Large animal - October 2021

Johne’s disease (JD) is a bacterial disease of cattle for which there is no cure. It is caused by the bacterium Mycobacterium avium subspecies paratuberculosis (MAP). Animals are susceptible from birth and once infected (mainly through the faecal-oral route), the disease is progressive. Johne’s prevention and control is important in both dairy and beef herds. Many herd owners do not fully appreciate that prevention is much easier and offers more certainty than trying to control infection once it has become established in a herd. For a variety of reasons, herd owners find the infection extremely difficult to eradicate once it has become well-established in a herd.

A 2016 study estimated the prevalence of JD in Irish dairy herds at 28 per cent (McAloon et al, 2016). Since the removal of milk quotas in 2015 and the resulting herd expansions, experts believe the prevalence has likely increased. Research confirms that, without effective control measures, the incidence of JD will continue to rise over time.

Once infected, an animal progressively becomes more unproductive. Initially, there is a reduced feed-conversion efficiency leading to a lower than expected weight gain, followed by weight loss, scour, and ultimately emaciation and death. These changes may not be recognised unless the animals are regularly condition scored and weighed, or milk production is being measured. Signs are often only noticed when animals are approaching peak productivity. In some cases, infected animals may never develop the full range of clinical signs but are still able to shed MAP.

Some animals have a sub-clinical infection, and these animals may also have reduced fertility and produce less milk while continuing to test negative. This affects dairy production directly and, in suckler herds, impacts calf growth. In beef herds, slaughter weight and carcase value may be reduced. Infected cattle may also be more susceptible to other diseases.

What are the economics of Johne’s disease

The impact of JD may be more subtle than the classical clinical signs described in the literature. Indeed, an infected herd may not display clinical signs like diarrhoea but still suffer from production failures that lead to premature culling.

A review of scientific literature on JD underscores its wide-ranging negative effects, including:

• decreased milk yield;
• premature culling, increasing replacement costs;
• lower market value of culled animals;
• elevated lameness levels;
• reduced feed conversion efficiency, leading to higher feed costs;
• compromised herd fertility;
• increased somatic cell counts in milk; and,
• elevated veterinary expenses and antimicrobial use.

Importantly, these losses tend to escalate as the level of infection within a herd increases over time, resulting in greater economic and welfare consequences for affected farms. Conversely, studies show that herds with low JD prevalence experience negligible financial losses, underscoring the importance of monitoring herd JD status and implementing control measures.

One specific area where JD causes economic loss is in milk production. An international scientific study, which included data from Ireland alongside other countries (Rasmussen et al, 2021), estimated that approximately one per cent of gross milk revenue is lost annually in MAP-infected dairy herds.

The study also compared economic losses across various countries using different assumptions, resulting in an estimated annual cost to the Irish dairy industry ranging from €11.24 million to €23.85 million. A key factor influencing these losses in all scenarios was the prevalence of JD within herds. Specifically, higher infection rates within herds significantly amplify the economic impact.

Johne’s disease – preventing introduction and within-herd spread

JD is usually introduced to a herd by an animal which appears healthy and may even have had a single test with a negative result. As the animal matures and it starts to shed MAP in its faeces, the cycle of infection continues as other susceptible animals in the herd come into contact with the infective faeces.

Preventing the introduction of JD

Preventing the introduction of JD and many other diseases relies heavily on strong biosecurity practices. Some of the main pathways of JD onto a farm are as follows:

1. Purchasing infected animals
   JD-positive animals may appear healthy and can even test negative, but they may still shed large quantities of MAP bacteria in their faeces, posing a major risk, especially to young animals. Bulls are a particular risk as they will have contact with most of the breeding herd and thus, if they are passing MAP bacteria in their faeces, can easily spread JD.
2. Importing contaminated colostrum
   Colostrum can carry MAP if harvested in unsanitary conditions or sourced from infected cows.
3. Bringing in slurry or manure
   Importing manure (excluding pig or poultry manure) from a MAP-positive farm risks contaminating your pasture, especially endangering young stock.
4. Off-farm animal movements
   Animals that leave your farm – for example, to shows, marts, or contract rearing units – can pick up JD by exposure to infected faeces. Adult animals are less susceptible to picking up JD; however, the risk is not zero.
5. Visitors and workers
   People can unknowingly carry MAP-contaminated material between farms on boots, clothes, or equipment, particularly endangering calves and young stock.
6. Contaminated vehicles and equipment
   Machinery, trailers, and other farm vehicles can spread infected faeces if not properly cleaned before entering your farm.

