Transmission of paratuberculosis.

Infection of the calf soon after birth by oral ingestion of M. paratuberculosis organisms from feces of infected cows is the most important method of transmission of paratuberculosis. The severity and rate of progression of the disease are dependent on the quantity of organisms in the exposure and the age of the animal. Only a small dose of organisms may be required to establish infection in a newborn calf, and overwhelming age-related resistance by introduction of a large dose of organisms to an adult cow is probably possible. Other means of transmission such as transplacental and direct excretion in the milk and colostrum should not be ignored and should be a particular concern primarily in advanced stages of infection when heavy fecal shedding is detected.