The bacilli themselves, on the other hand, show only very little resistance to heat and drying.
"Special Report on Diseases of Cattle"
U.S. Department of Agriculture J.R. Mohler
Moreover, the bacilli may be absent at one time and present at another in milk from the same cow.
"Special Report on Diseases of Cattle"
U.S. Department of Agriculture J.R. Mohler
Since the bacilli when dried can be carried by the air, it is not necessary that healthy animals come in direct contact with cases of disease to become infected.
"Special Report on Diseases of Cattle"
U.S. Department of Agriculture J.R. Mohler