Johnes Disease is caused by a bacterium called Mycobacterium Avium Paratuberculosis, or MAP for short. It causes chronic scour and wasting in older cattle. Once significant clinical signs appear then the disease is generally fatal. A cow may carry the bug in her small intestine for many years before it develops and multiplies enough to appear in her dung. Clinical cases usually show when they are aged at least 5 years.
Although the disease does not show until many years later, 80% of infected cattle catch the disease aged 1m or less, usually within the first 24h of life. At this stage the calf’s small intestine is temporarily much more permeable in order to absorb the antibodies in colostrum and MAP can sneak through as well. Unfortunately the stress of calving may bring on an increase in the amount of MAP that a carrier cow excretes in her dung, and she will often get this dung smeared on her teats.
An infected calf will usually grow normally. The small amount of MAP in the intestine is ‘ringfenced’ by the immune system and almost in a state of hibernation. However this means that we cannot find the MAP in the dung. Moreover because the level of disease is so slight and the bug is so well walled off in the gut lining, the white blood cells cannot identify it and produce enough antibodies to register on a blood test. We are not sure exactly why the MAP suddenly increases in number and causes clinical signs though stress does seem to be a trigger. It is only towards the time that an animal shows clinical signs that an accurate diagnosis can be made using a blood sample, a dung sample or both. In short, we have no accurate way to tell if an animal is carrying Johnes Disease.
There is no effective vaccine for preventing Johnes Disease.
The cost of Johnes disease to the UK cattle industry has been estimated by the Scottish Agriculture College (SAC) at £13 million. From the graphs below it is obvious that this disease is becoming increasingly prevalent in the South West.
The Cost of Johnes Disease on an Individual Farm
There are many factors to look at when looking at the financial effects of Johnes Disease. These factors include;
- Production losses
- Effect on fertility
- Increased replacement rate
- Reduced market value of stock
- Veterinary and treatment costs
- Effects on concurrent disease processes
- Increased labour costs and management time
The University of Reading has developed a sophisticated computer program that tries to take account of as many of these variables as possible before coming up with a figure for the financial effects that a disease will cause. These figures can never be exact but they do give us general idea of the costs. Some examples from this computer program are given below;
150 Cow Dairy Herd that is Johnes Disease free. 10 adult ‘low shedding’ Johnes Disease carriers are introduced into the herd. If no testing or control measures are put in place over the following ten years then losses are calculated to average £17,000 per year. If management strategies are optimized and a testing regime (eg Herdsure) is introduced then losses can be reduced to an average of £3100 per year.
50 Cow Suckler Herd that is Johnes Disease free. 10 adult ‘low shedding’ Johnes Disease carriers are introduced into the herd . If no testing or control measures are put in place over the following ten years then losses are calculated to average £4200 per year. If management strategies are optimized and a testing regime is introduced then losses can be reduced to an average of £2500 per year.