What are the signs of BJD?
Cattle are usually infected when less than 12 months of age. However, due to a long incubation period, clinical disease is often not seen until the affected animal is at least 4 or 5 years of age.
Signs may appear after a period of stress such as:
- poor nutrition
- heavy milk production.
As the bacteria lodge and multiply in the wall of the small intestine, the cow responds by producing inflammatory cells. This combination of bacteria and cells leads to a thickening and distortion of the gut wall. Eventually the gut fails to absorb water and nutrients.
In dairy cattle, the first sign is often a drop in milk production. Affected animals then develop chronic diarrhoea. Cattle gradually lose weight and become emaciated, while still maintaining a good appetite.
They may also develop 'bottle jaw', a swelling under the jaw.
How are cattle infected with Johne's disease?
Cattle up to 12 months of age are most susceptible to infection, particularly calves less than 3 months old.
Infected cattle shed bacteria in their manure and contaminate the environment before showing signs of BJD. However, the greatest contamination comes from animals showing signs of BJD.
Calves become infected as they drink from an udder contaminated with manure, or eat or drink feed or water contaminated by manure. Calves can also become infected while in the cow's uterus, however this is less common. The likelihood of this is increased if the cow is showing signs of BJD.
Infected cattle may also shed the bacteria in their milk. When calves drink this milk they can become infected.