Though initially the disease affects only the intestinal tract, like a cancer, it will spread to other parts of the body. Perhaps the most perplexing aspect of the disease is that it has a long incubation period. An animal may be infected the day it's born, but not show a symptom for up to 10 years, says Whitlock. It can shed the bacteria for months or years before it shows any clinical signs. Animals in Stage 1 can't be detected with current tests.
Infection progresses slowly.
It may take months or years for an animal to reach Stage 2. Stage 2 - subclinical shedders. Blood tests may not be reliable at this stage. Animals in Stage 2 pose a major, but often hidden, threat of infecting other animals. Through Stage 2, infected cattle appear and can function normally.