Carp edema virus in ornamental fish farming in India: A potential threat to koi carps but not to co-cultured Indian major carp or goldfish

Koi sleepy disease caused by carp edema virus (CEV) of pox virus is a potential carp killer and poses a constant threat to co-cultured fish. Only limited studies are available on its occurrence from the eastern part of India. Here, we report a large-scale mortality of koi carps (Cyprinus carpio koi) in ornamental fish farm of Choudwar, Cuttack, Odisha. The fish showed clinical signs of ulcers on body, patchy haemorrhages, massive necrosis of gills, and observed sleeping at the bottom of tanks before death. However, the goldfish maintained in the same tanks did not show any clinical signs or mortality. We noticed Trichodina infestation in the gills of affected animals and secondary bacterium Aeromonas hydrophila in kidney tissue. The gills, liver and kidneys of koi carp were found positive for carp edema virus (CEV) (Poxviridae) in PCR followed by sequencing and blast search, and phylogenetic analysis. The damages to the gills and kidney along with the presence of intracytoplasmic eosinophilic inclusions in gill epithelial cells upon histopathology suggested viral association. Experimental challenge with CEV failed to reproduce the disease in Indian major carp and goldfish, the commonly co-cultured fish species with koi carp, though, the disease was reproduced in koi carp. To our best knowledge, it is the first report of CEV infection from the farm of the eastern part of the country. Another mortality incidence in ornamental shop retailer in the same locality also revealed the presence of CEV in koi carp and pointed possibly towards its wide spread. Observations from this study suggest the need for CEV disease surveillance in the country.