Fish Poxviruses on the Rise: Prospects for Aquatic Health

Fish poxviruses are increasingly recognised as emerging pathogens of fish and should be considered in cases of unexplained gill or skin pathology. Carp edema virus (CEV), the causative agent of koi sleepy disease in common carp and koi (Cyprinus carpio), represents the first known example. Since then, additional members of the poxviridae have been described, including Plecoglossus altivelis poxvirus (PaPV) in ayu (Plecoglossus altivelis), seahorse poxvirus (SHPV) in Cape seahorse (Hippocampus capensis), salmon gill poxvirus (SGPV) in Atlantic salmon (Salmo salar), cod gill poxvirus (CGPV) in cod (Gadus morhua), black bullhead poxvirus in black bullhead (Ameiurus melas) and Japanese seabream poxvirus (JSPV) in red seabream (Pagrus major). Most poxviruses share a tropism for epithelial tissues, causing gill hyperplasia, lamellar fusion and in the case of SHPV, dermatopathy. Clinical presentation is often complicated by secondary infections due to the immunomodulatory effects of poxviruses. Diagnostic progress is hampered by their failure to replicate in cell culture, inconsistent electron microscopy results and the lack of broad molecular screening tools. Fish health professionals should remain vigilant and include poxviruses in differential diagnoses for gill and skin disorders.