Experimental Coinfection with Aquabirnavirus and Viral Hemorrhagic Septicemia Virus (VHSV), Edwardsiella tarda or Streptococcus iniae in Japanese Flounder Paralichthys olivaceus

Abstract

Japanese flounder Paralichthys olivaceus, farmed or wild, are ofoen infected with aquabirnavirus (ABV) but the virus does not usually exhibit pathogenicity to the fish under usual experimental conditions. The present study investigated the experimental coinfection in flounder with ABV and three flounder pathogens, i.e. viral hemorrhagic septicemia virus (VHSV), Edwardsiella tarda and Streptococcus iniae. When young flounder were injected with ABV (10^6.5 TCID₅₀/fish) and subsequently (1 week later) injected with VHSV (10^6.7 or 10^4.7 TCID₅₀/fish), higher survival rates were obtained in these flounder compared with those in fish infected with VHSV alone. In contrast, higher mortalities occurred in the flounder dually injected with ABV (10^6.8 TCID₅₀/fish) and E. tarda (1.4 × 10¹ CFU/fish) or S. iniae (1.7 × 10² CFU/fish) at a 1 week-interval than those in fish infected with bacteria alone. These results indicate that the primary ABV infection in flounder suppresses the secondary viral infection but facilitates the secondary bacterial infections.