Quantitative Risk Model for Predicting Mycobacterium avium subsp. paratuberculosis Contamination in Bulk Tank Milk on Dairy Farms

Introduction:  Mycobacterium avium subsp. paratuberculosis (MAP) is a pathogenic bacterium associated with Johne’s disease, one of the important endemic infectious diseases in dairy cattle. Contamination ny MAP can be through direct shedding in milk by cows (internal route), entering of feces in the bulk tank milk (fecal route), or via contact with soil and water containing MAP (external/environmental route). Considering that there are reports of its survival in milk after pasteurization, humans can be exposed to MAP via milk (raw or pasteurized) consumption.

Purpose: The goal of this study was to develop a quantitative risk model of MAP transmission on dairy farms, including all three aforementioned routes, with a focus on different infection statuses of the infected cattle.

Methods: A probabilistic modeling framework was used to predict the MAP contamination from cows’ shedding (internal route), feces, surfaces of milking parlor, and other environmental sources into the bulk tank milk. The data were extracted from a survey on dairy farms across the U.S. and published literature.

Results: The model estimated the final concentration of MAP in bulk milk to be, on an average, 3.07 log CFU/L with a maximum of 25 log CFU/L. Average dirt contamination containing feces played the most significant role in contributing MAP contamination in bulk milk, followed by herd size. Washing practice efficiency contributed to lowering the total contamination.

Significance: This study emphasized that good hygiene practices and proper herd management are crucial for maintaining the quality of raw milk on a dairy farm.