The organism that causes Johne’s disease is called Mycobacterium avium subspecies paratuberculosis (Mptb). It is in the same family of bacteria as the ones that cause tuberculosis and leprosy. It causes disease in ruminants – cattle, sheep, goats etc.
When a ruminant ingests the bacterium it can lead to infection of the gut. The disease can take a few years to develop in susceptible animals and can eventually cause weight loss and death of the animal.
Infected animals can look healthy for many years and it is these animals that ‘silently’ carry disease that are the most problematic. These animals can shed the bug in their faeces from time to time and spread the disease to the rest of the herd or flock.
The ultimate test for diagnosing Johne’s disease requires culture of intestinal tissues. Unfortunately this is not a practical test. There are two main types of tests for detecting Johne’s disease. Those that detect the bug and those that detect the response of the animal to exposure to the bug.
Tests that detect the bug mainly use faecal samples. Culture methods detect viable bugs but these tests take 8-12 weeks as Mptb grows very slowly. The new HTJ test developed by our group detects the DNA of the bug. This test only takes days.
Blood samples can be used to detect an animal’s response to the bug in tests like the antibody ELISA test. In most animals antibodies take years to develop and are only detected in animals in the later stages of disease. In many cases these animals are already infectious, shedding the bug in their faeces.
On-farm management strategies can be
Vaccination is another strategy used in the control of Johne’s disease. In Australia, the Gudair vaccine (Zoetis, formerly Pfizer) is registered for use in sheep. This vaccine can reduce mortality due to Johne’s disease by up to 90%. However some animals can still become infectious despite vaccination. Therefore it is recommended that all sheep are vaccinated. The vaccine Silirum (Zoetis, formerly Pfizer) has recently been registered for use in cattle.