The therapy frequency of antibiotics and phenotypical resistance of Escherichia coli in calf rearing sites in Germany

Abstract

Introduction: The association between antibiotic use and the occurrence of resistant bacteria is a global health problem and is subject to enormous efforts at national and international levels. Within the scope of the study "KAbMon", the resistance situation as well as the use of antibiotics in calf rearing farms in Germany was investigated. We hypothesized that the levels of resistance are associated with certain calf keeping farm types, such as pre-weaned calf farms, animal groups, and therapy frequency.

Methods: In total, 95 calf keeping farms were visited between October 2019 and April 2021. At each farm, up to three pooled fecal samples (10 freshly released feces each) were collected. One sample was taken in the youngest calf group, another in the oldest calf group, and one in the hospital box, if available. Escherichia coli was isolated from non-selective MacConkey agar. The therapy frequency reflects the average number of treatment days per calf in a half-year, while the resistance score is the sum of the relative minimum inhibitory concentration per substance over all 10 tested substances.

Results: The 1781 isolates from 178 samples showed high resistance rates against sulfamethoxazole (82%), tetracycline (49%), and ampicillin (40%). High resistance scores were mainly found in pre-weaned calf farms (purchasing calves from 2 weeks of life) and in the youngest animals. The therapy frequency showed an almost linear relationship with the resistance scores, and the age at purchase was negatively related to the resistance score.

Discussion: The high use of antimicrobials of young calves might be associated with a high risk for infectious diseases and might indicate that the current system of crowding 14-day-old calves from different farms in one group is not optimal. Further efforts are necessary to educate and motivate the calf keepers to ensure highest levels of hygiene and management as well as animal welfare conditions and to increase animal health.

Keywords: AMR score; antibiotics use; antimicrobials; calves; therapy frequency.

Figure 1

Structure of calf raising systems in Germany. Red, dairy or suckler cow farms (closed farming systems); green, farms raising pre-weaned calves; blue, farms raising weaned calves.

Figure 2

Boxplots of AMR score per isolate in different farm types (n = 1781). The box represents the interquartile range (central 50% of the data), the solid vertical line represents the median, and ◊ represents the mean. Dots are >1.5 standard deviations from the mean.

Figure 3

Boxplots of AMR score in the youngest and the oldest animal groups of dairy farms [n = 430 isolates (230 youngest, 200 oldest)], pre-weaned calf farms [n = 781 isolates (400 youngest, 381 oldest)], and weaned calf farms [n = 390 isolates (190 youngest, 200 oldest)]. The box represents the interquartile range (central 50% of the data), the solid vertical line represents the median, and ◊ represents the mean. Dots are >1.5 standard deviations from the mean.

Figure 4

Boxplots of AMR score in the hospital boxes [n = 170 isolates (120 pre-weaned calf, 50 weaned calf farms)]. The box represents the interquartile range (central 50% of the data), the solid vertical line represents the median, and ◊ represents the mean. Dots are values >1.5 standard deviations from the mean.

Figure 5

Association between the AMR score and the current therapy frequency: all farms, dairy farms, pre-weaned calf farms, weaned calf farms; n = 178 samples. Displayed is the estimate (blue line) and the 95% confidence intervals (dark gray area).

Figure 6

Association between AMR score and median age at purchase in days in farms with pre-weaned calves and weaned calves (n = 134 samples). Displayed is the estimate (blue line) and the 95% confidence intervals (dark gray area).