When infected and noninfected cows in the same herd were compared, some studies revealed that infected cows had lower milk production, longer calving intervals, and tended to be culled from the herd at an earlier age than noninfected herd mates. Clinical disease, though highly variable, usually appears in animals between 2 and 5 years of age. Usually symptoms first appear following calving, a severe stress, or poor management. A persistent diarrhea without straining is commonly the first clinical sign, followed by a gradual loss of flesh. The disease develops so slowly that by the time the owner becomes concerned, the herd may be in serious trouble.
Infected animals continue to eat, but have constant or recurring bouts of diarrhea.
Cattle can become emaciated in a short period of time, such as a few weeks, but often the wasting progresses over a few months. Some cattle may appear to recover temporarily, but relapse later, such as after calving. Infected cows may shed organisms for up to 12 to 18 months after showing signs of infection. Even in herds with widespread infection, at any one time only a few animals may be showing signs of disease. Infection results in invasion of the terminal portion of the small intestine.