Whitespot disease WSSV
The MPEDA/NACA study clearly shows that WSD is not caused by any one factor. Rather a number of risk factors influence the occurrence of WSD in the farm. These risk factors occur throughout the shrimp cropping cycle and in general terms, fall into the following categories during the different stages of the crop cycle:
• season of stocking;
• pond preparation;
• pond filling and water preparation;
• seed quality and screening;
• water management;
• pond bottom management;
• feed management; and
• disease treatments.
The risk factors at each stage of the cropping cycle and their relationship to WSD outbreaks are illustrated below in a “web of disease causation” in Figure 1.
The following summarize the main points shown in the “web”:
• A WSD outbreak is the end result of a series of actions or changes from healthy shrimp through to disease outbreak.
• At each stage of the cropping cycle, a number of factors influence the development of the disease in individual animals and also in the population of shrimp in each pond.
• WSSV can enter the shrimp and pond through different routes, including shrimp seed, water, carrier animals and transfer of infected animals and farm equipment from one farm to another.
• Adverse environmental factors combined with a high prevalence of infected shrimp among the pond population are necessary for a mass disease outbreak to occur.
Management factors can be used to control environmental factors and reduce risks of WSD occurring in the pond. To be successful in controlling shrimp disease, one has to manage all potential risks at different stages of the cropping cycle.
The results from the shrimp disease risk factor study clearly show a number of significant factors that influence shrimp disease outbreaks and shrimp yields at the pond level, many of which can be managed at the farm level. The risk factor study clearly demonstrates that WSD is not caused by any one factor but by a number of factors that interact and influence the occurrence of the disease. Thus, an integrated
management and extension approach is necessary to deal with the key factors that contribute to disease occurrence.
The findings provide a strong foundation for reducing shrimp disease losses to farmers, improving farm-level capacities and skills in shrimp health management, minimizing the risks of spread of shrimp diseases to other areas and improving shrimp farm productivity and profitability (MPEDA/NACA, 2003).
Source: Umesh, N.R., Mohan, C.V., Phillips, M.J., Bhat, B.V., Ravi Babu, G., Chandra Mohan, A.B. and Padiyar, P.A. 2008. Risk analysis in aquaculture – experiences from small-scale shrimp farmers of India.