Cutaneous angiomatosis in koi carp: clinico-pathological investigations and ongoing insights

In the period April-September of 2019 and 2020, five koi carp from different places (Belgium, The Netherlands and Italy) presented with multifocal, irregularly round, few mm to 1 cm, variably raised cutaneous reddened areas, located on the dorsal and ventral body aspects, without any apparent association with coloured areas of the skin. The fish displayed good general conditions, normal behaviour and appetite. After 7-12 months, the skin lesions underwent remission in three out of five carp regardless the treatments. Carp were reared in ponds supplied with water at temperature not controlled, ranging between 18 °C and 25 °C; carp of two ponds were treated with drugs against gyrodactylosis. A clinical-pathological investigation was performed to study the skin lesions. Cutaneous swabs were cultured at 28 °C for 36 hours on blood agar. Skin biopsies were obtained and preserved in buffered formalin and/or alcohol ≥96%. In addition to routine Hematoxylin & Eosin stain to study the morphology of the skin lesions, special stainings (Gram, acid-fast, Warthin-Starry) to investigate potential bacterial agents were performed. Molecular analyses for koi herpesvirus, carp edema virus, Cyprinid herpesvirus 1, Rickettsia-like organism were performed; the samples were also tested with 16S universal bacterial primers. Based on the molecular analyses, viral and bacterial fish pathogens were not found. Bacterial cultures from swabs were negative for primary pathogenic bacteria. Skin parasites or fungi were not detected histologically. The dermis presented a multifocal inflammatory process focused in the dermis characterised by well demarcated round to oval clusters measuring 30-400 μm in diameter, composed of densely packed vascular structures, occasionally forming whorls and inconstantly surrounded by fibrosis. The vascular spaces were occupied by a florid reduplication of small capillaries with aggregates of plump to elongate cells resembling endothelium or pericytes without atypia. In hypocellular areas, a continuum of the lesions could be appreciated, ranging from irregular clusters of plump cells to well-developed micro-vascular structures. Gross and histological findings, common to all cases, were consistent with a process centred on dermal vessels and morphologically consistent with angiomatosis reported in other species. Further morphological investigations are tentatively ongoing in order to elucidate the origin of proliferating vascular/perivascular structures.