At what age is Johne's disease typically diagnosed in cattle?

Johne's disease, caused by the bacterium Mycobacterium avium subspecies paratuberculosis, often remains asymptomatic in cattle for an extended period. The disease primarily affects the intestines, leading to chronic diarrhea and weight loss as it progresses. In many cases, clinical signs do not appear until the animal is older.

Typically, the diagnosis of Johne's disease occurs in cattle when they are over two years old, often between four to six years. This delayed onset is due to the long incubation period of the disease, where calves may be infected at a young age but do not show symptoms or detectable disease until they are considerably older.

Therefore, diagnosing Johne's disease at this later age aligns with the biological and clinical progression of the illness in cattle, reinforcing the significance of monitoring older animals for symptoms, especially in herds with a known history of the disease.