While skin TB testing is a very good test for herd screening, it can miss out on detecting individual animals, David Quinn, superintending veterinary inspector at the Department of Agriculture, told a recent Teagasc webinar.

There were over 30,000 cases of TB last year, an increase of 6,000 on the previous year, and a national herd incidence rate of 5%.

However, an estimated that one in 5,000 animals could be a false positive to the skin test,” Quinn said.

“If an animal comes back as positive, it is highly likely that it is a reactor, even if it doesn’t have lesions. This is due to the fact that the lesions could be microscopic, and not visible to the naked eye.

“When TB is detected, it is crucial to identify and confine the disease. Repeated positive tests are likely from an animal that has not been detected and is spreading the disease.”

There are a few reasons for false negative results when it comes to the skin tests, Quinn explained.

These include the animal having a compromised immune system or being under physiological stress, such as calving.

Another reason is that the animal could be in the pre-allergic phase. It takes three to six weeks after the animal is infected to develop an immune response so, until then, the immune response is not adequate to respond to the test, and it won’t come back as positive.

These undetected animals are the risk and can spread the disease.

When it comes to blood testing, the vet said that while “it is very good at picking out true positives, it does tend to pick up a lot of false positives”.

“The issue is when you get 10% of the herd infected, because sometimes what is being picked up on the skin test is the tip of the iceberg.

“The longer this remains in the herd, the harder it can be to root it out. Continually failing the skin test can lead to herd depopulation.

“Farmers can sometimes be reluctant because they are afraid of extra animals being removed, but if you get in early and take them out, the blood test can pick up animals at an earlier stage of the infection than the skin test.

“It is important to get those animals out or otherwise you could end up in a recurring cycle of test after test,” he said.

Residual infections

Recurring herd test failures are usually due to residual infection, where TB has become embedded in the herd.

“Often, it is an older animal that has the infection and probably has multiple lesions, and outwardly they are not showing any signs, but it is having an impact on their immune system and reducing their capacity to respond to the test.

“These animals can be highly infectious, so they are passing the annual herd test, but could be spreading infection to other animals that are responding to the test. So that’s how you get into the cycle of test after test.”

Quinn warned that “two clear tests don’t necessarily mean you are clear of TB. There could be animals that have been missed by the test.

“For the three years after you are at higher risk of breaking down than when you go beyond that period.”


On lesions found in animals in the factory, he said only around 30% of reactors have visible lesions.

“And that’s not to say the other 70% are not infected, it’s just that the lesions are microscopic, so you can’t see them with a naked eye. The more advanced the infection, the more lesions you’ll see.”

Quinn put the rise in TB in recent years down to the expansion of the dairy herd.

“The expansion of the dairy herd has probably acted as a disruptor.

“There was a lot of movement of animals around the country, so I think the increases we are seeing now are probably the expansion of the herd,” he said.

While the figures are going in the wrong direction in recent years, he said he is “optimistic that we might be about to turn a corner on it. Another way to look at it is that 95% of the herds are not locked up”.