29 September 2022

5 minute read

Cause

Johne's disease is a chronic enteritis of ruminants caused by M. paratuberculosis.

This bacteria embeds itself in the wall of the lower part of the small intestine known as the ileum. As an immune response, infected tissues attempt to regenerate healthy tissue which leads to visible thickening of the intestines. This prevents nutrient absorption, resulting in weight loss. Late in the infection, antibody production by the animal can be found in serum of animals and is an indicator that clinical signs of disease and death from the infection will soon follow.

M. paratuberculosis grows and multiplies inside the cells of an animal's immunie system and are excreted in the feces, and to a lesser extent in milk and saliva.

When the microbe is excreted, it can contaminate the soil or water. Outside the host animal, the organism multiplies poorly—if at all—but it can survive over a year in the environment because of its resistance to heat, cold, and drying.

The primary cause of the spread of Johne‘s disease is contact with the feces or saliva of an infected animal.

Prenatal exposure may be a source of infection for calves. Becoming infected before birth is possible for a fetus if its mother is in the late stages of Johne’s disease.

Another source of infection is milk from infected dams.

There are huge concerns in many beef and dairy herds across the world about the prevalence of Johne's disease.

Symptoms

Because of the slow, progressive nature of the infection, signs of Johne’s disease may not show up until years after initial infection. When they finally do occur, the signs are long-lasting diarrhea and weight loss despite good appetite.

Bottle jaw may also appear, which is fluid accumulation in the bottom jaw causing an abscess.

Once clinical signs appear the animal will not recover and will continue to deteriorate.

Treatment

There is no treatment for Johne's disease.

Prevention

Prevention is the most cost-effective way to manage Johne's disease. It is far less expensive to block introducing Johne's disease into a herd/flock than it is to control or eradicate the infection once it creeps in and invisibly starts to spread.

Testing ill looking animals can confirm whether Johne's disease is present.

If it is it may be more economical to cull infected animals, if not, the animal must be managed to ensure no young animals are exposed to their milk or manure.

Newborn animals must be protected from infection by being born and raised in a clean environment and fed milk and water free of M. paratuberculosis contamination. The primary source of contamination is manure from an infected adult animal.

Proucers are advised to purchase animals from a source herd free of Johne’s disease. Second best is to work with producer who knows the level of Johne’s disease in his or her herd, follows good infection control practices, and then purchase test-negative animals from test-negative dams.

Johne’s disease is a herd problem, and that knowing the test-status of numerous adults in the source herd will give you a much better sense of the risk of purchasing an infected animal than the one test result you might get on the one animal you wish to buy.

Vaccine

A vaccine has previously being made available in the US to prevent the risk of Johne's. The vaccine uses a mixture of killed mycobacteria and oil. It can sometimes cause large lumps at the site of injection, usually the brisket region. Occasionally these lumps will become draining abscess-like lesions. Although the vaccine is given to calves less than 30 days old, the tissue reaction at the injection site may last for the life of the animal.

Another vaccine available elsewhere made from live M. paratuberculosis (but not capable of causing disease).

The efficacy of vaccines is controversial. Studies in The Netherlands have shown that herd owners who follow the recommended management changes to control Johne's disease will be as successful, if not more successful than those who use the vaccine.

Diagnostics

Only a small proportion of animals will actually develop overt clinical signs that are easily identified and then removed from the herd. The identification of subclinical disease in animals, which can shed the organism over long periods and thus be the source of infection for other members of the herd, is crucial for disease control. Animals are usually infected at a young age, and it can take years until clinical signs appear. PCR and serology tests are available for M. paratuberculosis.

PCR tests are used to detect shedding animals. These tests are able to detect the pathogen itself (or rather the pathogens’ DNA) with very high sensitivity. However, PCR also detects transiently infected animals, i.e. animals which have ingested bacteria, but will not develop an infection and do not have to be removed from the herd. As a rule of thumb, repeated positive PCR tests are indicative for productively infected animals. In these animals, the amount of bacteria shed tends to correlate with the appearance of clinical signs. ELISA tests are used to measure the immune response of the animal against M. paratuberculosis, i.e. if the animal has been in contact with the pathogen. Animals can respond differently to the infection, there is therefore no silver bullet for M. paratuberculosis control programs. Different types of tests are needed to detect M. paratuberculosis infections depending on the stage of disease.

Using a combination of both ELISA and PCR offers the best solution for comprehensive herd monitoring that can be adapted to the operation’s specific needs – whether that’s testing high risk animals in your herd, individual animals or new additions coming into the herd. With both ELISA and PCR tests, results are available in just a few hours rather than days or weeks as seen with bacterial culture methods.

For more information about diagnostic solutions for M. paratuberculosis /Johne’s disease, click here.