The covert mortality nodavirus (CMNV) is an emerging pathogen that infects several species significant in aquaculture including marine shrimp, freshwater prawns, and crabs. CMNV has been detected in both wild Artemia and commercial Artemia cysts, and studies have demonstrated that Artemia could be a vector and reservoir host of CMNV. Utilization of Artemia from unknown sources can impose considerable biosecurity risks in hatchery operations, effective hatching methods to eliminate the potential infection from CMNV-positive (CMNV+) Artemia cysts are urgently needed.
Three treatments were applied to assess their efficacy of disinfecting CMNV+ Artemia cysts during hatching: (1) Decapsulation of Artemia cysts with Na2CO3 (sodium carbonate) and NaClO (sodium hypochlorite) at various durations before hatching, (2) Application of different concentrations of formalin in hatching water, (3) Combinations of decapsulating Artemia cysts and formalin-treated hatching water. Hatching CMNV+ Artemia with disinfected seawater only serves as the control.
Upon treatments, the total viral loads of cyst shells and Artemia (both larvae and adult) collected from post-hatching were quantified using a commercial diagnostic kit based on the specific TaqMan fluorescence probes. The results showed that: (1) The virus located on the cyst shells attached/infected Artemia larvae during hatching and remained active for a prolonged time (7 days), (2) The viral load of empty shells and the activity of infected larvae decreased with the decapsulation time. After a 45-minute treatment of decapsulation, CMNV on shells, as well as both larvae and adults, were eliminated successfully. Furthermore, decapsulation shortened the hatching time of the cysts to nearly half and improved the hatching synchrony, (3) 10ppm formalin in the hatching water could block the transmission of CMNV from the shells to the newly hatched larvae, although such a level was insufficient to eliminate the virus from the shells. Formalin of 30ppm or higher could eliminate CMNV, however, it also reduced the hatching rates of the Artemia cysts. (4) Combination of decapsulation and 10ppm formalin in hatching water effectively eliminated the CMNV and significantly enhanced the hatching synchrony. This study developed a practical, effective, and reliable treatment method for hatching Artemia to ensure biosecurity in aquaculture hatcheries.
Keywords: Covert mortality nodavirus (CMNV), Artemia cyst, Decapsulation, Biosecurity.