Submitted to: Journal of Applied & Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 22, 1997
Publication Date: N/A
Interpretive Summary: Johne's disease is a chronic, debilitating intestinal disorder in cattle characterized by diarrhea, reduced feed intake, weight loss, and death. Cattle usually become infected as young calves by ingesting feces containing the causative bacteria. However, symptoms of disease do not usually present themselves until the animals reach 3 to 5 years of age or even older. During this time the animal is infected and may be shedding the organism in its feces without showing any clinical signs of disease. In addition to reduced production by these animals through reduced milk production, they also present a potential infective threat to the rest of the herd. The causative agent of Johne's disease, Mycobacterium paratuberculosis, is shed in the milk of severely infected cows. It is not known whether human beings are exposed to the live microorganism through pasteurized milk or dairy products. This study shows that using industry conditions for pasteurization with a lab-scale pasteurization unit will kill M. paratuberculosis in milk. This information is important to the consumer, dairy producers, veterinary practitioners, and the dairy industry as it suggests that contamination of raw milk with M. paratuberculosis is unlikely to be a human health risk.
Technical Abstract: Mycobacterium paratuberculosis, an acid-fast bacillus which causes enteritis in ruminants, has been suggested as an etiological agent of Crohn's disease in humans. Mode of transmission is unclear, however, some evidence suggests that humans may become infected via contaminated milk. Currently, it is unknown whether commercial pasteurization effectively kills M. paratb in contaminated raw milk. Initial experiments used the holder-test tube method, simulating methodologies used in other labs, to determine optimal time/temp. combinations for effective inactivation of large numbers of M. paratb (10*8 CFU/ml). Results indicated a significant reduction in viable bacterial numbers at 72C, but mean time for optimal killing exceeded industry recommendations of 15 sec at that temperature. A residual population of viable M. paratb was detectable after 30 min incubation at 65, 72, 74, or 76C. Subsequent experiments evaluated a lab-scale pasteurizer unit designed to simulate the high-temp., short-time method (72C, 15 sec) currently used by commercial dairies. These results demonstrate that treatment of raw milk inoculated with live M. paratb (10*4, 10*6 CFU/ml) at 72C for 15 sec effectively kill all the bacteria. Comparison of these two models for testing heat inactivation of M. paratb indicate that turbulent flow of milk during the pasteurization process seems necessary to achieve complete inactivation of contaminating bacteria.