Susceptibility to cephalosporins of bacteria causing intramammary infections in dairy cows with a high somatic cell count in Germany

Mastitis is probably the most common cause for the antibiotic treatment of lactating dairy cows. Cephalosporins and other β-lactams are the most commonly used antimicrobial drugs used to treat bovine mastitis. The first objective of this study was to investigate the in vitro susceptibility of bacteria isolated from cows with elevated somatic cell count (SCC) to first (cefalonium, cefapirin) and fourth (cefquinome) generation cephalosporins. The data was also used to evaluate regional differences.

Farms in the North-West (18), the East (6) and in the South (19) of the country were randomly selected out of a pool that met the selection criteria (e.g. yield above average, participation in DHI and an average bulk milk SCC >150,000 cells/ml). Those farms were visited during the week after a DHI test and quarter foremilk samples were taken from all cows with SCC >200,000 cells/ml and no signs of clinical mastitis. The samples were sent to Hannover University for bacteriologic culturing. The antimicrobial sensitivity of the resulting pathogens was determined for cefalonium, cefapirin, and cefquinome by defining minimal inhibitory concentrations via microdilution method.

In total, 1,635 bacterial isolates were cultured from 6,936 milk samples. The most frequently isolated pathogens were in descending order: Staph. aureus, coagulase-negative staphylococci (CNS), and coryneform bacteria. Only 69 isolates of gram negatives were found.

The MIC of all cephalosporines found in this study were higher than reported from studies in clinical mastitis in Germany. Reason for this difference might be that subclinical mastitis tends to become chronic and these cows are more often repetitively treated than cows with clinical mastitis. This could lead to an increase of antibiotic resistance in pathogens causing subclinical mastitis.

Significant regional differences of the MIC were only detected for CNS. However, the antimicrobial activities against staphylococci differed significantly between the tested cephalosporins. The MIC of the fourth generation cephalosporin cefquinome against staphylococci were higher than the MIC of the tested cephalosporins of the first generation. Therefore, the results of the present study indicate that it would be wise not to use cefquinome as first choice for the therapy of bovine subclinical mastitis caused by staphylococci.

Susceptibility to cephalosporins of bacteria causing intramammary infections in dairy cows with a high somatic cell count in Germany. Wente et al. Preventive Veterinary Medicine, 2016

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