A study by Truong Thi My Hanh et al. 2018, published in the Journal of Fisheries Science and Technology in February 2018, identified risk factors associated with White Spot Disease, thereby proposing measures to reduce the risk of WSSV infection in farmed shrimp.
Spatial Factors
Water intake into ponds without a sedimentation pond system
Analysis results showed that in whiteleg shrimp farming areas in Nghe An, Nam Dinh, and Quang Ninh, farmers who took water directly from irrigation canals into their ponds had a 6.3 times higher risk of WSSV infection compared to those who took water into their ponds via a sedimentation pond system.
Untreated water entering the pond creates favorable conditions for pathogen introduction, as WSSV can survive outside host cells and persist in seawater at 30ºC for at least 30 days and in pond water for at least 3-4 days (Momoyama et al., 1998; Nakano et al., 1998). If the water source carries the WSD-causing virus, horizontal transmission will occur (Chou et al., 1998; Venegas1 et al., 1999). Poorly managed water intake into ponds will serve as a pathway for additional pathogen supply into the pond (Momoyama et al., 1998; Nakano et al., 1998; Kongkeo 1997).
Not using a filter net during water intake
Water taken through a filter net significantly contributes to limiting WSSV infection in shrimp during the culture cycle. Farmers who used a filter net when taking water into shrimp ponds had a 3.2 times lower risk of WSD infection compared to those who did not apply this technique.
Water taken into the pond without a filter net increases the likelihood of introducing WSSV-infected organisms into the pond (Lo et al., 1996). Many aquatic organisms, including crustaceans, invertebrates, and plankton (both zooplankton and phytoplankton), have been identified as WSSV carriers causing White Spot Disease in farmed shrimp (Jiang, 2012; Liu et al., 2007). In this study, for intensive farming models, supplementing water into the pond during the culture cycle is a risk factor for White Spot Disease infection in farmed shrimp, a finding consistent with the study by Takahashi et al. (1995). The author suggests that when water is supplied to the pond, especially in large volumes, it can shock the farmed shrimp, leading to an easy outbreak of White Spot Disease. However, this contradicts the improved extensive shrimp farming model.
In the improved extensive farming model, supplementing water into the pond is considered a protective factor against White Spot Disease outbreaks. This means that proper water regulation and intake, primarily supplying water to the pond in the previous month (dry season), helps limit the occurrence of White Spot Virus disease in the subsequent month of the pond during the culture cycle (Nguyen Van Hao et al., 2007).
Supplementing untreated water into the pond during the culture process
Furthermore, during the culture process, supplementing untreated water into the pond also poses a 3.6 times higher risk compared to ponds where this technique is not applied.
The research results are consistent with Umesh et al. (2008), who identified water sources as one of the risk factors for White Spot Virus infection in farmed shrimp. Thus, the analysis results indicate that water intake techniques into ponds are significant in minimizing the risk of WSSV infection in farmed shrimp. The study's findings further confirm that water intake and management of water sources supplied to shrimp ponds are crucial.
Temporal Factors
Ponds located in areas with diseased ponds
In areas where farmers reported White Spot Disease infection in shrimp, the remaining ponds in that area had a 4.6 times higher likelihood of WSSV infection compared to ponds in areas without WSSV.
Infrequent monitoring of pond water environment
Furthermore, the probability of WSSV occurrence in ponds where the environment is regularly monitored is 3.7 times lower compared to ponds where this technique is not practiced. The regularly monitored environmental parameters mentioned by farmers include temperature, pH, salinity, and dissolved oxygen.
It is evident that despite applying intensive shrimp farming models with high investment costs in both infrastructure and technology, the risk factors for WSSV infection in shrimp are not reduced. The reason identified is that farmers have not strictly adhered to technical guidelines related to the entire shrimp culture cycle for a given crop, corresponding to the intensive shrimp farming model.
Regarding the Subject
Stocking post-larvae smaller than post 10
Studies in farming areas of Nghe An, Nam Dinh, and Quang Ninh showed that using post-larvae smaller than post 10 resulted in a 7.5 times higher risk of WSSV infection compared to stocking post-larvae of size ≥ post 10. This indicates that the size of shrimp post-larvae introduced into farming plays a crucial role. Post-larvae size is calculated by their age after metamorphosis; it is recommended to stock post-larvae ≥ post 10 in ponds. At this size, they can exhibit better resistance to White Spot Syndrome compared to smaller post-larvae, while also promoting greater weight gain in the initial days in the pond.
A study by De Yta et al. (2004) monitoring the survival rate of stocked shrimp sizes confirmed that the highest survival rate was 79% (stocking post 30), decreasing to 77% when stocking post 20 shrimp, and 67% (post 10 size). The survival rate decreased proportionally with the size of the stocked post-larvae.
Not reducing stress during shrimp stocking
Additionally, the role of stress reduction activities for post-larvae during stocking is also clear: in ponds where stress reduction techniques for post-larvae are not applied, farmed shrimp have a 4.8 times higher risk of WSSV infection compared to ponds where this technique is applied. In reality, stress reduction techniques for shrimp during stocking have not received sufficient attention in farming areas, with many farmers not applying them, especially in Nam Dinh, Quang Ninh, and Nghe An, with respective rates of 75.6%, 58.1%, and 40%.
Sudden changes in water environmental factors will disrupt physiological functions in the post-larvae's body, causing shock, weakness, affecting growth, and reducing shrimp's disease resistance (Nguyen Van Thanh, 2017). When stocking post-larvae, technical experts recommend that farmers practice stress reduction solutions such as applying minerals, Vitamin C, and balancing the pond water temperature with the water temperature in the shrimp bags. Vitamin C is considered an antioxidant, immune stimulant, and iron absorption aid, helping to prevent common anemia and reduce stress in farmed shrimp (Le Cung, 2015).
Presence of other organisms in the pond
Furthermore, the presence of organisms other than farmed shrimp in the shrimp pond indicates a 3.9 times higher risk of WSSV infection in farmed shrimp compared to ponds without other organisms. Commonly found species in shrimp ponds include: red crabs, spiny shrimp, freshwater prawns, sand shrimp, mud snails, gobies, some species of mussels, clams, polychaetes, etc.
8 factors to help limit the risk of WSSV infection in shrimp farming
RESEARCH IDENTIFYING SOME RISK FACTORS RELATED TO WHITE SPOT DISEASE IN INTENSIVELY FARMED WHITELEG SHRIMP (Litopenaeus vannamei) IN SOME NORTHERN PROVINCES by Truong Thi My Hanh, Huynh Thi My Le, Pham Thi Yen, Truong Thi Thanh Vinh, Chu Chi Thiet, Phan Thi Van
NIMDA TH
