Results of the antimicrobial agent susceptibility study raised in a representative, cross-sectional monitoring study on a national basis

The use of antimicrobial substances in human and veterinary medicine inevitably results in a selection pressure for drug resistance in exposed bacteria. Preventive measures, apt to avoid the consequent development of new resistances and selection for existing ones, respectively, have to be elaborated. Moreover, it has to be ensured that neither resistant bacteria nor resistance genes are spread to and consequently via the food chain. Respiratory diseases as well as mastitis in dairy cattle belong to the most frequently occurring diseases in food-producing animals. For the first time in Germany, a comprehensive, cross-sectional study into the antimicrobial susceptibility of bacteria associated with these disease patterns in food-producing animals was conducted by the Federal Office of Consumer Protection and Food Safety (BVL) in 2001. The selection of examined bacterial species comprised Pasteurella multocida and Mannheimia haemolytica associated with respiratory disease in pigs, and Escherichia coli, Streptococcus spp. and Staphylococcus spp. causing mastitis in dairy cattle. Bacterial strains were collected following a representative sampling scheme, taking into account the total number of animals in the individual German federal Länder. In an analogous study conducted in 2002/2003, this selection was extended by the indication respiratory disease in juvenile cattle, caused by P. multocida and M. haemolytica, respectively. In comparison with data from 2001, MIC values determined in 2002/2003 suggested significantly lower or higher degrees of drug susceptibility only for a few antimicrobial agents. Comparison was carried out on the basis of bacterial species and individual federal Länder, respectively. Overall, the data raised in both studies revealed substantially lower resistance rates than published for Germany so far. This is particularly true for results from those Länder, whose animal health services had implemented preventive strategies to control infectious diseases. No correlation could be established between differing animal population densities and differences in the prevalence of resistance in corresponding Länder. However, the geographical distribution of occurrence of resistance against β-lactam antimicrobial agents suggests different therapeutic strategies employed in different sized animal flocks. In federal Länder marked by large-scale livestock farming, significantly higher resistance values could be measured for cephalosporins than for penicillins, whereas in Länder with rather traditional farming structures, resistance to penicillins was predominant. Assuming otherwise similar factors of influence on the emergence of resistance, this pattern suggests that cephalosporins are preferably used in large enterprises and penicillins in smaller farms, respectively. Currently, mechanisms effecting changes in antimicrobial resistance are being further investigated in a successive study.