Abstract:Viral covert mortality disease (VCMD), caused by the covert mortality nodavirus (CMNV), is a highly prevalent viral infection affecting the majority of cultured shrimp. Shrimp with this disease exhibit clinical soft shells, hepatopancreatic atrophy and necrosis, empty stomachs and guts, and slow growth. During the acute infection stage, infected shrimp display opaque abdominal muscles. Farmers have observed daily mortalities in the diseased population, with mortality rates increasing between 60 and 80 days post-stocking and reaching cumulative mortalities up to 80%. CMNV can infect major cultured crustaceans, including Penaeus vannamei, P. chinensis, Marsupenaeus japonicus, P. monodon, and Macrobrachium rosenbergii, as well as fish, sea cucumber. The prevalence of VCMD has resulted in considerable economic losses within the shrimp farming industry over the past decade. Recognizing its threat to the aquaculture animals and substantial economic impact on Asian shrimp farming operations, the World Organization for Animal Health has recently issued CMNV infection as an emerging disease in aquaculture. The occurrence of VCMD in shrimp is closely related to factors such as positive CMNV infection, culture density, and culture environment. After shrimp are infected with CMNV, the disease usually occurs when the water temperature is high and the environment changes sharply. However, accurately determining the extent to which each factor is associated with VCMD occurrence remains challenging. Therefore, this study aimed to assess the risk of VCMD occurrence during shrimp culture by establishing a comprehensive risk assessment framework using the Delphi method, analytic hierarchy process, and multi-objective comprehensive evaluation. The evaluation index system consisted of 1 target level (risk assessment and early warning of VCMD occurrence), 5 criterion layers (including health status, water quality and temperature, culture mode, culture management, and environmental status), and 20 indicator layers (including CMNV infection, soft shell, empty stomach and guts,, opaque abdominal muscle, dissolved oxygen, ammonia nitrogen, nitrites, pH value, water temperature, factory culture, greenhouse culture, high-altitude tank culture, soil pond culture, breeding density, compound feed, frozen bait, fresh food, surrounding disease, facility isolation, and water treatment). The relative weight and absolute weight of each risk indicator in the risk assessment framework of VCMD occurrence were calculated based on expert scoring results. The total matrix consistency ratio was CR ＜ 0.1, which passed the overall consistency test. The weight values of five criterion-level risk factors were W= {0.420, 0.127, 0.094, 0.214, 0.146}. The high weight values of risk factors, including CMNV infection (0.173), culture density (0.095), opaque abdominal muscle (0.086), and soft shell (0.084), indicated that these factors might be crucial for the incidence of VCMD in shrimp culture. Then, the results were preliminarily validated by studying the case of three shrimp farms in Shandong Province with methods of multiple objective comprehensive evaluation. The risk values of VCMD occurrence in these three shrimp farms were determined to be 0.550, 0.508, and 0.466, all indicating a high risk of VCMD outbreak. Consistently, within 4 days after monitoring and sampling activities, severe cases of VCMD occurred in all three farms with a mortality rate exceeding 80%. These findings demonstrate that the risk assessment results align with the monitoring results and confirm the accurate early warning capabilities of the model for assessing VCMD occurrence. In summary, this study established a convenient and feasible risk assessment model for VCMD occurrence in shrimp culture. The risk assessment model comprehensively considers various factors that contribute to VCMD occurrence during the culture process, ensuring accurate assessment results. This research suggests that the proposed risk assessment model could be used to estimate the risk of VCMD occurrence in shrimp culture and could serve as an early warning tool for prevention and control, thereby ensuring the healthy and sustainable development of shrimp farming.
