1) What is Johne's Disease?
Johne’s disease is a chronic, debilitating and irreversible disease of cattle and all other ruminants that affects the lining of their intestines, reducing its capacity to absorb both fluid and nutrients. While only a small proportion of cattle will show clinical signs of wasting or scour at any one time, it is likely that a much larger proportion of the herd are infected and only showing very mild (e.g. higher cell counts, reduced milk yields, and increased susceptibility to other diseases) if any, signs of the disease.
3) Where does Johne's disease come from?
The bacteria that cause Johne’s can be shed in the faeces, colostrum or milk of infected animals, and is able to survive in the environment for a considerable period of time. Johne’s disease is, however, most commonly introduced to a herd through purchasing infected replacement stock (including bulls). Importing slurry from other herds which may be infected, including stock from grazing off farm, and swapping colostrum between herds can also pose risk of introducing the disease into a herd.
5) How do calves become infected?
The main route of infection for calves is the ingestion of faeces contaminated with MAP. Although direct ingestion of faeces does occur, calves are more commonly infected through ingestion of muck contaminated bedding, udders, teats or buckets.
Calved can become infected by drinking colostrum or milk form an infected dam, or colostrum that is pooled and contains the milk from an infected dam.
7) Why, if they are infected as calves, don't you see signs of Johne's until later in life?
Despite picking up the infection when they are young, they can be infected for a variable length of time, often years, without showing any signs of disease or being a source of disease to other animals. At a certain point, the immune response of the cattle towards the bacteria can change and they will start to show outward signs of the disease. At this point, they also become infectious to other animals.
9) Why can an animal's test status change?
Due to the way the test works, despite being infected with MAP, young animals are unlikely to test positive because of the way their immune system responds to the bacteria. As they get older, this response changes and the animal will test positive. This is usually around the same time that they start to show signs of the disease and start to pose a risk of infection to others.
11) What should I do if I have an animal that tests positive for Johne's disease?
If you have an animal that has a positive test it is vital that you develop and put a Johne’s Management Plan in place. Due to the complex nature of the disease, it is something that should be discussed with your vet, to make sure you develop a plan that is most suited to your own farm system.
13) Can I test for Johne’s disease if short interval TB testing?
A primary concern for farmers undergoing short interval TB testing is the effect it will have on their Johne’s results. We often hear the argument that as they are TB testing every 60 days, they cannot test for Johne’s. This is not the case and almost 50% of herds undertaking quarterly Johne’s testing are on short interval TB testing.
For Johne’s testing using milk samples there must be an interval of at least 6 weeks between the TB test and Johne’s testing (for blood samples the interval must be longer, 70 days). The recommendation is to leave testing for Johne’s until just before the next TB test, but at least an interval of 6 weeks. Milk recorders are familiar with moving recording dates to accommodate this and if this procedure is followed then interference is minimal.
Johne’s results should always be interpreted with the assistance of the farm vet.
See the following paper for further information:
Kennedy, A. E., Da Silva, A., Byrne, N., Govender, R., MacSharry, J., O’Mahony, J. and Sayers, R. G. (2014) The single intradermal cervical comparative test interferes with Johne’s disease ELISA diagnostics. Frontiers in Immunology. 5, 564