The importance of the number of animals tested and threshold at which a herd is deemed positive for BVD when testing young-stock for BVDV antibodies

Objectives

The objective is to demonstrate some examples of why vets in practice should take into account the number of animals tested and the threshold of positives at which the veterinarian or, if it is part of a scheme, then the scheme decides to designate that herd as positive. It is likely that to some extent veterinarians do this intuitively but it is also useful to have quantitative estimates of this for reference.

Materials and Methods

Prior to the start of the BVD eradication scheme in Scotland, a random survey of Scottish beef herds was undertaken and blood samples from ten young stock per herd were tested for BVDV antibody. Using the results from this survey it was possible to randomly sample from the results to see how many of the herds would have tested the same way depending upon (a) the number of animals tested and (b) the threshold of positive animals at which a herd is deemed positive.

Therefore we are able to present evidence of the effective herd-level sensitivity and specificity across different combinations of the number of animals tested and thresholds chosen.

Many schemes aiming to eradicate BVD choose, as a pillar of the screening system, the testing of blood of ten young-stock animals for antibodies to BVDV. If one or more animals tests positive then the herd is deemed positive. In some herds, the number of young-stock animals available for this screening process is fewer than ten. This study makes use of data from a random survey of beef farms in Scotland, prior to the Scottish eradication scheme, to show that the number of animals tested and the threshold of positive animals at which a herd is declared positive has a hugely important effect on the overall herd level sensitivity and specificity of this method. Veterinarians should be aware of this when interpreting their results.

Results

The herd sensitivity (i.e. the probability of designating the herd as positive given that it is truly positive) is greatly dependent on both the number of animals tested and the threshold at which we call the herd as positive. Unsurprisingly the herd sensitivity goes up with the former and down with the latter and we employ graphs to demonstrate this pattern based on our specific estimates of those different scenarios.

The herd specificity (i.e. the probability of designating the herd as negative given that it is truly negative) behaves in a fairly similar but reverse manner and we employ graphs to demonstrate this.

Conclusions

Whilst many veterinarians and farmers, probably intuitively, account for both the number of animals tested and the number of animals testing positive for BVDV antibody when interpreting their results, it is possible to give some empirical evidence in order to estimate how they would do this in a more quantitative way and this may be of interest to vets, farmers or those planning eradication schemes.