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The Lowdown on Johnes Testing

What is Johnes disease?

  • Johnes disease is a bacterial infection that is picked up by calves and is followed by a long subclinical period during which the bacteria incubates in the gut cells.

  • Following a long incubation/subclinical period, during which there may be some low-level shedding, the disease presents clinically, with clinical signs including bottle jaw (fluid under jaw), low body condition and bubbly green scour.

  • Animals in advanced subclinical and clinical stages of the disease are shedding the most.

  • Animals are most susceptible to infection under the age of six months, though it is possible for infections to develop in older animals exposed to high loads of disease.


Interpreting your test results:

There are multiple disease statuses your animals may be classified into following testing.

  • High positive testing animals are those that are likely already showing or about to begin showing clinical signs of the disease. The recommendation for these animals is to cull as they are likely the main contributors to shedding of the bacteria onto your farm, therefore they’re likely the ones most at risk of spreading infections to calves.

    • Their results can be confirmed with a free blood test offered by LIC

  • A positive result often means that the cow is clinically infected with the disease but may not be showing clinical signs yet.

    • It is best to cull these animals before they develop clinical signs.

  • A suspect result indicates that they either have low levels of antibody (similar to positive testing animals; may be infected but not yet showing clinical signs), had a nonspecific reaction, or had some contamination in the sample.

    • Options here include waiting for the next milk test to recheck the disease status or running a blood test. However, due to the nature of Johnes disease, a blood test may return a negative result on an animal that is not yet showing clinical signs.

  • Animals rejected from testing either have too high SCC or too small of milk volume submitted for sampling.


The trouble with testing for Johnes?

  • The long subclinical period can make testing for Johnes more challenging than for other diseases.

  • The LIC herd test is looking for antibodies for Johnes - it is looking for confirmation that the cow has been exposed to the bacteria (through infection) and generated some sort of immune response (antibody) to the disease.

  • In general, until an animal is in the advanced subclinical stage or is presenting with clinical disease, neither the amount of antibody nor the amount of bacteria is actually detectable by testing.

    • Therefore, an animal may test negative in one year’s herd test and positive the following year- they move from being subclinical to clinical.

    • This is also how they can change their disease status (for example can move from being positive to high positive).

    • This is why repeat annual testing is important if you are serious about monitoring with the intention to eliminate Johnes on your farm.

  • When we test the blood, we are also looking for the antibodies against Johnes. This test allows us to confirm the milk test result.


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