Mr. Philip Breslin:

The bug that causes TB - mycobacterium bovis - and all the microbacteria is that they are quite unusual as bacteria. They grow slowly. It can take up to 20 days for them to replicate, whereas a bug like E. coli, with which we are familiar because of the difficulties it can cause in water, can replicate in 20 minutes. There is a certain resistance in some animals. Through the Irish Cattle Breeding Federation, sires have been identified in Ireland that confer more resistance to TB on their progeny than other sires. That information will go into the estimated breeding values, EBVs, in the new year. There is a resistance issue. The immune system is constantly in a battle with TB. Approximately half of all TB infections are conquered by the immune system for a period. They become latent. This phenomenon was especially recognised when TB was prevalent among people. In the 1950s and 1960s, 50% of people in Ireland had some evidence of TB in their lungs. In many of those cases, it was walled off and was no longer progressing. This question of latency, whereby the immune system stops TB in its tracks for a period of time, reduces the animal's chances of being infective to other animals.

The first point I would make in response to the question about the sporadic nature of some TB breakdowns is that there must be a transmission point - the animal must pick up the TB from somewhere. If many cattle are exposed to the same wildlife source - for example, if a badger contaminates some feed - the number of cattle infected could be large. By contrast, just one or two animals grazing in a field might be contaminated. The cattle in question will have various levels of infectiousness, which will affect their ability to infect other cattle. There can be significant variations in animal numbers. We get some explosive breakdowns. Thankfully, most of our breakdowns involve just one or two animals. That has increasingly been the case as our TB situation has improved over the years. Just one third of our breakdowns involve three or more animals. In such cases, there is a higher risk that TB will be spread onwards - back into badgers or back to other cattle. Transmission is the first issue when explaining why there is such variation. The behaviour of the bug in respect of the animal's immune system is the second issue. In human medicine, it is recognised that certain people are super-shedders, which means they have managed to infect way more people with TB than most people who get infected. It is thought that the same super-shedder phenomenon exists in the badger and bovine populations.