To scan or not to scan? The economics of transthoracic ultrasonography for ‘whole-flock’ ovine pulmonary adenocarcinoma screening in UK sheep flocks

Background
Transthoracic ultrasonography (TTUS) is currently the only widely used method to diagnose suspected preclinical or subclinical cases of ovine pulmonary adenocarcinoma/Jaagsiekte (OPA) in live sheep. However, the economic impact of using TTUS as a screening test has not been described previously.

Methods
Test characteristics for TTUS in a low-prevalence situation were obtained from a previous study of 1074 breeding ewes that underwent TTUS with an experienced operator. The economic impact was modelled using a 10,000-iteration partial budget simulation and probability sensitivity analysis to explore the relative influence of model variables.

Results
In flocks of this size, culling true-positive and false-positive cases resulted in an estimated median net loss of £4647 (interquartile range: £3537–£6006), determined primarily by replacement ewe value and the cost of TTUS per ewe.

Conclusion
The results of this study emphasise that great caution should be exercised by practitioners when determining if TTUS is appropriate as a screening test for OPA in low-prevalence flocks or subpopulations within a flock, such as younger age groups, where the losses incurred through the inadvertent culling of healthy sheep may significantly exceed any benefit derived from culling preclinical/subclinical cases.