Johne’s disease (JD) is an infectious bacterial disease that occurs globally. The most recent study in Ireland states a prevalence of approximately 20%. JD results from infection with Mycobacterium avium paratuberculosis (MAP) and causes a chronic wasting disease in cattle.

The disease primarily affects the wall of the gut, causing it to thicken, which leads to difficulty in absorbing nutrients from feed. This can lead to weight loss, diarrhoea and eventual death. Additionally, an as yet unproven theory has been proposed citing JD as a possible cause of Crohn’s disease in humans.

Although currently there is no evidence to support this theory, proof of a link in the future would have important consequences for the Irish dairy industry in terms of dairy exports. It is therefore crucial to implement control strategies to minimise levels of Johne’s disease on Irish farms.

Disease transmission

Young calves are most susceptible to JD infection. Infection occurs primarily when an animal ingests faeces contaminated with bacteria (for example, calves sucking an infected cow’s dirty udder). Infected cows can also shed the bacterium in their colostrum and milk, which can lead to infection of calves fed this milk. Finally, in a limited number of cases, calves may become infected in utero.

Once an animal becomes infected, JD progresses slowly. Clinical signs are most common in animals of three to five years of age. In the initial stages of the disease, animals appear healthy and the disease is virtually undetectable, even using diagnostic tests.

As the disease progresses, clinical signs may begin to develop and there is an increased likelihood that animals will test positive. It is therefore essential to complete whole herd tests to identify at-risk animals.

Controlling Johne’s

Animal Health Ireland launched a pilot JD control programme in 2013, which combines the use of diagnostic testing and on-farm risk assessments (VRAMP) to identify at-risk herds.

A number of vets have been trained in VRAMP protocols enabling identification of management practices associated with a risk of transmitting Johne’s. This allows the introduction of strategic management practices to minimise risk on individual farms.

If your herd is currently test-negative for JD, a good biosecurity and purchasing policy is essential. The most common route of entry for Johne’s on to a farm is through the purchase of an apparently healthy, but infected individual.

If possible, avoid purchase of any livestock on to the farm. If this is unavoidable, animals should be sourced from a low-risk herd – ie a herd with multiple negative whole-herd results, which has never recorded a clinical case of JD.

Additional biosecurity practices include avoiding importation of slurry or colostrum, and avoiding sharing of equipment, such as cattle trailers. If equipment is borrowed, it should be thoroughly disinfected before and after use.

Get your calving management right

In herds where evidence of infection exists, control is aimed at minimising transmission to calves. As contact with infectious faeces is a major risk factor for transmission of JD, hygiene is a key element in control.

Dry cows should be as clean as possible when entering the calving area. Regular cleaning and re-bedding of calving pens is essential to minimise the build-up of faeces in the calving pen.

Test-positive cows should be segregated within the dry cow area and calved separately from the main herd. An isolation box or sheltered paddock provides a good solution, although this paddock must not subsequently be used for grazing calves.

Consider raising to beef any calf born to a positive cow, or any calf born in the dry cow area, as their JD risk status is unknown.

Calves should be removed from the calving area quickly, preferably before standing. This again minimises the risk of calves coming into contact with contaminated udders when attempting to suck.

Each calf should only receive their own dam’s colostrum, with the exception of calves from test-positive cows, which should be fed colostrum from a low-risk (homeborn, test-negative) cow.

Colostrum or milk from a positive cow should not be used to feed any calf. Colostrum from multiple cows should never be pooled. Feeding equipment should be cleaned regularly.

Prior to weaning, calves should be managed in group pens situated away from adult cows and their manure. When calves are going to grass, use paddocks that have not been grazed recently by adult animals or had slurry recently applied.

High-risk practices

A nationwide survey by Teagasc showed the majority of Irish dairy farmers engage in a number of JD high-risk practices. This includes housing sick cows in the calving area, overcrowding and not cleaning the calving area.

Over 70% of farmers pool colostrum and use waste milk to feed heifer calves. While a number of these practices may appear labour- and cost-efficient, they are placing farms at undue risk of spreading JD.

It is essential with current herd expansion programmes that farmers evaluate any labour-saving practice in terms of increased disease risk.

Also, removal of quotas may lead to retention of cows that may otherwise have been culled due to sub-optimal productivity.

JD-infected cows may record sub-optimal performance and retention of such individuals could lead to an increase in prevalence of JD.

Available tests

Identification of positive animals aids herd management when implementing control strategies for JD. Individual milk and blood using an ELISA test, or faeces samples using PCR or faecal culture, can all be used for diagnosis. Bulk milk samples are an unreliable indicator of infection and should be avoided.

Interpretation of JD results is not clearcut and test results are best interpreted by a combination of the farmer and his/her vet on an individual farm basis.

Both false positive (not infected with JD, but yield a positive test result) and false negative (infected with JD, but yield a negative result) results can be generated during a testing programme. This stresses the importance of individual farm interpretation when additional risk factors – for example, previous history of stock purchases – can also be assessed.

It is recommend that a faeces sample be sent for culture from any animal yielding positive ELISA results to highlight whether or not the individual is shedding JD. The more test results available for an individual cow, the greater the level of confidence associated with her test status.

Sampling for ELISA testing using either milk or blood samples should not be conducted following TB testing, as false positive results will be recorded. Recent work from Teagasc showed that false positives due to the TB test can occur up to 43 days for milk samples and up to 71 days for blood samples.

Economics

The economic impact of JD will vary between farms and will also depend on the number of animals infected and how advanced the disease is in infected cattle.

In Ireland, however, the economic impact of JD on a number of farms may be minimal, perhaps promoting a reluctance to introduce a control programme.

Studies have shown an overall benefit to implementing JD control programmes on-farm, as a number of the control strategies discussed have led to improvements in general calf health.

Indeed, in light of the proposed link to Crohn’s disease, it is crucial to implement control strategies at a national level to minimise levels of Johne’s disease on Irish farms and maximise access to global dairy food markets.