Johnes Disease
Johnes Disease, a bacterial infection affecting cattle and other grazing animals, is caused by Mycobacterium avium paratuberculosis (MAP), a resilient bacterium that can persist in the environment for 12 to 18 months, particularly in damp, cool, and shaded conditions. Calves under six months old are particularly vulnerable, with young stock under 12 to 18 months at
risk. The disease has a long incubation period, often taking years before clinical signs manifest in adults. Adult cattle, especially heavy shedders and clinical cases, can excrete up to 1 million bacteria per gram of faeces, serving as the primary source of infection.
Typical signs of Johnes Disease include a drop in milk yield and reduced weight gain or condition loss initially. As the disease progresses, clinical signs may include profuse, watery diarrhoea, "bottle jaw" (swelling under the jaw), noticeable weight loss, muscle wasting, and ultimately, death within weeks or months.
While Johnes Disease commonly affects cattle over four years old, it can also be diagnosed in younger animals, particularly during times of stress such as calving, dry-off, or nutritional changes.
Unfortunately, there are no cost-effective treatments available for Johnes Disease. Although a vaccine is available for sheep, it interferes with tuberculosis (TB) testing in cattle and is thus not used in New Zealand. Prompt culling or euthanasia upon diagnosis is often recommended as a management strategy.
Testing for Johnes Disease can be conducted through milk testing during herd testing, with positive results often confirmed through blood testing. Blood testing individual suspected cows is the best approach at the individual cow level.
Managing Johnes Disease involves culling infected cows from the herd and implementing surveillance measures to limit its spread, particularly to young stock. Strategies include test-and-cull of high-risk cattle, careful management during calving and colostrum feeding, pre-weaning calf management, proper replacement heifer management, and strict biosecurity protocols for purchased stock.
Useful links:
Dairy NZ Johne's Disease Information
At the bottom of the Dairy NZ page, you can find two valuable PDFs on Johne's Disease, covering milk testing and management strategies.