Why do some reactors not show signs of disease?

Information on why some Tuberculosis (TB) reactor animals do not show any obvious signs of being diseased.

Not all TB reactors are expected to show signs of disease. This is because herds are tested at least every 12 months and disease is detected by the test before it has had time to become visible by post-mortem examination. Commonly, the TB lesion in an animal is smaller than the size of your thumbnail and may be in a site where it can be very difficult to detect.