How do you explain cattle with Johnes disease that do not show signs?
Any infectious disease, including Johne’s disease, typically passes through four stages. Stage I is the initial infection: the animal is infected, not showing signs of disease and may be shedding small numbers of microbes into the environment that are not detectable by diagnostic tests. In Stage II, the infection is progressing and the animal still does not show any clinical signs. Nevertheless, the organism is being excreted in very high numbers, probably enough to infect others nearby or in contact. Infection is detectable by fecal culture techniques but not often by blood tests. In Stage III, the animal is showing the early signs of disease and many diagnostic tests can also detect the infection. Stage IV is the obvious clinical disease and readily recognized by the trained observer and detected by diagnostic tests. One difference between diseases is the time required for a disease to progress through these stages. It may be less than 12 hours as in some forms of calf scours, 1-5 da
