August 2026 – Understanding Johne’s Herd Test Results

The first is the ELISA test, which looks for signs that an animal’s immune system has responded to Johne’s disease. This test can be carried out using a milk or blood sample.

The second is the Faecal Ancillary PCR (ANC) test, which looks for the DNA of the bacteria that causes Johne’s disease, known as MAP, in a faecal sample.

Johne’s disease develops slowly. An animal can be infected for several years before either test picks up signs of infection. This is why it is important to look at test results alongside the animal’s history and the wider herd picture.

The ELISA test looks for antibodies in a milk or blood sample. Antibodies are produced when an animal’s immune system responds to infection.

In dairy herds, milk samples collected through milk recording can be used for ELISA testing. Individual cows can be tested, and the results are used to build up a picture of Johne’s disease risk within the herd.

However, a single test result does not always tell the full story. Because Johne’s disease develops slowly, an infected animal may test negative for a number of years before the infection can be detected.

Your vet will look at the bigger picture when interpreting your results, including:

• How many animals have tested positive or inconclusive.
• How strong the positive test results are.
• Previous test results for individual animals and the herd.
• The herd’s history of Johne’s disease.
• Animals that have been bought into the herd and the risk associated with their source herd.

This information can help you and your vet make decisions about areas such as breeding, calf management and which animals may need further investigation or removal from the herd.

The Ancillary PCR test uses a faecal sample to look for the DNA of MAP, the bacteria that causes Johne’s disease.

In herds where MAP infection has not previously been confirmed, the ANC test may be carried out on animals that have tested positive on the ELISA test.

The test can help in two ways.

• Confirming MAP is present in the herd
  A positive ANC result confirms that MAP is present in the animal and in the herd. Once MAP has been confirmed in the herd, the information from ELISA testing can be used alongside your herd history and veterinary advice to manage the risk of infection.
• When an ELISA-positive animal has a negative ANC result
  If an animal tests positive on ELISA but negative on the ANC test, it means that no MAP DNA was found in that faecal sample. This is encouraging, but it does not completely rule out infection. The animal may still be infected but may not yet be shedding enough MAP to be detected in its faeces. These animals are classified as suspect animals and should be considered as a possible infection risk.

Johne’s disease testing can provide valuable information, but the results need to be looked at in the context of the individual animal and the herd as a whole.

Your vet can help you understand what the results mean for your farm and use the information to develop a plan to reduce the risk of Johne’s disease in your herd.

If you have questions about your Johne’s disease test results, speak to your veterinary practitioner. They can help you get the most value from the information provided through your herd testing and turn those results into practical management decisions for your farm.

For more information on the Irish Johne’s Control Programme, visit Animal Health Ireland’s Johne’s Disease Programme.