High stocking densities on pasture units heighten contamination rates on the forages being utilized and in the drinking water resources. Production units with active shedders can potentially infect other species, such as birds and rabbits, which can act as reservoirs for re-infection despite the cleansing effects of seasonal changes. In cattle, infection is usually acquired early in life, but clinical signs are not commonplace before the age of two years unless the herd is massively infected. Fortunately, resistance does increase with age, so exposed adults are not as likely to become infected. Most calves are infected soon after birth from a source of fecal contamination from their mothers or herdmates or by ranging in contaminated paddocks.
Most cases become evident by manifestation of clinical signs in cattle two to six years of age.
Semen can contain the organism, therefore the disease can be transmitted sexually. Infected embryos implanted artificially could act as a source of infection. Intrauterine infection has been described and colostrum can contain higher densities of the organism than milk if the supramammary lymph nodes are infected. The progression and severity of disease is much more rapid than in cattle and bison. Actual clinical symptoms are not typically similar to those observed in cattle or bison.