Fish Viruses — Survival and Inactivation of Infectious Pancreatic Necrosis Virus

The survival of infectious pancreatic necrosis (IPN) virus under conditions pertaining to its retention in the laboratory, and its inactivation by several physical and chemical means were examined.Approximately 99.9% of infectivity was lost when washed IPN virus (106.5–107.0 TCID50/ml), suspended in phosphate buffered saline (PBS) and Eagle’s Minimal Essential Medium diluted 1:10 with PBS, was held at 4 C for 1 yr. After 2 yr more than 102 TCID50/ml remained in both suspensions. The virus survived equally well for 1 yr in 20 and 50% glycerol.Thermal inactivation of IPN virus at 60 C proceeded in a two-component manner, each component following first-order kinetics. At neutral pH, the infectivity of 107 TCID50/ml was reduced by 99.9% in the first 30 min of exposure. After 5 h, 0.0001% still survived. Inactivation rate of the initial rapid component was increased by adjustment of pH to 3 and 10.The curve for the 1:4000 formalin inactivation of 106.5 TCID50/ml of IPN virus was multiphasic. When formalin-treated virus was titrated, cytopathic effect was delayed.Under a UV intensity of 2000 μW/cm2, the infectivity of 107 TCID50/ml of virus was lost exponentially during the first 6 min of exposure after which it abruptly leveled off. The curve was less steep and curvilinear under an intensity of 440 μW/cm2. The level of tailing was the same in both cases.IPN virus was predictably stable at pH 7 at 4 C. A surprising 0.001% of an original concentration of 107 TCID50/ml survived incubation at pH 9 and 2 for 5 wk. The reported survival of IPN virus in homeothermic animals was related to its ability to withstand prolonged exposure to low pH and to its relative heat stability.