There are two different tests used on cattle within the IJCP. The first is the ELISA test which looks for the presence of antibodies (the animal’s immune system response to the presence of infection) in a blood or milk sample. Secondly, we have the Ancillary PCR test (ANC) which looks for the presence of DNA from the causative bacteria Mycobacterium avium subspecies paratuberculosis (MAP) in a faecal sample. MAP infection generally progresses slowly whilst hidden from the immune system, meaning that animals are typically infected for years before either the ELISA or ANC test will detect the respective antibodies or DNA, however typically the detection of antibodies precedes the detection of the MAP DNA.
Testing in the Irish Johne’s Control Programme (IJCP): ELISA and Faecal ANC Tests
By: Liam Doyle
Johne's Disease Programme Manager
ELISA Test (Milk or Blood)
The ELISA test checks for antibodies, evidence of the immune system’s response to infection, in either a blood or milk sample. Milk samples for ELISA testing are widely used across IJCP herds, particularly through milk recording, where samples from individual cows are tested and detailed herd reports are generated.
However, interpreting ELISA results isn’t always straightforward. The disease typically progresses slowly and remains hidden from the immune system, meaning animals may be infected for years before testing positive. Generally, ELISA detects antibodies before MAP DNA becomes detectable via other testing methods.
Veterinary interpretation is essential. Your vet will consider:
- The number of animals with positive or inconclusive ELISA results.
- The strength of positive results (S/P values), which indicate the likelihood of infection.
- Historical test results, both herd-wide and for individual cows.
- The herd’s history with Johne’s disease.
- Movement of animals into the herd, including source herd risk factors.
With expert analysis, ELISA results can help guide decisions around breeding, calf care, and strategic culling to manage Johne’s risk effectively.
Faecal Ancillary PCR (ANC) Test
The ANC test detects DNA from MAP in faecal samples. It is used in herds that haven’t previously confirmed MAP infection and is only conducted on animals that test positive in the ELISA test.
The ANC test serves two main purposes:
- Confirming the Presence of MAP in the Herd
A positive ANC result confirms that MAP, and therefore Johne’s disease, is present on the farm. Once MAP infection is confirmed, any animals testing positive or inconclusive on ELISA are likely infected.
- Clarifying ELISA-Positive Results in Previously Uninfected Herds
When an animal tests positive on ELISA:
- A positive ANC confirms the animal and herd are infected with MAP.
A negative ANC means no MAP DNA was found in the faecal sample. This is encouraging but doesn’t rule out infection. The disease may still be in early stages, with antibodies present but not yet shedding DNA. These animals are classified as “suspect” and pose a possible infection risk
Key Takeaway
Work closely with your veterinary practitioner to interpret your test results. They can provide critical insights based on herd history, disease progression, and risk factors, helping you make informed decisions to protect and manage your herd
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