Final answer:
Johne's disease in cattle is marked by a thickening of the intestine due to chronic inflammation from Mycobacterium avium subspecies paratuberculosis, with diagnosis confirmed by identifying the bacteria in various samples.
Step-by-step explanation:
The pathognomonic lesion of Johne's disease in cattle is characterized by thickening of the intestinal wall due to chronic inflammation, which is a result of infection by the bacterium Mycobacterium avium subspecies paratuberculosis (MAP). This condition leads to malabsorption, diarrhea, and wasting, and is often chronic and fatal. In advanced cases, a veterinarian may palpate the corrugated or cobblestone texture of the intestine due to the granulomatous lesions that are formed. Identification of MAP bacteria in stool samples, tissue biopsies, or through DNA-based tests such as PCR can be used to confirm the diagnosis.