are only of value if all role players are properly integrated into the process.    


Karzis, J., Petzer, I-M., Donkin, E.F. & Naidoo, V., 2018. Proactive udder health management in South Africa and monitoring of antibiotic resistance of Staphylococcus aureus in dairy herds from 2001 to 2010.Journal of the South African Veterinary Association 89(0), a1490. jsava.v89i0.1490