Johne’s disease (JD), also known as paratuberculosis, is a chronic infection caused by a bacterium called Mycobacterium avium paratuberculosis (MAP). The bacterium infects the gut of cattle and other ruminant animals, causing the intestinal wall to gradually thicken and become inflamed. This results in leakage from the gut wall and prevents uptake of vital nutrient by the animal.
Johne’s disease is widespread amongst NZ dairy herds. Most infected herds harbour low levels of JD with occasional clinical disease. In some herds Johne’s disease becomes problematic with annual losses of 1% and more in the milking herd.
What are the symptoms?
Calves and young stock are particularly susceptible to infection, however the disease has a long ‘incubation period’ so clinical signs of Johne’s disease typically do not appear until several years later in the adult cow.
Falling milk production (lower lactation worth) may precede overt clinical disease. Clinical Johne’s disease is characterised by ill-thrift and weight loss with profuse diarrhoea – the animal basically starves in spite of a good appetite. “Bottle jaw” (swelling under the jaw) is the result of severe protein loss. There is no cure for this disease and the condition is fatal.
How is it spread?
MAP bacteria are primarily spread through faeces and ingested with contaminated pasture, colostrum/milk, feed, and water.
Infected cows start shedding bacteria some time before clinical signs of disease appear. As the infection progresses, increasing amounts of bacteria are excreted in faeces of heavy shedder cows. Calves may pick up the infection in the calving paddock and remain at risk if they are exposed to faeces or effluent from the adult herd.
Cows with advanced infection (both clinical JD cows and subclinical) will also transmit the bacteria to the unborn calf in utero and via colostrum or milk.
Download MAP bacteria lifecycle diagram
How can Johne’s disease be managed / controlled?
During the early stages of Johne’s disease infection diagnostic tests are unreliable, so JD control can be a challenge. It is possible to minimise the impact of JD by reducing the exposure of disease to replacement heifers.
The severity of Johne’s disease in an individual (ie time until clinical signs develop and the amount of faecal shedding) depends on several factors including the age when infection occurred, the amount of bacteria ingested and the number of times the animal has been exposed to JD. Therefore any management measures that reduce exposure (eg avoiding effluent paddocks and paddocks grazed by the adult herd) will help limit the spread of Johne’s disease.
The risks and control measures vary from farm to farm and good veterinary advice is critical to control JD. Key to a successful risk management plan are:
- Eliminating the source of bacteria by early removal of clinical JD cows and other high-risk shedders.
- Removing susceptible calves and young stock from possible sources of infection as soon as practicable.
LIC Animal Health offer whole herd Johne’s testing for dairy cows on herd-test milk samples to identify high-risk cows with advanced Johne’s disease.
These cows shed high numbers of bacteria and are a major cause of the spread of the disease to other animal’s especially young stock and calves. Our testing may detect “super –shedders” that are not showing signs of the disease at the time of testing, but they are likely to develop clinical JD in the near future.
Test Limitations and timing
The Antibody ELISA test detects the immune response of the animal to the MAP infection. While it generally does not identify cows during early stages of subclinical infection (ie low-risk non-shedders or intermittent / low shedding), the test performs very well in advanced stages of JD and will identify most heavy shedders.
The test will identify 8 to 9 out of 10 cows with clinical disease or that are excreting large amounts of bacteria (heavy and super shedders).
The screening test should be carried out later in the season (Feb/March herd test for NI herds and April/May herd test in the SI) to maximise the benefits and identify as many high-risk cows as possible to be culled before calving.
Low milk volume can cause false positive results, so avoid testing too close to dry off or when part of the herd has dried off.
How to book
All animals at the herd test will be tested.
This test must be booked by the herds veterinarian to ensure follow up interpretations of the results and implementation of a disease management plan.
For further information, please contact the Animal Health Advisor’s team at LIC on 0800 436 362 or email firstname.lastname@example.org
Download a PDF about Johnes testing.