Johne’s Disease (JD) – Controlling the Spread on an Infected Farm

As noted in our previous bulletin, the most recent estimate of JD prevalence in Irish dairy herds was 28%, based on a 2016 survey. Although no new national estimate has been published since, the trend is suspected to be increasing.

It spreads when MAP is passed from infected or shedding animals to young calves and other vulnerable stock. Typically, older animals shed the most bacteria, while calves in their first few months are most susceptible to infection.

JD can circulate silently for years before any clinical signs appear. Animals become infected primarily in two ways:

1. Through dung, colostrum, and milk – Calves may ingest MAP bacteria from contaminated milk, colostrum, teats, or bedding.
2. Before birth – Infected dams can pass the bacteria to their calves in utero. This risk increases as the dam’s disease progresses.

Once MAP is shed, it can survive in the environment—including slurry—for many months, and in some cases, over a year. This long-term environmental persistence contributes to ongoing transmission without visible signs. By the time the first case of JD becomes apparent, it’s likely that:

• Several other animals in the herd are already infected but undetected.
• Calves and other young animals are at significant risk.

As illustrated in Figure 1, infection can gradually escalate: more infected animals shed more MAP into the environment, which in turn increases the infection rate among susceptible stock—a classic snowball effect.

The most important group to protect on your farm is the calves—especially those selected as future breeding stock. Preventing infection in this cohort has the greatest long-term benefit for reducing Johne’s Disease prevalence in your herd.

Certain common practices can significantly hasten the “silent” spread of JD on infected farms, particularly those that allow one infected animal exposure to multiple calves. Key high-risk practices include:

1. Feeding pooled colostrum or milk to calves.
2. Using group calving pens.
3. Inadequate cleaning and disinfection of calving pens between uses.
4. Allowing adult cows to share housing with calves or young stock.
5. Treating sick adult cows near calf pens or young animal areas.
6. Spreading slurry from adult animals on pastures grazed by calves or young stock.
7. Poor hygiene when entering calf pens or handling calves—especially without cleaning and disinfecting boots and hands.

Each of these practices increases the likelihood of widespread, undetected MAP transmission.