Epidemiological insight into disseminated neoplasia in Mytilus chilensis from southern Chile

[Display omitted] •Neoplasia cells are smaller in the early stages of the disease compared to advanced stages.•Early stages of the disease are frequently detected in branchial hemolymphatic vessels.•In advanced stages of DN, neoplastic cells fill the connective tissues, hemolymphatic vessels and gonadal follicles.•Advanced stages of DN cause castration in mussels.•Nuclei of some neoplastic cells have emarginated chromatin. This study investigates the prevalence and characteristics of disseminated neoplasia (DN) in Mytilus chilensis in the Los Lagos Region of southern Chile. DN is a malignant proliferative disease that has been described in several species of the genus Mytilus. More than 4.000 adult mussels, both cultured and natural, collected between 2016 and 2022 in different geographical areas (Calbuco-Hueihue, Castro, Quellón) and seasons (winter-spring, summer-autumn) were histologically analysed. The prevalence was assessed and three levels of severity of DN were defined, based on number of neoplastic cells and organs invaded. Morphology and morphometry of neoplastic cells and hemocytes were evaluated. Cell and nucleus diameters and the nucleus/cytoplasm ratio of granulocytes and hyalinocytes were analysed in healthy and neoplastic individuals. The size of neoplastic cells was significantly larger in stage 3 of neoplasia than in stage 1. The overall mean prevalence of DN in M. chilensis was 3.4%, significant differences were found between sites, being higher in Calbuco-Hueihue (5.59%) than in Castro (3.78%) and Quellón (1.40%). The prevalence was significantly higher in the winter-spring season (7.79%) than in the summer-autumn season (3.95%) in Calbuco-Hueihue. The sex, gametogenic stage, and Adipogranular (ADG) cells status were recorded for each mussel, and related to the severity stages of DN. For stages 1 and 2 of DN, females and males could be recognized, but in stage 3 almost no evidence of sex remained. The same trend was identified for the gonadal status, the advanced stages of DN were associated with castration of the mussels. The ADG cells status was highest at stage 1 of DN, declining in number and staining till stage 3. The sex ratio of healthy mussels was 53.5% female, 43.7% male, 1.6% hermaphrodite, and 1.3% indeterminate, for mussels with DN it was 34.1% female, 16% male, 2.2% hermaphrodite and 47.8% indeterminate. Occurrence of DN was higher in females, but the severity stages affected equally female and male mussels. Although the ov