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To study antibiotic use in livestock in a temporal context with the development of antimicrobial resistance, long-term changes in antibiotic use must be mapped and their possible causes must be explored. Therefore, the present work assesses the changes in antibiotic use over time in German livestock husbandry. In addition, factors associated with antibiotic use were analyzed to identify possible strategies for further reducing antimicrobial usage. For 2011, 2013 and 2014, antibiotic usage data were collected and examined within the VetCAb project. Three hundred participating pig holdings provided information on their antibiotic use based on obligatory application and delivery forms (ADFs) filled in by their veterinarian as well as information on their current stabling capacities for each production type held. Data on sow, piglet, weaner and fattening pig holdings were described separately, using the semi-annual treatment frequency (TF) to measure antibiotic consumption. Multiple linear mixed models were used to investigate the effects of time, farm size, region and farm management category on the treatment frequency. The study yielded significant time changes with p-values below 0.001 in antibiotic administration with a decreasing median TF in piglets from 3.8 in the first half of 2011 (IQR = 1.1-10.6) to 1.7 in the second half of 2014 (IQR = 0.2-4.5) and in fattening pigs from 5.1 in the first half of 2011 (IQR = 0.2-15.4) to 0.7 in the second half of 2014 (IQR = 0.1-6.7). Meanwhile the TF fluctuated between 8.2 and 12.2 in weaners during the observational period (IQRs between zero (lower quartile) and 37.9 (upper quartile)). Piglet, weaner and fattening pig holdings belonging to the upper third of the holdings in size used significantly more antibiotics than the other holdings investigated. Particularly for weaner and fattening pig holdings, a higher TF was noted for farms without breeding units. The region was only a significant factor in weaners. In conclusion, for 2011, 2013 and 2014, the present study shows a clear reduction in antibiotic treatment frequency in German pig holdings. In addition, the association with various factors such as herd size and farm organization on the antibiotic usage frequency is indisputable. Therefore, these factors should be included in monitoring systems and considered when evaluating intervention measures.

To appreciate the significance of sales data of antibiotics, data on antibiotic consumption (which were collected for selected food-producing animals within the representative VetCAb pilot study in Germany in 2011) were extrapolated for the total German animal population, and subsequently compared to the antibiotic sales data, which were collected throughout Germany in 2011 for the first time and which were published by the Federal Office of Consumer Protection and Food Safety (BVL). The extrapolation was realized based on the average dosage and the number of treatment units determined in the VetCAb pilot project for each active ingredient and animal species. It was extrapolated that 1302 tons of antimicrobial substances were consumed for treating cattle, pigs and broilers--this amount is accompanied by the amount of 1706 tons sold for all animal species. In Germany, tetracyclines and beta-lactams are the most commonly used and sold antimicrobial drugs. In third place are the potentiated sulfonamide, in fourth and fifth place, macrolides and polypeptides. To describe and evaluate the antibiotic usage for the several food-producing animals, detailed information is needed. Therefore, the number of treatments per active ingredient per animal and per day (treatment unit), and the number of treatment days per active ingredient per animal (treatment frequency) were calculated in the VetCAb project. These values represent scientifically collected orientation values for the treatment frequency of food-producing animal with antimicrobial substances in Germany. For example, a fattening pig is treated at 4.2 days (median) with one active ingredient within its 115-day fattening period. A dairy cow is treated at 3.5 days with one active ingredient within one year. A broiler is treated at 10 days with one active ingredient within its 39-day production period.

To be able to analyze the relationship between the level of resistance and the use of antimicrobials, it is necessary to collect detailed data on antimicrobial usage. For this reason, data on antimicrobial use on 495 pig farms from entire Germany were collected and analyzed. In Germany, each application and dispensing of medicines to food-producing animals is documented in detail obligatorily by the veterinarian. This information was collected retrospectively for the year 2011. The analyses undertook separate examinations on the age groups sow, piglet, weaner and fattening pig; both the route of administration and indication per active ingredient, and active ingredient class, were evaluated. In total, 20,374 kg of antimicrobial substances were used in the study population. Tetracyclines were used in highest amounts, followed by beta-lactams, trimethoprim-sulfonamides and macrolides. Concerning the frequency of using an active substance per animal, polypeptides were most commonly administered. In all age groups, respiratory infections were the main indication for using antimicrobials, followed by intestinal diseases in piglets, weaners and fattening pigs and diseases of reproductive organs in sows. Over a period of 100 days, the median number of treatment days with one antimicrobial substance for piglets was 15 days, for weaners about 6 days, for fattening pigs about 4 days and for sows about 1 day. A multifactorial ANOVA was conducted to investigate which factors are associated with the treatment frequency. The factors "veterinarian" and "age group" were related to the treatment frequency, just as the interaction between "veterinarian" and "farm size" as well as the interaction between "veterinarian" and "age group".

In order to represent relationships between the use of antibiotics in food-producing animals and the extend of bacteria with resistance characteristics, it is necessary to have access to information about dispensary and consumption of antibiotic drugs. This does not only require suitable detection methods, but also the establishment of figures which adequately describe the use of antibiotics both on the level of the enterprise, respectively veterinarian, as well as in cumulative form. In various countries different calculations for the analysis of data on antibiotic use are applied, therefore the results are not always comparable.The aim of this publication is to provide an overview of the national and international terminology and variables, to give concrete definitions and to identify those variables most suitable for particular objectives. Hereby a contribution to the standardization of evaluations should be made, to enable a comparison of data on use of antibiotics from various sources of information.