What factor does waste milk play for antibiotic resistance in calves?

Feeding of waste milk may be one factor increasing the amount of bacteria resistant to antibiotics in dairy calf guts, say researchers.

An international team of researchers from Spain and the US explored the use of pasteurized waste milk in the diets of dairy calves. The group published its work in the Journal of Dairy Science​​.

“The aim of the present study was to assess whether feeding pasteurized waste milk (pWM) increases the prevalence of resistance in fecal Escherichia coli isolates at both phenotypic and genotypic levels, and whether this potential resistance would decrease after weaning when the antimicrobial pressure from pWM is no longer present,”​ said the researchers. “With this objective we have identified the genes involved in the resistance and evaluated the presence of antimicrobial resistance in fecal E. coli from calves before and after weaning from both types of milk feeding regimens, pWM and milk replacer (MR).”​

The researchers found that feeding the waste milk increased the presence of phenotypic resistance to some antimicrobials, they said. However, there also were indications that more factors were involved in the selection for antimicrobial resistant genes.

“[Waste milk] produced by cows treated with β-lactam antimicrobials and used to feed dairy calves contained drug residues in concentrations sufficient to select for resistant E. coli in the calf gut,”​ they said. “In addition to lincosamide, β-lactams were the only antimicrobial residues that could be present in WM. However, FFC-resistant E. coli were also selected in WM-fed calves, suggesting the potential of antimicrobial residues in milk to select for resistance to antimicrobials other than those present in milk.”​

Why examine waste milk use?​

Antibiotic resistance has been an increasing concern globally, said the researchers. The prevalence of bacteria resistant to antibiotics in livestock has been associated with the administration of antibiotics.

But some studies exploring farm management practices have suggested that other factors play a role, including the diet animals get and farm environment, they said. A common practice on dairy farms is to use milk from cows treated with antibiotics to feed young calves, as the milk cannot be sold.

Feeding waste milk (WM) may increase the amount of bacteria resistance to the antimicrobials used on the farm and ones not in use, they said. Previous research has demonstrated the ability of resistant strains to transfer resistance genes to other bacteria treated with different types of antibiotics.

It also has been reported that bacterial isolates from animal, human and environmental sources have displayed genetically similar plasmids offering resistance to different antimicrobial classes, the researchers said.

“Because of the potential risk of antimicrobial residues in WM, several studies have examined the effect of feeding WM to calves on the occurrence of phenotypic resistant bacteria in the gut microbiota,”​ they said. “However, only a limited number of studies have focused on the effect of milk feeding practices on antimicrobial resistance at genotypic level, and even fewer studies have evaluated shifts on the prevalence of antimicrobial resistance occurring at weaning from nonsalable milk with consistent result.”​

Methods and materials​

In the feeding study, 52 calves were given one of two milk-based diets for a period of 49 days and textured calf starter from day 1 through 56, said the researchers. Calves were weaned on day 49.

The pre-weaning diets included milk replacer (MR) without antimicrobials or pasteurized waste milk with β-lactam residues, they said.

Fecal samples were taken from calves on days 0, 35 and 56 of the study and three E. coli​ isolates were analyzed, they said.

A panel of 12 antimicrobials was used to check for phenotypic resistance and PCR was used to assess 13 resistance genes including ampicillin (AMP), cephalothin (KF), ceftiofur (EFT), enrofloxacin (ENR),  florfenicol (FFC), penicillin G (P), imipenem (IMP), pirlimycin (PIR), trimethoprim/sulfamethoxazole (STX), erythromycin (E), tetracycline (TET) and streptomycin (S), said the researchers.

Results​

It was found that WM generated by cows treated with β-lactam antimicrobials and used as feed for dairy calves contained drug residues and was able to select for E.coil resistant to the antibiotics, said the researchers. But, calves getting the milk diet also carried bacteria resistance to other types of antibiotics.

“FFC-resistant E. coli were also selected in WM-fed calves, suggesting the potential of antimicrobial residues in milk to select for resistance to antimicrobials other than those present in milk​,” they said. “Moreover, the high levels of resistance to TET and aminoglycoside antimicrobials found at both phenotypic and genotypic levels, and in calves from both feeding regimens, supported the hypothesis that antimicrobial pressure is not required for the emergence and maintenance of resistance in gut bacterial populations.”​

Almost all calves, regardless of feeding type had E. coli​ that were resistance to erythromycin, penicillin and pirlimycin, they said. But none were resistant to ENR or imipenem.

“The prevalence of resistance to AMP and KF did not differ in calves fed MR during the study period, whereas in calves fed pWM, the prevalence increased from d 1 to 35 (before weaning), and decreased from d 35 to 56 (after weaning),”​ they said. “No E. coli resistant to EFT was isolated at d 1 and 56 in calves fed MR, and no E. coli resistant to FFC was isolated in calves fed pWM at d 1.”​

No determinations could be made about the connection of feed type and resistance to EFT and FFC phenotypes, the researchers said. However, the amount of bacteria resistant to EFT was larger and tended to be greater for FFC in calves that received the pWM diet.

The presence of bacteria resistant to TET and S grew during the trial, but did not appear to be linked to a diet, they said.

“Calves fed MR had a greater percentage of E. coli isolates grouped in cluster B and D, which showed resistance to aminoglycosides (S) and TET, than pWM-fed calves,” ​the researchers said. “In contrast, calves fed pWM had more E. coli colonies grouped in cluster C, E, and G, with resistance profiles against β-lactams (AMP, EFT, and KF), than calves fed MR.”​

The results indicate that more research is needed to understand the process of bacterial selection for antibiotic resistance, they said.

“These findings highlighted the need for further studies evaluating the specific factors involved in the widespread antimicrobial resistance in bacteria isolated from farm animals, such as genetic linkage of resistance genes in mobile genetic elements and the transfer among different bacterial populations,” ​they said.