Young stock are at the greatest risk of contacting Johne's disease, a notifiable disease in Ireland, but farmers can take measures to minimise the exposure to disease.

Johne's disease is spread via the bacteria, Mycobacterium avium subspecies paratuberculosis (MAP).

It is a persistent bacterium that can survive for months on pasture and significantly longer in sheds.

The predominant clinical signs of Johne's in cattle are a chronic severe watery diarrhea and a severe loss of body condition in spite of a good appetite.

Johne's disease is spread via contact with infected faeces but is also commonly passed on through colostrum and suckling. Calves fed on whole milk or suckler calves are at higher risk of infection as a result.

New research from the Royal Veterinary College has shown that cows passed Johne's antibodies onto calves via colostrum.

The research concluded that while these antibody measurements are not a diagnostic test for infection in newborn calves, they may eventually complement existing herd surveillance and risk assessment strategies that could help improve disease management and animal welfare.

The UK has a higher prevalence rate than Ireland for Johne's disease, with more than 50% of herds affected.

Feeding calves

Farmers suffering from drought conditions are often feeding out silage, straw or hay to weanlings.

If these calves are being fed indoors in sheds that were previously used to house mature animals, a good power-hosing and disinfection will reduce the risk of transmission.

Farmers in the northern and some western counties will still have high grass growth rates. These areas should not that grazing calves and weanlings are at increased risk of infection where slurry has been spread recently.

Where calves are grazing, it is important to ensure older mature stock have not grazed recently, as undisturbed dung can carry infection.

For this reason, leader-follower systems offer the best protection against infection. This is not practical in most dairy systems but works well in suckler enterprises.

Symptoms

Johne's infected cows can become infectious from two years-of-age, but symptoms can take years to develop. Clinical symptoms often occur only after five years-of-age.

Farmers must not wait for Johne's positive cows to show symptoms before taking precautionary measures.

In dairy herds with high replacement rates, farmers may never see Johne's symptoms despite persistent infection.

Farmers are advised that even when subclinical, Johne's is estimated to cost the farmer €33/cow. For a 100-cow herd, this equates to €3,300/year.

According to AHI, Johne's disease costs the Irish dairy industry between €11.24 million and €23.85 million per year through lost production and culled cows

Testing

The dairy herd is monitored via milk collections. Through co-ops, the Department of Agriculture, Food and the Marine (DAFM) conducts two yearly Bulk Tank Milk (BTM) tests during spring and autumn from each supplier. This programme began in 2019.

The BTM tests' reliability is open to debate, as it requires a higher level of antibodies than a standard faecal test.

Initial data from 2022 suggests that it only identified 16% of positive cases based on the estimated national herd incidence of 30% at the time.

AHI confirmed to Agriland that identifying an exact number for prevalence is hard, as testing at a national scale is expensive and time-consuming, as blood or faecal tests are required to give accurate numbers.

However the BTM test is an indicator of high levels of infection in a herd.

It is estimated that the tests sensitivity improves where the herd's prevalence rate is greater than 9% (9+ cows out of every 100 infected).

False positives are also possible with an estimated 5% of herds being falsely identified. TB tests or other similar bacteria exposure in particular can impact test accuracy.

AHI's Irish Johne's Control Programme helps farmers who apply to join by providing support and advise, on minimising future risk, organising secondary tests if BTM tests positive, and other supports.

There is no widespread comparable testing for beef herds by factories with a herd level approach adopted to control the disease.

New Zealand approach

New Zealand tackles Johne's differently to Ireland. It is not treated as a notifiable disease, but culling upon identification is advised.

DairyNZ advises farmers to cull where infection is detected or as soon as possible, do not calve Johne's-positive cows, and avoid feeding colostrum from untested cows.

As a pasture-based system, DairyNZ advises farmers similarly to Animal Health Ireland in that biosecurity is crucial to avoid importing the disease.

Importing disease is more likely in New Zealand as 50% of herds have Johne's positive tests but DairyNZ admit the real figure is likely higher due to subclinical cases being undetected.