Preventing within-herd spread of JD

One of the major challenges in managing JD is its slow and silent progression. An infected cow will often show no signs until they have had three or more calves, by which time the infection may already be well-established in the herd.

Calves are typically infected early in life by drinking or eating milk or food containing, or contaminated with, MAP. On occasion, calves are born infected, most often when their dams have advanced Johne’s disease. One infected cow in a calving pen or housed with other cows and calves can infect a number of calves that come into contact with her faeces. Calf-to-calf transmission can also occur in calf pens, and dairy farmers are advised to take particular care with the rearing of any stock which will become future breeding animals.

In order to prevent within-herd spread, a control programme for JD must aim to break the infection cycle, focusing primarily on reducing the spread of JD among calves.

The following areas are vital in terms of preventing within-herd spread:

1. Ensure pre-calving cows are clean

Cows with dirty coats, udders, and tails pose a greater risk of transferring faecal material to their calves and contaminating the calving area.

• Clip tails and clean udders well before calving to reduce contamination risks.
• Keep cows on straw or other clean bedding before they enter the calving area.
• Maintain clean coats and udders to minimise the risk of calves ingesting contaminated material while suckling or moving around the cow.

2. Manage enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction (PCR) test-positive cows separately

Cows that test positive for MAP using the faecal ancillary PCR (ANC) test are actively shedding bacteria and should be prioritised for immediate culling.

ELISA-positive and inconclusive animals, even if testing negative on ancillary tests, are still considered high-risk and should have specific management plans.

• If not culled before calving, these cows should calve in separate areas, away from where other calves are held or test-negative cows are calving.
• Implement physical barriers, such as solid fences or walls, to separate PCR- and ELISA-positive animals from the rest of the herd.
• Regularly change or top up calving pen bedding to prevent the build-up of infective faeces. MAP is susceptible to the standard iodine and phenolic based disinfectants used against bovine TB but resistant to chlorine and QAC based disinfectants.
• If space is limited, consider dividing a larger calving area into smaller sections using temporary fences or bales.
• By segregating high-risk animals, the chances of contaminating calving pens and spreading infection through faecal transfer are significantly reduced.

3. Remove calves quickly to minimise exposure

The longer a calf remains in the calving pen, the greater its exposure to MAP bacteria.

• Calves from PCR- and ELISA-positive cows should be removed immediately after birth to limit their risk of exposure.
• Calves born in cubicle areas or locations where pooled faeces from multiple adult cattle is present are at high risk and should be considered exposed.
• Pay extra attention to future breeding stock. If possible, relocate them as soon as possible to a separate, biosecure area away from adult cattle. Where practical, consider assigning them dedicated personnel.

The Irish Johne’s Control Programme

In Ireland, the Irish Johne’s Control Programme (IJCP) functions as the national programme used by vets on farms around the country to control JD. This programme started in 2013 as a pilot version and has over the years moved through various versions, currently being launched in a new, updated format in 2026. The JD Technical Working Group (TWG) made up of practising vets and representatives from academia and Department of Agriculture, Food and the Marine (DAFM) have developed the IJCP based on a set of agreed objectives:

• Objective 1:
  To establish a framework for farmers of low risk/low prevalence herds to demonstrate higher confidence in the infection status of their herd.
• Objective 2:
  To support farmers of infected herds both in reducing the rate of new infections and in appropriate management of existing infected animals on their farm.
• Objective 3:
  To identify farms at high risk of introducing JD due to high-risk trading practices and to afford these farmers an opportunity to manage this risk with input from a trained approved veterinary practitioner (AVP).

From these objectives, the TWG have developed a programme with the aim of bringing about their delivery. The required components of the programme for members are:

1. Nomination of an AVP, these being private veterinary practitioners who have undertaken JD training delivered by AHI.
2. Regular completion of a herd visit by a nominated AVP to complete a risk assessment and develop a tailored programme to meet the needs of that herd.
3. An annual whole herd test (WHT) of eligible animals (those aged two years and above) using either a milk or serum sample from each animal.
4. Ancillary testing of faecal samples (by PCR) of all animals following ELISA test-positive result in herds where infection has not already been confirmed (i.e., absence of previous faecal-positive result).
5. Members undertake not to move any animal that is inconclusive, positive, or suspect based on testing for JD, except directly to a knackery, licensed slaughter premises, feedlot, or herd from which animals are exclusively sent to slaughter.

