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OBJECTIVE To determine the survivability of Mycobacterium bovis on salt and salt-mineral blocks in typical weather conditions in Michigan over two 12-day periods at the height of summer and winter.

SAMPLE 4 salt (NaCl) and 4 salt-mineral blocks inoculated with pure cultures of a strain of M bovis currently circulating in Michigan livestock and wildlife.

PROCEDURES In the summer and again in the winter, inoculated blocks were placed in secured outdoor facilities where equal numbers of each block type (2/type/season) were exposed to shade or sunlight. Samples were collected from randomly selected areas on the surface of each block beginning within 1 hour after placement (day 0) twice a day for the first 4 days and once a day from days 7 through 11. Bacterial culture of samples was performed to detect viable M bovis.

RESULTS Depending on the exposure conditions, salt blocks yielded viable M bovis for up to 2 days after inoculation and salt-mineral blocks yielded viable M bovis for > 3 days. Survival time was greatest on salt-mineral blocks kept outdoors in the shade during the winter. The odds of recovering viable M bovis from salt-mineral block samples were 4.9 times as great during the winter (vs the summer) and 3.0 times as great with exposure to shade (vs sunlight).

CONCLUSIONS AND CLINICAL RELEVANCE Results from this study indicated that salt and salt-mineral blocks should be considered potential sources of bovine tuberculosis when designing risk mitigation programs for cattle herds in areas with wildlife reservoirs of M bovis.

Full access
in American Journal of Veterinary Research


OBJECTIVE To determine whether Mycobacterium bovis remains viable in ensiled forages.

SAMPLE Alfalfa, mixed mostly grass, and corn silages.

PROCEDURES For each of 10 sampling days, six 250-g replicate samples of each feedstuff were created and placed in a film pouch that could be vacuum sealed to simulate the ensiling process. Within each set of replicate samples, 4 were inoculated with 10 mL of mycobacterial liquid culture medium containing viable M bovis and 2 were inoculated with 10 mL of sterile mycobacterial liquid culture medium (controls) on day 0. Pouches were vacuum sealed and stored in the dark at room temperature. On the designated sampling day, 1 control pouch was submitted for forage analysis, and the other pouches were opened, and forage samples were obtained for M bovis culture and analysis with a PCR assay immediately and 24 hours later.

RESULTS None of the control samples had positive M bovis culture or PCR assay results. Among M bovis-inoculated samples, the organism was not cultured from alfalfa and corn silage for > 2 days but was cultured from mixed mostly grass silage for 28 days after inoculation and ensiling initiation. Mycobacterium bovis DNA was detected by PCR assay in samples of all 3 feedstuffs throughout the 112-day observation period.

CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that properly ensiled forages would be an unlikely source for M bovis transmission to cattle. Further research is necessary to determine whether ensiling kills M bovis or forces it to become dormant and, if the latter, elucidate the conditions that cause it to revert to an infectious state.

Full access
in American Journal of Veterinary Research