The lesion most commonly associated with bovine TB is the presence of a granuloma, a collection of inflammatory cells within the suspect tissue. When the animal's immune system recognizes the bacteria within the tissues, special inflammatory cells (macrophages) are sent to dispose of it.
Mycobacterium is resistant to destruction and once ingested by the macrophages may replicate and kill the macrophage instead of being killed. The animal's immune system continuously sends more macrophages to help destroy the bacteria, resulting in an accumulation of living and dead macrophages at the site of the bacteria. The accumulation of the living and dead macrophages, bacteria, and tissue cells in a focal area is called a tubercle. Over time, a thick capsule may form around the tubercle, walling it off from other tissues, forming the granuloma.
In addition to looking for specific lesions under the microscope, a special stain can be used to identify organisms that are compatible with Mycobacterium bovis, the bacteria that causes bovine TB. This is called an acid-fast stain. The cell wall of bacteria belonging to the Mycobacterium family, contain certain structural elements for which the acid-fast stain is specific. Following acid-fast staining, bacteria that take up this stain, including Mycobacterium bovis, will appear as short red or pink rods when examined under a microscope. The finding of acid-fast staining organisms in tissues is only suggestive of infection with bovine TB as other bacteria may also take up the acid-fast stain. More definitive laboratory testing is required to make a definitive diagnosis of bovine TB infection.
Based on the examination of the tissues for lesions and the acid-fast staining, three possible outcomes can occur:
- Mycobacterium bovis histopathology non-compatible - No lesions or acid-fast organisms compatible with Mycobacterium bovis are seen.
- Mycobacterium bovis histopathology suggestive - Lesions compatible with Mycobacterium bovis are seen but no acid-fast organisms are present.
- Mycobacterium bovis histopathology compatible - Lesions and acid-fast organisms compatible with Mycobacterium bovis are seen.
Regardless of the outcome of the acid-fast staining and histopathological examination, tissues are submitted for further diagnostic evaluation by PCR and bacterial culture. The results of all test findings are used to make a definitive diagnosis of bovine TB.
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