What age group of cattle is most frequently affected by Johne's disease?

Johne's disease, caused by the bacterium Mycobacterium avium subspecies paratuberculosis, predominantly affects cattle that are between 2 to 4 years of age. This age group is particularly significant because the disease has a long incubation period, often taking several years post-infection for clinical signs to manifest.

Young calves are usually exposed to the bacteria through contaminated feed or water and can remain asymptomatic for a long period. By the time the cattle reach the ages of 2 to 4 years, they are more likely to display the characteristic signs of the disease, such as chronic diarrhea and weight loss, which are indicative of advanced infection.

As cattle continue to age, the likelihood of displaying symptoms increases, making the 2 to 4-year age group the most affected during the diagnostic assessment of Johne's disease. Consequently, understanding the typical age range for clinical manifestations can help in monitoring and managing herds, particularly for early intervention and control of the disease.