Effect of waste milk pasteurization on fecal shedding of Salmonella in preweaned calves

Abstract

The objective of the current research was to determine if pasteurization of nonsaleable waste milk influences fecal Salmonella concentrations and prevalence, or antimicrobial susceptibility and serotype of the cultured isolates. Holstein dairy calves (n = 211) were housed on a single commercial dairy in the southwestern United States and randomly allotted to be fed either pasteurized (PWM; n = 128 calves) or nonpasteurized waste milk (NPWM; n = 83 calves). Fecal samples were collected via rectal palpation or from freshly voided, undisturbed fecal pats, weekly during the first 4 wk of the animal's life and then again at weaning. Eight total collections were made and 1,117 fecal samples cultured for Salmonella. One isolate from each culture-positive fecal sample was preserved for antimicrobial susceptibility screening and serotyping. Sixty-nine percent of the fecal samples were culture positive for Salmonella with no difference due to treatment (67.7 and 69% Salmonella positive for PWM and NPWM treatments, respectively). Few fecal samples (178/1,117; 15.9%) contained Salmonella concentrations above the limit of detection (∼1 cfu/g of feces) with concentrations ranging from 1.0 to 6.46 cfu (log10)/g of feces. Concentration was not affected by treatment. Seventeen different serotypes were identified, the majority of which were Montevideo and Anatum. A greater percentage of Typhimurium (87 vs. 13%), Muenchen (88 vs. 12%), and Derby (91 vs. 9%) were recovered from calves fed PWM compared with NPWM-fed calves. Conversely, Newport (12.5 vs. 86%), Bredeney (22.2 vs. 77.8%), and Muenster (12.5 vs. 87.5%) were lower in PWM compared with NPWM treatments. The majority (66.7%) of isolates were susceptible to all of the antibiotics examined. Results from this one commercial dairy suggest that milkborne Salmonella is not an important vector of transmission in dairy neonates, nor does pasteurization of waste milk influence fecal shedding of this pathogen. Caution should be used, however, when extrapolating results to other farms as Salmonella contamination of milk on farm is well documented. The potential benefits of pasteurization in disease prevention outweigh the potential risks of feeding a nonpasteurized product and warrants incorporation into any calf-rearing program using nonsaleable waste milk for feeding young dairy neonates.