Testing in the IJCP – the ELISA test (milk/blood) and the faecal ancillary PCR (ANC) test

There are two different tests used on cattle within the IJCP. The first is the ELISA test which looks for the presence of antibodies in the blood or milk sample. Secondly, we have the faecal ancillary PCR (ANC) test which looks for the presence of DNA from the causative MAP bacteria in a faecal sample. Typically, the detection of antibodies precedes the detection of the MAP DNA. It should be noted that bovine TB testing can interfere with the results of JD milk or blood ELISA testing, by causing false positive results in the JD test and, thus, the IJCP recommends a 90-day separation between them.

The ELISA test (milk/blood)

The majority of herds in the IJCP now implement the ELISA test through milk recording, where a milk sample from each animal is submitted for analysis. This means that a detailed report is generated giving information on each cow sampled. The type of information returned from an ELISA test which is considered when interpreting a herd test is:

• The number of animals in the herd with positive or inconclusive ELISA results.
• The strength of the positive ELISA result (known as the sample-to-positive (S/P) ratio/percentage), with higher values associated with higher likelihood of the animal being infected.
• Test results from previous years at both herd level and for the animals with an ELISA-positive or inconclusive result.
• Disease history for the herd, especially whether there has been a prior diagnosis of JD.
• The history of animal movements into the herd, including number of animals, number of source herds, and risk factors for those source herds (infection status, test results, trading history).

ELISA results can be used to assess management activities, such as breeding, calf care, and priority culling.

The faecal ancillary (ANC) test

In the IJCP, the faecal ANC test is used on cattle (in herds which have not previously disclosed MAP infection) and which have had an ELISA-positive result in the milk or blood test. It fulfils two different functions in the IJCP:

1. It attempts to prove if MAP is present in the herd.

A positive ANC test demonstrates the presence of MAP, and thus JD, on the farm. Once a herd has a positive result from an ANC test this confirms infection in the herd and any animals returning a positive or inconclusive to an ELISA test can reasonably be interpreted as being infected.

2. It assists in clarifying the infection status of ELISA-positive animals in herds with no previous evidence of JD.

Two results are possible following an ANC test in an animal that previously tested positive to the ELISA test. A positive result confirms infection in the herd while a negative is good news but does not prove the animal or herd are uninfected.

How will the updated version of the IJCP improve JD control in Ireland?

This new version of the IJCP is designed to build upon and enhance previous versions of the programme. One of the aspects focused on is the manner in which epidemiological information related to JD control is communicated to farmers and vets. With the new design, the TWG took the important epidemiological aspects of JD and used these to produce a disease-focused reporting system which vets can use to deliver effective, tailored control plans on farm.

This new reporting system available on the ICBF homepage has three important parts:

1. A new herd disease-focused report which will give farmers and vets all the up-to-date herd JD epidemiological information they can track over time.
2. A Johne’s Programme progression report which will allow herds in the IJCP to track progress over time in gradually reducing the risk of JD in their herd.
3. An individual animal report, which will list out, in detail, animals in a herd at higher JD risk. This report has all the information for an animal in one place, allowing better decision-making on culling.

This new IJCP reporting system will work alongside the targeted advisory service on animal health (TASAH)-funded veterinary farm visits. This will allow vets design tailored control programmes for individual farms. It also provides a solid foundation for future enhancements and responsive changes. AHI will be embarking on provision of training to vets in the functioning of the new IJCP.

All photographs courtesy of Animal Health Ireland.

Any farmer who wants to join the IJCP should simply scan the QR code in the right-hand column or go to the Animal Health Ireland (AHI) link, also provided, and follow the instructions. Membership of the IJCP is free to join and it includes an array of benefits, such as access to the ICBF Johne’s homepage which has invaluable information, and the new reports which will benefit anyone involved in JD control. Go to: https://portal.animalhealthireland.ie/programs/

or scan the QR code: