Estimating Bacterial Pathogen Levels in New Zealand Bulk Tank Milk.
Zoonotic bacteria such as Campylobacter, Listeria, and Shiga toxin-producing Escherichia coli have been found in bulk tank milk in many countries, and the consumption of raw milk has been implicated in outbreaks of disease in New Zealand. Fecal contamination at milking is probably the most common source of pathogenic bacteria in bulk tank milk. Raw milk was collected from 80 New Zealand dairy farms during 2011 and 2012 and tested periodically for Campylobacter, E. coli O157, Listeria monocytogenes, and Salmonella. Milk quality data such as coliform counts, total bacterial counts, and somatic cell counts also were collected. By treating the total bacterial count as a proxy for fecal contamination of milk and utilizing farm and animal level prevalence and shedding rates of each pathogen, a predictive model for the level of pathogenic bacteria in bulk tank raw milk was developed. The model utilizes a mixture distribution to combine the low level of contamination inherent in the milking process with isolated contamination events associated with significantly higher pathogen levels. By simulating the sampling and testing process, the predictive model was validated against the observed prevalence of each pathogen in the survey. The predicted prevalence was similar to the observed prevalence for E. coli O157 and Salmonella, although the predicted prevalence was higher than that observed in samples tested for Campylobacter.
Institute of Fundamental Sciences, Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Palmerston North 4474, New Zealand. j.c.marshall@massey.ac.nz.
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Marshall
Ministry for Primary Industries, Pastoral House, The Terrace, Wellington 6011, New Zealand.
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Soboleva
MilkTestNZ, 1344 Te Rapa Road, Hamilton 3288, New Zealand.
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Jamieson
mEpiLab, Infectious Disease Research Centre, Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Palmerston North 4474, New Zealand.
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French
D000818:Animals; D003612:Dairying; D019453:Escherichia coli O157 / Q000302:isolation & purification; D008892:Milk / Q000382:microbiology*; D009520:New Zealand; D012475:Salmonella / Q000302:isolation & purification