Over 80% of bovine tuberculosis (bTB) breakdowns in NI have only one strain of the disease in the herd, a new study has found.

Scientists from the Agri Food and Biosciences Institute (AFBI) examined the genetic make-up of bTB infections in almost 30,000 reactor cattle in NI over a nine-year period.

Only one strain of bTB was isolated in 83% of herd breakdowns, and just 0.7% of breakdowns were caused by five or more different genetic forms of the disease. According to the AFBI scientists, the results indicate that “highly localised factors operating over a small geographical extent were primarily influencing the epidemic” of bTB in the herds.

Three key examples of local bTB sources are highlighted in the peer-reviewed paper, namely neighbouring herds, wildlife and recurrence of an existing infection from within the breakdown herd itself.

One breakdown herd in the study did have 12 different strains of bTB identified.

However, the AFBI scientists point out that high levels of different bTB strains were only seen in a limited number of herds and these were most likely to be beef-finishing units.

“[This] is indicative of purchasing cattle from a wide geographic extent, or possibly the spreading of non-local contaminated slurry,” the researchers suggest.

As part of the study, NI was subdivided into 123 patches, with the average patch size equating to 110km2.

There were no patches which showed only one type of bTB strain, 24% of patches were associated with 10 or fewer genetic strains and 82% had 20 or fewer types of bTB.

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