Why do some cattle escape TB or liver fluke infection?

Opening her presentation at the Irish Cattle Breeding Federation’s (ICBF’s) Genetics Conference, Siobhan Ryan of Teagasc asked the audience: “Why do only a handful of animals in a herd become tuberculosis (TB) reactors, even though all animals in the herd should – in theory – be exposed to TB?”

Research trials

Speaking about trial work on the topic, Siobhan concluded that genetics are a major component responsible for this animal health mystery. Therefore, the prevalence of this disease can be reduced by breeding.

“Some sires will have nearly no progeny affected with TB, whereas up to 40% of a different sire’s progeny [in the same herd] will be infected with TB. This is the case even though the progeny from both sires are in the same herd,” she explained.

In the case of liver fluke, Siobhan said that – on average – 27% of a sire’s progeny will be diagnosed with liver fluke infection in the factory.

“However, there is a lot of variability around this figure, with up to 75% of a particular sire’s progeny infected with liver fluke,” she added.

Source: ICBF

From her research she found that the heritability of TB was 12% and liver fluke was 1%.

She explained that: “If we could breed solely for TB resistance, then the potential reduction in TB prevalence would be 1.9% per year; for liver fluke it would be 0.09% per year.

On average, as you select higher Economic Breeding Index (EBI) or higher replacement index bulls, you are also selecting for lower TB and liver fluke prevalence.

“However, if you select for higher terminal index bulls – on average – the cattle from those bulls would have higher TB and liver fluke prevalence.

“The reason for this is due to the survival traits, which are not included in the terminal index,” Siobhan explained.