Excretion of Antibiotic Resistance Genes by Dairy Calves

Twenty-eight Holstein and crossbred calves of both genders were used to evaluate the effect of milk replacer antibiotics on abundance of selected antibiotic resistance genes (ARG) in the feces. Calves were blocked by breed, gender, and birth order, and assigned to one of three treatments at birth. Treatments were control (containing no antibiotics in the milk replacer), subtherapeutic (neomycin sulfate and oxytetracycline hydrochloride each fed at 10 mg/calf/d), and therapeutic (no antibiotics in the milk replacer until d 36, then neomycin sulfate and oxytetracycline hydrochloride each fed at 1000 mg/calf/d for 14 d). Calves were fed milk replacer twice daily at 0600 h and 1800 h. Fecal and respiratory scores and rectal temperatures were recorded daily. Calves were weighed at birth and weaning to calculate average daily gain. Beginning at six weeks of age fecal grab samples were collected from heifers at 0600 h, 1400 h, 2000 h, and 2400 h for 7 d, while bull calves were placed in metabolism crates for collection of all feces and urine. DNA was extracted from feces, and ARG corresponding to the tetracyclines (tetC, tetG, tetO, tetW, and tetX), macrolides (ermB, ermF), and sulfonamides (sul1, sul2) classes of antibiotics along with the class I integron gene, intI1, were measured by quantitative polymerase chain reaction (qPCR). No tetC or intI was detected. There was no significant effect of antibiotic treatment on the absolute abundance (gene copies/ g wet manure) of any of the ARG except ermF, which was lower in the antibiotic-treated calf manure probably because host bacterial cells carrying ermF were not resistant to tetracycline or neomycin. All ARG except tetC and intI were detectable in feces from 6 weeks onwards, and tetW and tetG significantly increased with time (P < 0.10), even in control calves. Overall, the majority of ARG analyzed for were present in the feces of the calves regardless of exposure to dietary antibiotic. Feed antibiotics had little effect on the ARG monitored; other methods for reducing the ARG pool should also be investigated.