A Scoping Review of Naturally Occurring Xenomas in Fish: Clinical Features, Diagnostic Approaches, and Knowledge Gaps

Xenomas are distinctive hypertrophic host-cell lesions caused by intracellular parasites and represent a recurrent pathological finding in wild and farmed fish. Their presence has implications for fish health, diagnostic workflows, aquaculture productivity, and in some cases product quality and consumer acceptability. Despite this relevance, information on xenoma diversity, associated pathogens, and diagnostic practices remains fragmented across decades of literature. This scoping review synthesised available evidence on naturally occurring xenomas in fish, following PRISMA-ScR guidelines. Eligible sources included studies reporting clinical, pathological, or diagnostic information on xenomas in fish. Data were charted on host species and families, taxonomic identification of the aetiological agents, xenoma morphology, and diagnostic approaches. Across 114 publications published between 1968 and 2024, xenomas were reported in a wide range of teleost families and were attributed mostly to microsporidian infections, particularly species of Glugea, Loma, Spraguea, Pleistophora, and Microgemma, although myxosporean-associated cases (Kudoa, Myxidium, Nephrocystidium) were also documented. Light and electron microscopy were the most frequently applied diagnostic methods, whereas molecular techniques were used less consistently, with increasing use in the most recent decade. Macroscopic xenomas were typically described as whitish, rounded to oval, and well delimited, yet substantial morphological variation occurred across hosts and tissues. Overall, the review highlights major heterogeneity in pathogen identification and diagnostic pathways, underscoring the need for more standardised and integrative approaches.