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Infectious hematopoietic necrosis virus (IHNV) is a rhabdovirus that produces an acute, lethal infection in rainbow trout (Oncorhynchus mykiss). Fish that survive infection cease to produce detectable infectious virus at approximately 46 days after infection, yet there is evidence that survivor fish continue to harbor virus particles (B. S. Drolet, P. P. Chiou, J. Heidel, and J. C. Leong, J. Virol. 69:2140-2147, 1995). In an effort to determine the biological function of these particles, the kidneys and livers from IHNV survivors were harvested and divided into samples for nested reverse transcriptase PCR analysis and explant culture. Sequences for the IHNV nucleoprotein and polymerase genes were detected in 50 and 89%, respectively, of the organs from survivor fish. When explant tissue cultures were infected with purified standard IHNV, the liver tissues from survivor fish produced up to 10-fold less virus than naive control fish liver tissues. In addition, immunosorbent electron microscopy analysis of the supernatant media from the cultured explants of survivor fish revealed truncated particles, whereas the control tissue supernatants contained only standard viral particles. These results suggest that the truncated IHNV particles observed in persistently infected fish are defective interfering particles that may mediate virus persistence.