Development of superficial lung lesions monitored on farm by serial ultrasonographic examination in sheep with lesions confirmed as ovine pulmonary adenocarcinoma at necropsy.

Background: This ultrasonographic study monitored lesions involving the lung surface suspected to be the early stages of ovine pulmonary adenocarcinoma (OPA) tumours over 4 months in commercially farmed sheep. The enlargement of these lesions defined ultrasonographically, which likely represent the development of OPA tumours, have important implications for ultrasound screening schedules in veterinary management plans attempting to eliminate OPA by test-and-cull.

Results: The lungs of 58 adult Scottish Blackface sheep with ultrasonographic changes at the lung surface consistent with early OPA tumours were examined two to six times over 40 to 290 days. Lesion development, represented in early video recordings by 2-3 mm lesions involving the visceral pleural and comet tails, then a decreasing length of the hyperechoic line representing the normal visceral pleura and increasing depth of the sharply-demarcated and largely uniform hypoechoic areas into the lung parenchyma, was found in 26 of the 58 sheep. The rate at which the sonographic lesions progressed varied considerably and in 10 of 17 Group 1 sheep developed quickly from an estimated depth of 2-30 mm up to 70 mm between 60 and 120 days later. These sonographic lesions were confirmed as OPA at necropsy; histological changes of concurrent bacterial infection were detected in one of these 10 Group 1 sheep. Thirty-one sheep had sonographic changes ≤30 mm consistent with very early OPA at the first examination which had reduced or were not observed at subsequent examination. Five of these 31 sheep were necropsied, 3 had small OPA lesions while 2 had no significant pathology.

Conclusion: Lesions involving the visceral pleura, with sonographic changes consistent with previous published findings of early OPA, developed over 40-120 days to large masses in 10 of 17 Group 1 sheep with the provisional sonographic diagnosis confirmed histologically at necropsy. While it is possible that atalectic lung could have caused some of the minor sonographic changes there was no microscopic evidence of pathologies other than OPA in nine of 10 Group 1 sheep. We conclude that some small tumours progress to large tumours within 3 months questioning the assumption that OPA is a slow growing tumour in adult sheep taking several years to cause clinical disease. The findings that a proportion of small ultrasonographic lesions are not found again at subsequent scanning illustrates the challenges of interpreting small (< 1-2 cm) lesions during rapid whole flock ultrasonographic examination and we continue to recommend re-scanning suspicious sonographic changes 2 months later.