Detection of Infectious Hematopoietic Necrosis (IHN) Virus in Rearing Units for Steelhead before and during IHN Epizootics

Abstract

Abstract Nursery tanks and Burrows ponds at Dworshak National Fish Hatchery, Ahsahka, Idaho, containing age-0 steelhead Oncorhynchus mykiss were sampled for IHN virus before and during epizootics of IHN. The hatchery water supply was also sampled weekly from March to September. By use of a precipitation method involving polyethylene glycol (PEG) to concentrate virus from water samples, IHN virus was detected in nursery tanks and Burrows ponds during the early stages of epizootics at concentrations ranging from 0.1 to 0.3 plaque-forming units (pfu)/mL. Peak virus concentrations were 1–5 pfu/mL in nursery tanks and exceeded 50 pfu/mL in Burrows ponds. When tangential flow filtration was used in conjunction with PEG precipitation, virus was detected in Burrows ponds at concentrations of 0.007–0.07 pfu/mL, 3–12 d before periods of rapidly increasing mortality. The results indicate that IHN epizootics begin with waterborne IHN virus at relatively low concentrations. Virus was not detected in the hatchery water...