About one of every three U.S. dairies feeds waste milk to calves. Over approximately the past decade, on-farm pasteurization of waste milk has become an increasingly popular practice.Initially, the primary goal of pasteurization was to reduce the load of possibly pathogenic bacteria fed to calves, thus minimizing the risk of disease transfer. But researchers at Cornell University have recently explored another potential benefit of waste-milk pasteurization. Dr. Lorin Warnick and his colleagues were concerned about the exposure of calves' gut bacteria to low concentrations of antibiotics. Could that practice be creating a perfect environment for creating antimicrobial-resistant bacteria? The Cornell research team conducted an initial study that showed calves fed low doses of antibiotics in unpasteurized waste milk did often show higher levels of fecal E. coli bacteria resistant to several antibiotics, compared to calves fed milk without drug residues.
They also found the development of E. coli bacteria from the treatment group resistant to an antibiotic that was not even included in the treated milk (streptomycin), suggesting low-dose antibiotics in waste milk can result in a heterogeneous increase in antimicrobial resistance, because resistance to a drug class may occur even without its presence in the milk.
Veterinary researcher Richard Van Vleck Pereira, a member of the Cornell research team, suggested pasteurization is a viable means of degrading antibiotic residues to considerably lower levels, which could significantly reduce the influence of antibiotic presence in calves' gut flora.
The Cornell study is thoroughly summarized by Pereira in the May-June issue of Bovine Veterinarian magazine.