Johne’s disease is a chronic disease that causes cattle to waste and die, typically from the ages of three to five years old. Once an animal contracts Johne’s, treatment is futile.
Therefore, the approach needs to be to eliminate infected animals as soon as possible and break the transmission between infected and susceptible cattle.
Vet Kaz Strycharczyk, of Black Sheep Farm Health, answers some of the key questions about the disease.
What causes Johne’s?
The disease is caused by Mycobacterium avium subspecies paratuberculosis, known as MAP. It is closely related to the bacterium that causes bovine and human TB, and shares the resilience and tenacity of its cousins.
It is shed in dung and milk and can survive for up to a year on pasture. It affects other mammals, including sheep, goats, deer and rabbits.
It is widespread – data from south-west England in 2006 found that 97.9% of dairy farms and 78.9% of beef farms had Johne’s-positive cattle.
How does the disease present in cattle?
Affected cattle lose condition despite good appetite, with a characteristic bubbly scour. The final stages of the disease are often precipitated by a period of stress, such as calving.
Another characteristic it shares with bovine TB is the protracted period between infection and actual signs of disease, often measured in years. It also takes a long time before the bacteria are detectable in laboratory tests from infected animals.
For this reason, no farm is described as “Johne’s-free”; instead we talk of risk levels. These levels are in comparison with other diseases such as bovine viral diarrhoea (BVD) or leptospirosis, from which we can declare freedom with reasonable confidence.
- Level 1:
There have been three clear annual herd tests. This is the lowest level of risk.
- Level 2:
There is a current clear herd test, but it has not yet gained Level 1 status.
- Level 3:
At the most recent herd test there are reactors in the herd at the level of 3% or less.
- Level 4:
More than 3% reactors at the most recent herd test.
- Level 5:
Those herds without a health plan for Johne’s disease and which do not adhere to the mandatory elements of the health plan are at Risk Level 5. This is the highest level and additionally applies to herds that carry out no testing.
How can it be managed?
The control of Johne’s on cattle farms is swiftly moving into the mainstream, as both breed societies and farm assurance schemes recognise the value and start to make testing compulsory.
Most dairy farmers should now be familiar with the National Johne’s Management Plan (NJMP), as most milk purchasers – representing about 80% of milk bought in the UK – now require suppliers to commit to deliver on the NJMP’s recommendations.
While there are costs associated with membership and laboratory fees, these are outweighed by the benefits, even when the disease is at a low level in the herd.
A cheaper but more reactive alternative would be to test any suspect animals – any adult cattle which lose condition and/or start scouring. The causes of scouring in adult cattle are relatively few, and cases should not automatically be chalked up to liver fluke.
Can you vaccinate against it?
A Johne’s vaccination exists and there is great scope for wider use of this vaccine in UK sheep. However, the vaccine interferes with the bovine TB skin test and so its use in cattle in the UK is a last resort and only with strict veterinary supervision.
How can it be prevented?
Prevention relies on several methods:
- Annual testing, as previously discussed, along with swift removal of test-positive animals is the foundation of most control programmes. Bear in mind that the skin test for TB test can trigger false positives if Johne’s testing is carried out too soon after the tuberculin injection; blood tests should be left at least three months after the first day of the TB test.
- For suckler herds, the first step is to establish your status. The best way to do this is to join a health scheme and start testing every animal over two years of age annually. This can easily be combined with other tasks such as pregnancy diagnosis.
- If a cow is heavily in-calf and cannot be moved off the holding, it should be calved down in isolation, then sold as a cull.
- Calves from Johne’s-positive animals should not be retained for breeding as there is a strong chance they will have been infected in utero or via colostrum.
- Once herds reach the top risk level and have remained at this level for several years, you may opt to move to biennial herd testing, plus screening any cull cows.
- Your replacement policy is also key. Breeding your own replacements is ideal; replacements should not be retained from Johne’s-positive cows.
- If buying in, source from herds with the best risk level you can find. When bringing any cattle in, they should have blood and dung samples taken as a quarantine screen, unless they are from an R1 herd (the lowest risk).
- The wildlife element in transmission is probably the most difficult to counter. Providing piped water and limiting access to ponds and streams would be a good start.
How prevalent is it in the sheep sector?
Very few sheep flocks test for Johne’s, but it is estimated that about two thirds of UK flocks are infected.
We still have a lot to learn about how Johne’s is transmitted between cattle and sheep on farms which co-graze.
Given that uncertainty, it seems prudent to manage potential routes of infection by performing cull ewe screens to monitor the Johne’s level in the sheep flock, using the Johne’s vaccine to reduce shedding, and keeping calves off sheep-grazed pasture for as long as you can.
Each month, we bring practical independent advice from an XLVets practice on a range of different subjects. This month Kaz Strycharczyk, of Black Sheep Farm Health gives advice on Johne’s. Kaz joined Black Sheep in 2018 and has been involved in a calving nutrition study with the University of Edinburgh, and run several ‘Mastering Medicines’ courses.