Infection: Goats get infected with Johnes by eating contaminated faeces. Young stock are more likely to become infected than old. The bacteria sits in the lymph nodes (often for years) until it is released under a period of stress (entering a new herd, kidding etc) and causes disease.
Signs: Unlike cattle, goats do not get diarrhoea. A Johnes goat loses weight, has a rough coat and becomes depressed. You can see swelling under the jaw sometimes. Typically signs are seen at 2-3 years old. Animals can be infected and release bacteria in their faeces without showing any signs.
Diagnosis: It’s very hard to say for sure that an animal has Johnes. Clinical signs can arouse suspicion. An animal that is showing signs is far more likely to come back positive at the lab, while animals with no signs are often falsely negative (20-50% of animals will be missed). The best diagnosis is made once the goat is dead and either the infection can be seen/samples can be sent away.
Treatment: There is no treatment and for the sake of the herd, culling is probably the best option.