Pathological Spectrum of Ovine Pulmonary Adenocarcinoma in Small Ruminants: A Focus on the Mixed Form

SIMPLE SUMMARY: This paper describes pathological findings in small ruminants with ovine pulmonary adenocarcinoma, a contagious respiratory tumor caused by the jaagsiekte sheep retrovirus. Infection with JSRV leads to the formation of lung tumors, progressive respiratory failure and, ultimately, the death or culling of the affected animals. OPA has traditionally been divided into two forms: classical and atypical. However, in this study, we examined 27 animals with OPA, and found that most of the observed tumors displayed a mix of characteristics from both the classical and the atypical form, and were classified as mixed. Grossly, the lesions were found mostly in the cranial lobes of the lungs, appearing as firm, flat-to-slightly-raised masses that varied in size and color. Histologically, the cases were categorized based on the predominant architectural patterns, with the mixed pattern being the most common. The objective of this study was to provide a detailed description of the gross and microscopic spectrum of ovine pulmonary adenocarcinoma in small ruminants from Spain, focusing on the mixed form. It is important to note that the mixed form of OPA is less frequently reported, and can be mistaken for other concurrent lung diseases. ABSTRACT: Ovine pulmonary adenocarcinoma (OPA) is a contagious respiratory tumor of small ruminants, manifesting in chronic weight loss and respiratory failure. Infection with the betaretrovirus jaagsiekte sheep retrovirus (JSRV) is the cause of OPA. Here, we describe the gross and microscopic features of twenty-six sheep and one goat with naturally occurring JSRV-associated OPA. All the animals included in this study had pulmonary lesions morphologically consistent with OPA, but the majority of the observed lesions demonstrated features of both the classical and the atypical form of OPA, and were, therefore, classified grossly as mixed. The gross lesions were located mainly in the cranial pulmonary lobes, were multifocal to coalescing, variable in number and size, flat to slightly raised, firm, and white to grey. Histologically, the cases were classified according to the predominant architectural patterns as lepidic, papillary, acinar, or mixed; the mixed histological pattern was the most prevalent. The aim of this study was to describe the gross and microscopic spectrum of OPA in naturally infected small ruminants from Spain. The mixed form of OPA is less commonly reported, and can be confused with other concurrent pulmonary pathologies (such as BALT hyperplasia in SRLV-associated pneumonia or lungworm granulomas).