Johne's disease and milk: Do consumers need to worry?

Mycobacterium paratuberculosis, an acid-fast bacillus that causes enteritis in ruminants, has been suggested as an etiological agent of Crohn's diseas e in humans. The mode of transmission is unclear; however, some evidence su ggests that humans may become infected via contaminated milk. Currently, it is not known whether commercial pasteurization effectively kills M. paratu berculosis in contaminated raw milk. Using a laboratory-scale pasteurizer u nit designed to simulate the high-temperature, short-time method (72 degree s C, 15 sec) currently used by commercial dairies, we previously demonstrat ed that treatment of raw milk inoculated with 10(4) to 10(6) cfu of M. para tuberculosis/ml reduced numbers to an undetectable level. However, M. parat uberculosis is an intracellular pathogen that resides within the macrophage s of the host and evades destruction. We subsequently performed further exp eriments examining heat treatment of milk inoculated with mammary gland mac rophages containing ingested M. paratuberculosis, Heat treatment of these s amples under high-temperature, short-time conditions demonstrated that the macrophage does not protect the organism because we were unable to recover any viable M. paratuberculosis from the samples. Conversely, other research ers have demonstrated that a residual population of M. paratuberculosis may survive heat treatment of milk. In addition, a recent news report stated t hat viable M. paratuberculosis organisms have been cultured from retail-rea dy milk in Ireland. A summary of past and current studies concerning this i ssue along with a discussion of methodologies used to recover M. paratuberc ulosis from experimentally inoculated milk will be presented in this paper.