To scan or not to scan? Efficacy of transthoracic ultrasonography for ovine pulmonary adenocarcinoma screening in a large commercial UK sheep flock

Abstract

Background Transthoracic ultrasonography (TTUS) is currently the only widely used method to diagnose preclinical or subclinical ovine pulmonary adenocarcinoma (OPA) in the live sheep. However, little is known about the test characteristics of TTUS. Methods One thousand and seventy‐four breeding ewes in a flock with evidence of low OPA prevalence underwent TTUS by an experienced operator. Fifty‐one sheep were diagnosed with OPA and underwent gross postmortem examination (PME). Results Lesions consistent with OPA were found in only 24% (12/51) of the culled ewes. Thirty‐five percent (18/51) of culled ewes had gross lesions consistent with other pulmonary disease and 41% (21/51) had no detectable gross lesions on PME. Histopathology and immunohistochemistry confirmed OPA in only the 12 animals identified with OPA lesions from PME. Conclusion Great caution should be exercised when deciding if TTUS is an appropriate screening test in groups of sheep where OPA prevalence may be anticipated to be low. TTUS is a subjective test and thus individual operator ability will influence the sensitivity and specificity of TTUS for OPA diagnosis while the underlying prevalence influences the eventual positive predictive value.

How useful is transthoracic ultrasonography for detecting ovine pulmonary adenocarcinoma in sheep?

“…The results of Davies and colleagues' study are not supportive of transthoracic ultrasonography being used as a screening tool for OPA in low-prevalence populations, with a relatively low proportion of animals with significant abnormalities detected on the scan being confirmed as having OPA at postmortem examination and over 40 per cent of animals identified as OPA-positive ultrasonographically having no detectable lesions at postmortem examination. 14 This contrasts with the very high sensitivity and specificity previously reported by Cousens and Scott. 12 However, in Cousens and Scott's study, the animals scanned were all thin, and many came from flocks with a confirmed history of OPA.…”

“…In a study summarised on p 122 of this issue of Vet Record, Davies and colleagues sought to address this knowledge gap by assessing the efficacy of using transthoracic ultrasonography to screen a large flock for OPA. 14 In this study, all animals determined to have lesions consistent with OPA by the veterinary surgeon performing the scanning were sent for slaughter. A pathologist then examined the lungs, and representative sections were taken for histopathological examination.…”

“…Furthermore, Davies and colleagues found that a large proportion of animals diagnosed as OPA-positive by ultrasonography had lesions that postmortem examination confirmed as being of non-OPA origin. 14 In a previous study of fallen stock, OPA was found in 5.6 per cent of ewes, while chronic suppurative pneumonia was found in 6 per cent and bronchopneumonia Although transthoracic ultrasonography may be a useful additional diagnostic tool for use on sheep exhibiting clinical signs consistent with ovine pulmonary adenocarcinoma (OPA), it is not recommended for routine 'whole flock' screening or pre-sale screening of individual animals due to its low sensitivity in populations with low OPA prevalance RESEARCH COMMENT in 7 per cent. 7 These figures suggest that in a population where the prior prevalence of OPA is unknown, cranioventral lung consolidation detected ultrasonographically should be interpreted cautiously, with the findings checked by postmortem examination and histopathology.…”

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

“…The test performance of the TTUS technique was obtained from a previous study, 7 where 1074 breeding ewes in a flock with evidence of low OPA prevalence underwent TTUS by an experienced operator. Fifty‐one sheep were diagnosed as ‘OPA cases’ by TTUS and assigned a grade (Grade 0 = no abnormalities detected, Grade 1 = minor abnormality detected but not indicative of OPA, Grade 2 = likely OPA lesions of small to moderate size, Grade 3 = larger, more advanced lesions with the highest supposed likelihood of being OPA).…”

“…In a previous study, 7 we described the performance of TTUS as a screening test in a flock with a low prevalence of disease of 1%–2%. In that study, the positive predictive value of TTUS ranged from 12% to 36%, estimated sensitivity ranged from 70% to 93% and specificity estimates ranged from 96% to 99%.…”