Johne’s Disease: Cattle
Obvious symptoms, or clinical signs, in cattle include weight loss (even with a normal appetite) and diarrhea. The diarrhea may come and go, or may eventually become chronic, and does not respond to treatment. Many animals produce less milk than expected. Some animals may develop a low grade fever and, as the disease progresses, swelling under the jaw. Once symptoms appear, death occurs in weeks to months.
Clinical disease (presence of symptoms) has been seen in animals as young as 6 months and as old as 15 years. Symptoms often don't show until the first or second lactation or later, even though animals often become infected when calves. Age when exposed to bacteria; amount of bacteria to which exposed; stress caused by factors like calving and moving to new barns; and genetics all appear to play a role in when an animal shows symptoms.
Many cattle have "subclinical" infections. They may not show symptoms of diarrhea or weight loss, but they may not perform as well as expected. For example, they may produce less milk, or they may be more susceptible to problems such as infertility and poor health. Animals with subclinical infections can still shed bacteria and serve as a source of infection to others.