Johnes Disease
February 2023

Johnes disease is caused by the bacteria Mycobacterium avium paratuberculosis. It is a disease which causes a cow to rapidly lose condition while still eating normally. She will have watery bubbling diarrhoea.

While cows can catch Johnes at any age, they are much more susceptible and will usually catch it as calves when they are under 6 months of age. However, we don’t see the wasting (clinical) side of the disease until they are 5-7 years old after a period of stress. If the wasting form of the disease has been diagnosed in a heifer, then this can mean there is a high burden of Johnes on your farm.


Herd testing in the milk is a useful way to go about testing your whole herd to see which cows are currently shedding Johnes. Timing of milk testing should be considered. February is the most popular time, so it does book up quickly. The benefits of doing it in February are due to a high milk production which makes the results more accurate, and it is before the cull cows are sent. If you have a herd test coming up it can still be done at this time of year. The downside of doing it too close to dry off is that it will miss any cows that are dried off early and when there is low milk production or high SCC (>1million) there is a risk of false positives. Any positive cow on milk test can be confirmed with a blood test. Getting this information before the cows calve down next season is very important for the management of this disease.

No Johnes test is going to find all the infected cows so there will be false negatives but testing does give us a useful management tool. This is because cows do not usually shed or shed intermittently

unless they are near the clinical stage. Testing clinical cows (wasting disease) for Johnes is very accurate. If that test comes back negative the wasting is unlikely due to Johnes.

The results from the herd test will come back as:

  • High Positive
  • Positive
  • Suspect
  • Not detected

It is best to discuss with your vet a management plan for each category for what will fit with your farm.

Reducing Risk in Calves

Because the risk of catching Johnes is higher in calves, management around the disease involves ways to protect these calves from exposure. The calves of the infected cows should not be kept as replacements. There is a high risk that they are already infected with Johnes. Effluent should not be used on any paddock the calves graze on or anywhere near the calf pens.

Yearly testing will help identify the cows that are shedding before calving. But there will always be some that are missed. Therefore, these cows will still be contaminating the environment and the pooled colostrum/ milk. Keeping the calves in a clean environment and keeping them off effluent paddocks will help reduce the risk and each year should reduce the number of cows shedding Johnes.