Why do the diagnostic tests for Johnes disease appear to give us variable results?
The blood test currently used to diagnose Johne’s disease is similar in accuracy to those that we use for both brucellosis and TB. However, the difficulty in the diagnosis of Johne’s disease lies in the biology of the disease itself. There are several disease facts that complicate diagnosis including the long disease incubation period, the lack of a specific early response by the animal’s immune system and the difficulty and time taken to grow the bacteria in the laboratory environment. The Johne’s disease bacterium is very slow growing and stays within the animal’s own cells in the intestine lining. It takes several years before there is significant damage to the intestine. When this occurs, the immune system is exposed to the Johne’s disease organism and only then is an antibody response stimulated. Following this, enough bacteria are shed in the faeces to be detected by the faecal culture test.
