What is Johnes disease?
Johnes disease is a chronic, untreatable wasting disease of adult cattle. Affected adult animals shed the disease in their dung particularly at calving time, and newborn calves (both from infected cows and from other cows in the same calving pen) ingest this as they get to their feet and begin to suckle, especially if the teats are contaminated.
These calves are infected but test negative for a number of years by which time they may be part of a breeding herd. As adults they begin to test positive, shed the disease in their dung, later lose body condition and eventually start to scour.
Why is it a concern for my herd?
Animals that begin to scour will continue to do so in spite of any treatment. This may mean that a heifer is reared to the point of first service, she may well get in calf and may calve without difficulty, however as she gets older she will
- Begin to shed the disease without being clinically affected, infecting her calves and potentially many others (your replacement heifers)
- Eventually scour and lose weight – this may be when she is in early pregnancy and therefore she may need to be culled before she calves, or she may reach the point of calving but in such poor body condition that she cannot rear the calf.
- Lose value even as a cull animal as she loses weight.
- This may occur when she is a heifer, or at any stage thereafter but typically when she is 3 – 6 years of age ie she has a very short breeding life and has to be replaced earlier.
- Subclinical Johnes disease in the Dairy herd is closely associated with higher SCC’s, poorer fertility, increased lameness and lower milk yields. Clinical Johnes disease may be less apparent in the dairy herd as many Johnes positive females may leave the herd before they become clinical cases.
For herds breeding quality replacement heifers for sale, the value of these animals is greatly increased if buyers can be assured of the likely Johnes status.
How do I know if there is a problem in my herd?
There may be a suspicion of disease within a herd if there are animals displaying signs of weight loss and scour. These animals may have been culled in the past without testing. If one test positive animal has been identified then the scale of the problem should be assessed by herd screening. For example if one homebred animal has been identified as a Johnes positive, there is likely to be at least one cow (and often many more) that was shedding the disease when this animal was born. All other homebred cattle that were born around the same time would have been at risk of Johnes disease too. So in order to determine if there is Johnes in a herd and to assess the extent of the problem, a herd screen is indicated.
How can I screen my herd and how does this help?
Beef herds – Herd screening for Johnes is carried out by blood testing all breeding animals over 2 years of age (including bulls). This would indicate any animals currently shedding (or about to start shedding) the disease so that they may be removed from the herd at the earliest opportunity, eg testing at the end of the calving period would identify animals that could be removed instead of being put back in calf. Offspring of these animals are usually removed from the herd too as they are likely to have contracted the disease as calves.
This herd test is repeated annually in order to identify positive animals before they start to shed large amounts of disease, when they are still in good condition and can fetch a reasonable cull value. Animals under 2 years of age are not tested, as they are very unlikely to test positive yet although they may do so as they get older.
Dairy herds – Bulk tank sampling is likely to be a relatively insensitive method of screening the herd. Screens of 30 individual milk samples are recommended instead. By selecting older individuals, higher SCC cows, chronically lame animals and those with poorer fertility, we can increase the chance of identifying Johnes positive animals on a herd screen
In both beef and dairy herds, the initial aims of herd screening are:
- Minimising spread of the disease within the herd
- Minimising losses associated with clinical disease
- Maximising the value of homebred heifers sold for breeding
Once the disease is under control, annual testing usually continues for the same reasons. Johnes disease can be transmitted by wildlife, and on clothing, hands, vehicles etc, however these sources are much less significant than infected animals in the herd.
What about joining a health scheme?
Membership and accreditation with a health scheme brings particular benefits when selling breeding cattle, however certain biosecurity measures are required in order to join a CHeCS scheme. Any purchased breeding animals would need to be either sourced from an accredited herd, or isolated and tested on arrival. Certain grazing biosecurity measures are also required; see www.cattlehealth.co.uk, or www.biobest.co.uk for further information.
It is not essential to join a health scheme however in order to gain valuable information from a herd screen. For example the early identification of one Johnes positive cow by blood testing may allow that animal to be removed from the herd and sold for a reasonable price before she starts to lose condition. This financial saving compared being forced to cull a sick animal (possibly during pregnancy) who is spreading the disease to female replacements, would easily cover the costs involved in herd screen sampling.