DETECTION OF TRUNCATED VIRUS-PARTICLES IN A PERSISTENT RNA VIRUS-INFECTION IN-VIVO

Infectious hematopoietic necrosis virus (IHNV) is a rhabdovirus which causes devastating epizootics of trout and salmon fry in hatcheries ar ound the world. In laboratory and field studies, epizootic survivors a re negative for infectious virus by plaque assay at about 50 days post exposure. Survivors are considered virus free with no sequelae and, th us, are subsequently released into the mild. When adults return to spa wn, infectious virus can again be isolated. Two hypotheses have been p roposed to account for the source of virus in these adults. One hypoth esis contends that virus in the epizootic survivors is cleared and tha t the adults are reinfected with IHNV from a secondary source during t heir migration upstream. The second hypothesis contends that IHNV pers ists in a subclinical or latent form and the virus is reactivated duri ng the stress of spawning. Numerous studies have been carried out to t est these hypotheses and, after 20 years, questions still remain regar ding the maintenance of IHNV in salmonid fish populations. In the stud y reported here, IHNV-specific lesions in the hematopoietic tissues of rainbow trout survivors, reared in specific-pathogen-free water, were detected 1 year after the epizootic. The fish did not produce infecti ous virus. The presence of viral protein detected by immunohistochemis try, in viral RNA by PCR amplification, and in IHNV-truncated particle s by immunogold electron microscopy confirmed the presence of IHNV in the survivors and provided the first evidence for subclinical persiste nce of virus in the tissues of IHNV survivors.