LAUSR

Abstract Oomycetes and filamentous fungi are predominantly causing disease outbreaks in cultured fisheries worldwide. Mucormycosis are infrequently occurring in an immunocompromised or immunosuppressed fish host, predisposed mostly by undesirable changes in aquatic habitats. Few reports have shown mucor mostly occurring in water and soil, thus caused infection in freshwater and marine fish. In this study, we report Mucor hiemalis infection from cultured fish farms infecting gills and skin of carp fish (Cyprinus carpio communis) and rainbow trout (Oncorhynchus mykiss). The etiological pathogen was identified by morphological features of pathogen and molecular approaches using large subunit of ribosomal RNA as the fungal barcode. Pathogenicity was characterized by experimental exposure of carp fish to M. hiemalis, followed by histopathological approach. The histopathological results revealed that the pathogen managed to establish initial infection on and inside the gills, cutaneous and subcutaneous surfaces. The infection post initial establishment through systemic blood circulation managed to suppress the defense system and predisposed host to heighten infection and systemic mycosis. Further, the M. hiemalis was also able to cause severe infectivity post two weeks of exposure which resulted in mortality of fish host. The study provides the first insight into the pathogenicity patterns of M. hiemalis on the fish host and its disease-causing potential.