Efficacy of dairy on-farm high-temperature, short-time pasteurization of milk on the viability of Mycobacterium avium ssp. paratuberculosis

Abstract

Feeding pasteurized milk to suckling calves is a popular practice used increasingly on dairy farms. Waste milk is frequently fed to calves because of its high nutritional value and economic benefits compared to milk replacement products. However, one of the disadvantages of feeding waste milk is the potential for exposure to a high number of bacterial contaminants, which may lead to serious illnesses or infections in calves. One of these contaminants is Mycobacterium avium ssp. paratuberculosis (MAP), the causative agent of Johne's disease (paratuberculosis). The transmission and distribution of paratuberculosis in dairy herds occurs mostly through the feeding newborn calves with contaminated colostrum or milk, because this age group is believed to be most susceptible to infection. To reduce the risk of transmission of pathogens, on-farm pasteurization of milk has become increasingly popular. In this study, we analyzed the efficacy of a new commercial high-temperature, short-time pasteurizer (73.5°C for 20 to 25 s) in terms of MAP inactivation under experimental on-farm conditions. The pasteurizer uses a newly developed steam-heating technique, allowing for the pasteurization of the transition milk without clumping. In 3 independent trials, we spiked fresh raw milk samples to a level of 10⁷ or 10⁴ viable MAP cells/mL before pasteurization. We examined the thermal inactivation and viability of MAP using culture and a D29 bacteriophage-based assay. To verify the identity and number of MAP cells, we also performed PCR assays. Pasteurization of the inoculated milk (10⁷ and 10⁴ MAP cells/mL) resulted in a remarkable reduction in viable MAP cells. The mean inactivation rate of MAP ranged from 0.82 to 2.65 log₁₀ plaque-forming units/mL, depending on the initial MAP amount inoculated and the addition of conservative agents to the pasteurized milk. Nevertheless, approximately 10³ MAP cells/mL remained viable and could be transferred to calves after high-temperature, short-time pasteurization of milk.

Keywords: Mycobacterium avium ssp. paratuberculosis; pasteurization; phage assay; waste milk.