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Variance components for b TB were estimated using animal linear and threshold mixed models and co-variances were estimated using sire linear mixed models.Using a linear model, the heritability for susceptibility to b TB in the entire dataset was 0.11 and ranged from 0.08 (heifers in dairy herds) to 0.19 (heifers in beef herds) among the sub-populations investigated.The UK and Ireland have yet to gain an official b TB free status , although both countries introduced eradication programs during the late 1950s.In the UK, b TB is an expanding epidemic, while in Ireland, it forms a stable endemic with substantial year-on-year reductions, particularly recently .Since b TB is of economic importance and because all animals are routinely tested at least once annually in Ireland and some other countries, the presence of genetic variation for b TB susceptibility suggests that b TB susceptibility should be included in a national breeding program to halt possible deterioration in genetic susceptibility to b TB infection.], due mostly to the lack of b TB control in developing countries, where infection is endemic, resulting in reduced productivity of livestock.
In restricted herds, a lower threshold (so-called, severe interpretation) of the SICTT is used, such that all animals with a bovine-avian difference of at least 2 mm are considered a `reactor under severe interpretation’ and animals with a bovine reaction less than 2 mm greater than an avian tuberculin reaction are considered an `inconclusive reactor under severe interpretation’.
However, this approach requires information on the extent of genetic variation for b TB susceptibility within the Irish cattle herd. However, these studies were confined to dairy cattle and, to date, no study has been undertaken on beef cattle.
Moreover, the potential of genotype by environment (Gx E) effects for susceptibility to infection between herds of different disease prevalence levels has not been quantified.
The objective of the present study was to estimate variance components for b TB susceptibility in both dairy and beef cattle and to quantify whether Gx E for b TB susceptibility exists across environments that differ in b TB prevalence.
In the present study, tuberculin skin test results from both dairy and beef cattle were used as a measure of b TB susceptibility.