A Practical and Effective Artemia Hatching Method to Eliminate Covert Mortality Nodavirus (CMNV)

The covert mortality nodavirus (CMNV), an emerging pathogen that infects several species severely in aquaculture, including marine shrimps, freshwater prawns, and crabs, has been detected in both wild Artemia and commercial Artemia cysts. Utilization of Artemia from unknown sources can impose considerable biosecurity risks in hatchery operations; therefore, effective hatching methods to eliminate the pathogenic potential from CMNV-positive (CMNV+) Artemia cysts are urgently needed. In this work, we assessed the efficacy of three treatment methods of disinfecting CMNV+ Artemia cysts during hatching: (l) 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 served as the control. The virus located on the cyst shells attached/infected Artemia larvae during hatching and remained active for a prolonged time of 7 days. The viral load of empty shells decreased with the decapsulation treatment time. After a 45 min treatment of decapsulation, CMNV on shells, as well as larvae, were eliminated successfully. Furthermore, decapsulation shortened the hatching time of the cysts from 19 h to 12 h. Ten ppm formalin in the hatching water could block the transmission of CMNV from the shells to the newly hatched larvae, although at this level it was insufficient to eliminate the virus from the shells completely. While use of 30 ppm formalin or higher dosage could eliminate CMNV, however, it also reduced the hatching rates of the Artemia cysts. A combination of decapsulation (treated with Na2CO3-NaClO for 15 min) and 10 ppm formalin in hatching water effectively eliminated the CMNV. This study developed a practical, effective, and reliable treatment method for hatching Artemia to ensure biosecurity in aquaculture hatcheries.