Since 2015, there has been a steady increase in both the number of herds infected with TB, as well as the number of animals that are slaughtered as a result.

In the 12 months to September 2020, 20,930 animals have gone down with TB and 4.24% of herds tested have had at least one reactor.

There are a number of reasons for the worsening disease trends in the last five years, according to senior Department vet Eoin Ryan.

One of them is the expansion of the dairy herd. Scientific evidence shows dairy herds are at higher risk of TB than other herds because of larger herd sizes, more older animals and lots of animals in close proximity to one another.

Ryan stressed it was not about attributing blame on any one cohort of farmers

Another issue he pointed to was cattle movements. There are approximately 2.8m movements recorded in Ireland per year. The more herds an animal moves through, the greater the chance it will be exposed to TB.

One anecdotal piece of evidence reported by Department staff in the field was that the 2018 drought drove an increase in interaction between livestock and wildlife, increasing TB spread.

Ryan stressed it was not about attributing blame on any one cohort of farmers or saying “who was at fault”. He said it was important that farmers understood the risks in order to lower them.

TB testing: the facts and figures

One of the greatest areas of farmers’ frustration, along with compensation, is the TB testing programme itself.

At a recent Department of Agriculture briefing, its senior vet overseeing TB, Eoin Ryan, explained some of the common misconceptions about the testing regime.

“Traditionally, we have done a bad job at communicating what a clear test means. It does not mean there is definitely no TB – it means animals are probably TB-free. The space between probably and definitely is where the problem lies.”

Ryan explained in cases where the test did detect a reactor, it was correct 99.98% of the time and resulted in just one in 5,000 animals being wrongly judged to have TB.

The real issue, he said, was false negatives. These are animals which pass clear but actually have TB. The test only picks up 80% of infected animals, missing one in five.

A second common misconception, Ryan explained, was a belief that animals not showing lesions in the factory were, in fact, TB-free

The test fails to detect animals for a number of reasons. These include a 60-day incubation stage during which TB cannot be detected in the initial phases of infection, an immunosuppressed animal failing to respond to the test which requires an immune response, and poor testing.

This inability to pick up all TB-positive animals is what makes it a difficult disease to eradicate.

A second common misconception, Ryan explained, was a belief that animals not showing lesions in the factory were, in fact, TB-free. Only 30% of animals display lesions at slaughter, he said. These were animals in the advanced stage of infection and were a sign of a serious outbreak.

The majority of animals had no lesions as they are discovered in an early stage of infection before lesions form.