The efficacy of ozonated seawater for surface disinfection of haddock (Melanogrammus aeglefinus) eggs against piscine nodavirus

Piscine nodavirus, also known as viral nervous necrosis (VNN), is a lethal, vertically transmitted virus that causes severe mortality in fish. It affects primarily marine species, including cultured haddock (Melanogrammus aeglefinus). Ozone has been used successfully to disinfect Atlantic halibut (Hippoglossus hippoglossus) eggs against nodavirus. Fertilized eggs from different species of fish can tolerate varying levels of dissolved ozone, so specific exposure levels need to be determined for individual species. The objectives of this study were to investigate the tolerance of newly fertilized haddock eggs to dissolved ozone and to determine if this exposure is sufficient to disinfect against piscine nodavirus. Eggs were exposed to an ozone concentration of 3.0(±0.3) mg/l of total residual oxidants (TRO) of Cl2 for CT units (TRO × duration of exposure in min, mg/l/min) of 0, 5, 10, 15, 20, 25, 30, 40 and 50. A decrease in survival was observed when the exposure exceeded 30 CT units. Following this, other fertilized haddock eggs were submerged in nodavirus suspensions with densities of 102.5 and 103.5/0.1 ml TCID50 units for 30 min, followed by exposure to ozonated seawater at a concentration of 3.0 mg/l for CT units of 0, 10, 20 and 50. Viable VNN was detected by cell culture using striped snakehead (SSN-1) cell lines. The positive controls (exposed to 0 CT units) all tested positive for nodavirus, while all but one of 24 egg samples exposed to ozonated seawater tested negative. This indicates that ozone can be successfully used to disinfect haddock eggs against nodavirus at a concentration of 3(±0.3) mg/l TRO as Cl2 for 3.3-6.7 min.