Johne's Disease Factsheet
Johne's disease is an infectious wasting condition of adult cattle and other ruminants, including sheep and goats. It is caused by Mycobacterium avium subspecies paratuberculosis (MAP). This organism is closely related to Mycobacterium bovis, the organism that causes bovine tuberculosis. Infection with MAP leads to progressive damage of the intestines, and in cattle this leads to profuse and persistent scour, with severe weight loss. Eventually, an animal with clinical Johne's disease will die.
Awareness of Johne's disease is important because the classical signs of the disease will result in affected animals being prematurely culled from the herd, and therefore replacement rates need to be higher. This can lead to having to source additional replacements from outside the herd, which in itself can pose the risk of introducing other new diseases.
Infection with MAP occurs most commonly as a young calf. Infection is spread via the faeces, with an animal becoming infected through exposure to and ingestion of contaminated faeces, such as on the udder of the calf's mother, or through contamination of colostrum. The calves of an infected herd are most susceptible, but if the environmental infection levels are very high older animals can be susceptible too.
Spread of the disease between herds is usually as a result of purchasing infected replacement breeding stock, both bulls and heifers/cows. These animals can be incubating the disease without showing any clinical signs of disease.
In addition to farmed livestock, MAP has also been identified in many wildlife species including rabbits and deer. It may be that these animals are implicated in the spread of disease.
When an animal is first infected with MAP it is usual for a long period of time to lapse before the onset of clinical signs and disease. This is because the organism is very slow growing. The peak incidence for the onset of clinical signs in cattle is between 3 and 6 years of age, but occasionally both younger and older animals can go develop clinical signs of disease.
Confirmation of disease can be difficult, but diagnosis depends on either demonstrating the presence of MAP in the animal's faeces or detecting MAP specific antibodies in the blood. NMR offer screening of herds by looking for antibodies in individual cow milk samples. None of the tests give total accuracy, however; test results are quite often negative when an animal is incubating the disease, and an infected animal may test negative for many years before it tests positive. This means that for every animal which tests positive in an infected herd, there are likely to be several others which test negative but are still infected.
This difficulty, together with the prolonged incubation period of the disease and the ability of MAP to persist in the environment, makes the eradication of Johne's disease from infected herds - particularly heavily infected herds - a challenging and long-term commitment. However, the benefits of eradication and Johne's-free status are obvious, both from reduced culling and replacement stock, and also in herds where cattle are sold for breeding: being able to sell cattle as 'Johne's free' can add financial value to the stock.
If you suspect Johne's disease in your herd there are a number of measures which should be taken. These include:
Isolating all animals showing clinical signs of diarrhoea and weight loss. Veterinary assistance should then be sought to assess the animal and tests taken as appropriate;
Animals confirmed to be suffering with Johne's disease and those where clinical suspicion is high but disease cannot be ruled out should be culled;
Offspring of affected animals should be traced and not kept as replacements;
Cows and their environment should be kept as clean and free from faecal contamination as possible, especially during the calving period and for the first 3 months of the calves' lives;
Prevent faecal contamination of feed and water supplies and keep troughs clean;
Provide mains water and fence off other water sources, particularly areas of stagnant water;
Spread slurry or manure on arable land. If this is not possible grass that has had slurry or manure applied should not be grazed for up to a year following application.
In herds where disease levels are high, vaccination can be considered as part of the control measures for reducing the incidence of disease in the herd. However, there are disadvantages of using the vaccine: although it may help to reduce the numbers of animals in which disease develops, it does not prevent infection with MAP and a small percentage of animals that are vaccinated will still go on to develop clinical signs of disease. Also, vaccinated animals will react to tests for the disease, meaning eradicating the disease in a vaccinated herd becomes further complicated. In addition, vaccination against Johne's disease can interfere with the diagnosis and control of bovine tuberculosis.
There is an increasing public awareness of Johne's disease amongst the general public. This is because it has been suggested, although not proven that MAP maybe the cause of Crohn's disease in people. Although MAP is killed in milk by the pasteurisation process, many milk and beef buyers, especially the supermarkets, are putting increasing pressure on producers for the milk and beef that they purchase to be from disease and from accredited sources. There are various schemes which offer disease-free accreditation for Johne's disease including the VLA, SAC and Biobest. If you are interested in joining one of these schemes then please feel free to contact us at the surgery.