To avoid excessive cost, in larger herds it has been accepted that it will be reasonable to sample a statistically derived proportion of the herd. This proportion will be calculated by Surefarm Ltd using xxxx(stat tool/methodology) based on the number in the whole herd and using the same inputs in terms of prevalence, sensitivity and specificity as used for the statistical package used to interpret the results as discussed above. Clearly, not sampling all animals does increase the risk that infection may be missed and, where targeted sampling occurs, the owner will have to sign that they accept this risk.
Given the inherent difficulties in predicting where bovine tuberculosis might exist in a herd, there are no specific answers as to how to determine which animals should be tested where whole herd sampling is not performed. The general advice is that the sampling of animals should be designed to target the specific cohorts that exist in the herd in terms of age and group i.e. ideally samples should be taken from all the different groups of animals and to reflect the age structure of that group.
As this is a voluntary scheme, there is no compulsion on the owner to present specific groups but ultimately it is in the owner’s interest to ensure that freedom of disease is tested for as thoroughly as possible.