Johne's Disease (Paratuberculosis) Antibody ELISA

Johne's disease, also called paratuberculosis, is a long-lasting infection of the gut caused by Mycobacterium avium subsp. paratuberculosis, often shortened to MAP. Calves usually pick up the bacteria early in life from dung, contaminated colostrum or milk, or the dirty udder and teats of an infected mother. The disease grows very slowly, and signs often appear only years later in adult animals. These signs include weight loss even though the animal keeps eating, long-lasting diarrhoea, a drop in milk and, in some animals, swelling under the jaw.

This test is an ELISA, a lab method that uses a colour change to show whether antibodies are present. Antibodies are proteins the body makes after meeting a germ. The test looks for antibodies to MAP in serum or milk. Infected animals often start making these antibodies only late in the disease, when they may also be shedding more bacteria in their dung. Because of this, the antibody ELISA is commonly used to screen herds, to check animals with signs of Johne's disease and to check animals before they are bought or added to a herd.

Early infections often test negative, so a negative result in a young or healthy-looking animal does not mean it is free of Johne's disease. Herd screening is often repeated over time, and your vet may add a faecal PCR or culture, which look for the bacteria in dung. A positive result is usually followed up with these tests or a repeat, as your vet decides, because occasional false positives can happen. A registered veterinarian reads the result together with the animal's age, signs, herd history and other tests.