Lambs born into these contaminated environments are most susceptible to infection. Dirty udders and wool tags, barn yards, bedding, etc are all sources of infection to the curious lamb. Plowing infected grazing land will dilute the bacteria and help to kill them. Snow cover will help their survival whereas sunlight, drying or exposure to intense cold will reduce their numbers. Disinfectants must be suitable against mycobacteria to be effective against Johne's bacteria.
Few disinfectants work well if used in the presence of organic material.
Although Johne's disease is primarily a disease of the intestine, during the advanced stages of disease the agent may spread throughout the body. Fetuses are very susceptible to infection while still in the womb. This means that even removing lambs at birth will not guarantee freedom from infection if the dam is infected. Milk and colostrum sometimes have small numbers of organisms present but it is not known how important milk is as a source of infection. Semen In cattle, the bacteria have been isolated from semen from bulls with advanced Johne's disease.