Table 11.

Antimicrobial resistant bacteria in faeces of calves and dairy cows on farms feeding calves milk from treated cows

	Dolejská et al. (2008)	Randall et al. (2014)	Duse et al. (2015)	Kyselková et al. (2015)
Study design	Field studya	Field studya	Field studyb	Field studya
Country	Czech Republic	England/Wales; 2011	Sweden	Czech Republic
Farm	2 dairy farms	3 dairy farms with 200–500 adult milking cows and 30–50 unweaned calves; weaning occurred at 7–9 weeks, in 2 farms only to beef calves	243 dairy farms with a median herd size of 72 cows, ranging from 28 to 1,175 cows	1 dairy farm with 200 cows
Analysis	Observational study on farms feeding calves waste milk; 183 samples from calves and 95 samples from cows analysed	Observational study on 3 farms that about 10 months previously had been positive for both cefquinome and ESBL‐producing Enterobacteriaceae in waste milk. Waste milk from each farm was tested for cefquinome and ESBL‐producing Escherichia coli	Observational study to investigate statistical correlation between treatment from questionnaire and AMR shedding; samples from 729 calves analysed	Observational study on a farm where chlortetracycline (intrauterine suppository) is prophylactically used after each calving; 56 samples from 21 cows; 12 samples from 12 heifers and calves analysed
Feeding calves	Milk and colostrum from cows treated during lactation and dry period	Calves were fed waste milk containing milk or colostrum from cows treated at dry‐off, lactating cows treated with antimicrobials and for which the first milk after treatment was not discarded and milk with high cell counts	Waste colostrum and waste transition milk (from the second milking till the 4th day after calving) from cows treated with antimicrobials at dry‐off Waste milk from cows treated with antimicrobials during lactation	Milk during the withdrawal period from cows treated during lactation
Antimicrobial concentration	Not determined	In samples of waste milk taken from each farm on 3 occasions mean cefquinome concentration was 3,763 μg/kg and the maximum concentration 27,000 μg/kg	Not determined Colostrum might contain only penicillins and aminoglycosides, the only antimicrobials used in dry cow therapy in Sweden	Not determined
Method and criteria applied to interpret resultsc	E. coli, Disk diffusion (Literák et al., 2007) in accordance with CLSI (NCCLS, 2002)	E. coli, plating to ESBL selective and testing representative colonies for CTX‐M ESBL genes, by the method of the British Society for Antimicrobial Chemotherapy (BSAC, 2011 (http://www.bsac.org.uk/)	E. coli, MIC broth microdilution, in accordance with CLSI, 2013a	Polymerase chain reaction (PCR) (calves n = 26; heifers n = 45; cows n = 56)
AMR	12 antimicrobials	CTX‐M positive E. coli	12 antimicrobials	Tetracycline resistance genes: tet(M), tet(Q),tet(W)
Time/animals	Calves – cows age not specified	For each farm, samples from calves and adult cows (n = 90) taken in 2011 Calves were not classified by age, but as on milk machine, receiving colostrum; receiving milk powder; unweaned and weaned	Calves week 1–week 4	Calves (week 2; week 4–6)/–heifers/cows
Result	Calves: 40% AMR E. coli (n = 183). Cows: 3% AMR E. coli (n = 95).	Presumptive/confirmed CTX‐M‐positive E. coli in 33.3% to 74.4% of all faecal samples. For some groups of calves from two farms 100% of faecal samples positive for ESBLs. A greater proportion of faecal samples from calves were positive for CTX‐M‐positive E. coli compared with faecal samples from older animals	Colostrum: no effect on AMR E. coli. Waste milk: significantly more nalidixic and streptomycin res E.coli. Treatment or withdrawal period: no ≠ Older calves had less AMR E. coli	tet (M): calves week 2 log 1–2 >  calves week 4–6 tet(Q), tet(W): calves week 2 +  week 4–6 log 1–2 > heifers, cows
Main relevant results as reported in abstract	‘The prevalence of resistant E. coli in calves compared to adult cattle was much higher and probably was influenced by oral antimicrobial usage in calves, feeding with milk and colostrum from treated cows, as well as mechanisms unrelated to antimicrobial drug selection’	‘All three dairy farms fed waste milk to calves, and both cefquinome and ESBL‐producing E. coli were detected in the waste milk samples tested. For farms A and B, 100% of faecal samples from calves classified as “weaned”, “unweaned”, “on milk machine” and “receiving colostrum” were positive for CTX‐M E. coli. For farm A, a greater proportion of faecal samples from calves were positive for CTX‐M‐positive E. coli compared with faecal samples from older animals. A combination of serotyping and PFGE suggested the same strains in waste milk and in calves could occur’	‘Feeding colostrum from cows treated with antimicrobials at drying‐off did not affect the prevalence of resistant E. coli. In contrast, feeding milk from cows treated with antimicrobials during lactation resulted in significantly more nalidixic acid‐ and streptomycin resistant E. coli; no significant effect was seen for other resistance traits. In general, the prevalence of resistance was lower for older calves and calves on small farms. We detected no significant difference between feeding waste milk produced during the withdrawal period and feeding waste milk during both the treatment and the withdrawal period’	‘Calves acquired the tetracyclin‐R genes in their early age (1–2 weeks). The relative abundance of the tet(W), tet(Q),and tet(M) genes in excrements of calves was about 1–2 orders of magnitude higher compared to heifers and dairy cows, possibly due to the presence of antimicrobial residues in milk fed to calves. The occurrence and abundance of tetracycline resistance genes in fresh excrements of heifers and adult cows remained unaffected by intrauterine chlortetracycline applications’
Main uncertainties	Relationship between AMR E. coli shedding and feeding waste milk is only hypothesised, not based on experimental data. Only two farms were included in the study limiting the representativeness of the results	Because no control group was included in the study, no conclusions can be made on the causal relationship between feeding milk containing antimicrobial residues and the shedding of CTX‐M E. coli	Statistical analysis of a large data set reduces the uncertainty on the obtained results. Uncertainty remains due to the design of the study, lacking experimental control groups. Also only farms, voluntarily accepting to contribute were included, creating a volunteer selection bias The conclusion on the colostrum is restricted to penicillins and aminoglycosides as these are the only antimicrobials used in dry cow therapy in Sweden. The study is restricted to antimicrobial‐resistant E. coli	Relationship between AMR shedding and feeding waste milk is only hypothesised, not based on experimental data; only resistance to one antimicrobial is investigated. The study is based on the presence of AMR genes detected by PCR and therefore no information is available on the identity of bacteria carrying the AMR genes. Study presents results of only a limited amount of samples taken from one dairy farm, limiting the representativeness of the results

ESBL: extended‐spectrum beta‐lactamases; CTX‐M: Cephalosporinase‐type cefotaximase; MIC: minimum inhibitory concentration; AMR: antimicrobial resistance.

^aStudy on the occurrence of AMR bacteria in faeces of calves on farms feeding calves' milk from cows treated with antimicrobials. No relationship between this feeding and the induction of AMR was investigated.

^bStudy on the effect of feeding calves milk from treated cows using statistical correlation between shedding results of AMR and treatment as indicated in questionnaires to farmers.

^csee Appendix E.