Comparative Virulence of Spring Viremia of Carp Virus (SVCV) Genotypes in Two Koi Varieties

Spring viremia of carp virus (SVCV, species Carp sprivivirus), is considered one of the most lethal freshwater pathogens of cyprinid fish. Common carp Cyprinus carpio L. and koi C. carpio koi are the most susceptible host fish species. The virus was formally described in the 1960's after outbreaks occurred in carp species on the European continent, but there has been a global expansion of SVCV primarily in Asia and North America. Genetic typing of the SVCV isolates separates them into four genogroups correlated with geographic origin: Ia (Asia), Ib and Ic (Eastern Europe), and Id (Central Europe). In this study we compare the virulence of eight SVCV strains including representatives of each genogroup: Ia (20040741, 20070165, 20100910, and 20120450, and NC2002), Ib (RHV), Ic (P4-7), and Id (Fijan). These were tested in two koi varieties, Beni Kikokuryu; long fin semi-scaled fish and Sanke; short-fin fully scaled fish. Koi fry challenged by immersion were observed for mortality and assessed for infection. Mortality ranged from 4 to 82% and 0 to 94% in the Beni Kiko and Sanke koi breeds respectively. The five genotype Ia isolates of Asian origin had a wide range in virulence/mortality (0 to 94%). Single isolates representing the European genotypes Ib and Ic, or Id were moderately virulent (38 to 56%) or highly virulent respectively. Each virus strain had similar levels of mortality in both koi breeds with the exception of high virulence SVCV Id strain, whose mortality rose from 60% in Beni Kiko koi to 87% in Sanke koi. Overall SVCV strain virulence appears to be a dominant factor in determining disease outcomes, whereas intraspecies variation, based on koi variety, has less of an impact. This is the first side-by-side virulence comparison of global SVCV strains representing each genogroup in an infected principal host species.