Assessing the Role of Farm Hygiene as Predictor of Milk Contamination by Mycobacterium avium subsp. paratuberculosis (MAP) in Dairy Farms

Introduction: Mycobacterium avium subsp. paratuberculosis (MAP) is associated with Johne’s disease in dairy cows, and is suspected to be linked to Crohn’s disease in humans. MAP can be secreted by cows with milk (internal route), or contaminate milk via contact with feces or farm equipment (external route). Humans can be exposed to MAP via milk consumption, since MAP is not fully inactivated by pasteurization. While hygiene measures aim to limit milk contamination, to-date the relative importance of different contamination routes is poorly understood.

Purpose: This study aims to: 1) Survey hygiene practices in Northeast U.S. dairy farms, 2) Identify farm practices and hygiene interventions that impact MAP contamination in bulk milk, and 3) Develop a model of MAP transmission in dairy farms, with focus on herd and environmental hygiene practices.

Methods: Cross-sectional data on MAP and corresponding antibodies in bulk milk, as well as herd and farm hygiene metrics were collected for 292 dairy farms in the Northeast U.S. Classification and regression tree (CART) analyses were used to identify major risk predictors.

Results: Milk quality metrics, in particular somatic cell counts and plate counts in milk, cow-side somatic cell counts, and levels of E. coli and Streptococcus in milk were the main predictors of MAP antibody levels in milk, as assessed by ELISA assay (CART importance rank  = 4-7). Age of cow housing facilities, breed, detection of Mycoplasma in milk, and high use of pasture were also of intermediate importance (rank = 3). Only in 2.1% of farms was MAP detected in milk by PCR, making this variable unsuitable for CART analysis.

Significance: Findings suggest that both animal health and farm hygiene can play an important role in MAP contamination in milk. Further quantifying these variables could inform risk assessment models and improve milk quality.