Research Article: Resistance to penicillin of Staphylococcus aureus isolates from cows with high somatic cell counts in organic and conventional dairy herds in Denmark

Date Published: November 24, 2006

Publisher: BioMed Central

Author(s): Torben W Bennedsgaard, Stig M Thamsborg, Frank M Aarestrup, Carsten Enevoldsen, Mette Vaarst, Anna B Christoffersen.


Quarter milk samples from cows with high risk of intramammary infection were examined to determine the prevalence of Staphylococcus aureus (SA) and penicillin resistant SA (SAr) in conventional and organic dairy herds and herds converting to organic farming in a combined longitudinal and cross-sectional study.

20 conventional herds, 18 organic herds that converted before 1995, and 19 herds converting to organic farming in 1999 or 2000 were included in the study. Herds converting to organic farming were sampled three times one year apart; the other herds were sampled once. Risk of infection was estimated based on somatic cell count, milk production, breed, age and lactation stage.

The high-risk cows represented about 49 % of the cows in the herds. The overall prevalence of SA and SAr among these cows was 29% (95% confidence interval: 24%–34%) and 4% (95% confidence interval: 2%–5%) respectively. The prevalence of penicillin resistance among SA infected cows was 12% (95% confidence interval: 6%–19%) when calculated from the first herd visits. No statistically significant differences were observed in the prevalence of SAr or the proportion of isolates resistant to penicillin between herd groups.

The proportion of isolates resistant to penicillin was low compared to studies in other countries except Norway and Sweden. Based on the low prevalence of penicillin resistance of SA, penicillin should still be the first choice of antimicrobial agent for treatment of bovine intramammary infection in Denmark.

Partial Text

Staphylococcus aureus (SA) is the most commonly occurring pathogen in udder quarters with elevated somatic cell counts (SCC) in Denmark and accounts for approximately 50% of the intramammary infections of lactating cows [1]. Experimental infections with SA have shown that infected cows develop high SCC, though both the SCC and the number of bacteria shed in the milk vary considerably both between cows and within quarters over time [2]. The control of SA infections in dairy herds often includes a combination of preventive measures to reduce the number of new infections, dry cow treatment of all cows with antibiotics, treatment of infected animals, and culling of chronically infected animals [3,4]. Frequent use of antibiotic treatment in dairy cows has been proposed to comprise a risk for development of or selection for SA resistant to antibiotics [5]. However, results of susceptibility patterns for commonly used antibiotics indicate that the prevalence of β-lactamase producing SA which are resistant to penicillin seems to have remained at a fairly constant level (40–60%) for the last twenty years. Norway, Sweden and Denmark are exceptions because they have had a consistently lower proportion of penicillin resistant isolates (10–20%) than other countries [6,7]. Comparison of susceptibility data from different surveys is complicated because both the selection of isolates and the methods used for susceptibility testing differ. Often a few clones of SA dominate in the single herd due to the contagious nature of the bacteria. Therefore, surveys only including few herds might provide invalid estimates of the general prevalence [1,8,9]. Similarity of phage types among quarters from the same cow and analysis of infection patterns in the quarters of a cow indicate that the multiple SA isolates from the individual cow are most often a result of an infection from the initially infected gland, and consequently isolates from the same cow cannot be regarded as independent [1,10].

No difference in prevalence of penicillin resistant SA or in the proportion of SA resistant to penicillin was found between conventional and old organic herds or before and after converting to organic farming. The overall prevalence of SAr was low, at about 4% of the cows with high infection risk and the proportion of resistant isolates at about 12%. The low level of resistance makes penicillin a good choice for treatment of intramammary infections in Danish dairy herds. However, based on the changes in prevalence over time and the possible differences in strains causing high SCC and clinical mastitis milk, it can be recommended to monitor the antimicrobial susceptibility on a regular basis. A regular sampling at the herd level will also provide the necessary information for choosing the most effective preventive measures for controlling udder infections in general.

CMT: California Mastitis Test

The author(s) declare that they have no competing interests.

TWB, FMA, SMT and CE have been involved in the initial design of the study and protocols. ABC has been responsible for the microbiological work in the laboratory at the Cattle Health Laboratory. TWB has been the main responsible for data analysis in coorporation with TWB, CE, SMT and FMA. All authors have contributed substantially to the editing of the manuscript.




Leave a Reply

Your email address will not be published.