A longitudinal field trial assesing the impact of feeding waste milk containing antibiotic residues on the prevalence of ESBL-producing Escherichia coli in calves.

A longitudinal field trial was carried out on a farm known to harbour cefotaximase (CTX-M)-positive Escherichia coli, in order to assess the impact of feeding waste milk containing antibiotic residues (WM+AR) on the prevalence of these bacteria in the faeces of calves. Fifty calves were alternately assigned to one of two groups at birth and fed either milk replacer (control group) or WM+AR (treatment group). Faecal samples were collected from all calves daily for the first week after enrolment, twice weekly until weaning, then weekly for a further six weeks. Environmental samples from the calf housing were collected weekly. WM+AR and powdered milk samples were examined for antibiotic residues and CTX-M-positive E. coli. Total E. coli and CTX-M-positive E. coli in faecal samples were enumerated using selective media. Regression analyses were performed on the bacterial count data using a population-averaged approach based on generalised estimating equations (GEE) to account for repeated measurements on individual calves over time. Cefquinome, a fourth generation cephalosporin, was detected in 87% of WM+AR samples at a mean concentration of 0.746 mg/l. All environmental sampling locations yielded CTX-M-positive E. coli. Significantly more pen floor samples were positive in the treatment group. Calves in the treatment group shed greater numbers of CTX-M-positive E. coli than calves in the control group throughout the study, and shedding decreased at a slower rate in the treatment group. CTX-M-positive E. coli persisted in a larger number of calves fed WM+AR compared with calves fed milk replacer where the prevalence in the treatment group declined significantly slower over time. There was no difference between calves fed WM+AR or calves fed milk replacer in the proportion of E. coli isolates that were CTX-M-positive. These findings indicate that feeding WM+AR increased the amount of resistant bacteria shed in the faeces. Shedding of CTX-M-positive E. coli persisted for longer in calves fed WM+AR, and persisted after weaning.

Evaluation of an autogenous vaccine in cattle against Escherichia coli bearing the CTX-M-14 plasmid.

Enteric bacteria with resistance to third and fourth generation cephalosporin antibiotics, especially Escherichia coli bearing the blaCTX-M gene, have been detected in a wide range of food producing animals. However, commercial vaccines for these organisms are not currently available. An autogenous vaccine was prepared from E. coli bearing the blaCTX-M-14 gene and evaluated as a potential control measure to reduce shedding and dissemination of these organisms in cattle. Calves (n=30) received either an autogenous vaccine prepared from E. coli serotype O33 bearing the blaCTX-M-14 gene or a placebo by intramuscular injection on three separate occasions. Two weeks after the final vaccination, all calves were challenged by oral gavage with the O33 CTX-M-14 strain of E. coli (1×10(10) CFU). Faeces, intestinal mucosa and blood samples were taken for enumeration of total and CTX-M-14 E. coli and for assessment of the humoral immune response. The cumulative number of total E. coli excreted at 7 days post-challenge was significantly (p=0.006) lower in the vaccinated group than the placebo group. However, there was no significant difference in the shedding of either CTX-M-14 E. coli or total E. coli between vaccinated and placebo calves throughout the study period. The systemic immune response to E. coli O33 antigen was tested by ELISA and was significantly higher (p<0.001) in vaccinated than placebo calves. However, there was no significant difference in the mucosal immune response. These findings do not support the use of autogenous vaccination for the control of CTX-M-14 E. coli in calves.