Breeding Shrimp for Disease Resistance : Challenges and Opportunities for Improvement

After the 1992 Taura syndrome virus (TSV) epizootic in Ecuador, shrimp farmers in the Western Hemisphere began implementing a variety of strategies to mitigate crop loss from this pathogen, including selective breeding programs to develop TSV-resistant shrimp using between-family selection. Although TSV-resistant shrimp initially improved production and profitability for those farmers who were experiencing a TSV outbreak, genetically improved stocks were not a panacea for the broader disease problems plaguing the industry. In fact, breeding shrimp for resistance to a single viral pathogen, using current breeding strategies, may not be the most effective course of action for the long-term viability of the shrimp farming industry. There are a number of concerns associated with current breeding strategies. As in other organisms, there appears to be a trade-off between disease resistance and growth. In addition, disease-challenge tests typically used to estimate breeding values are based on a threshold character (dead or alive) and such tests are inefficient and low in statistical power. Also, there are concerns that performance in laboratory challenge tests may not be predictive of survival in commercial ponds. Importantly, there are growing concerns about viral mutations, whereby previously resistant shrimp strains may become susceptible to evolving viruses. To mitigate some of these concerns, we suggest that the current practice of selecting survivors of challenge tests (or the relatives of survivors) be replaced by selection based on an index which includes both survival and viral load. The selection objective would be to reduce the viral load in surviving shrimp, with viral strain or serotype being defined with whatever specificity that the situation requires. Viral load is both a cause and indicator of survival, so the practical result of the genetic improvement program would be a line of shrimp resistant to disease, just as it is now. This can be achieved more effectively when the selection objective is re-defined to include a continuous indicator variable (viral load), in addition to survival. In the future, we anticipate that more effective breeding strategies will emerge as we gain a better understanding about the genetic basis for disease resistance. Moss, S.M. and R.W. Doyle. 2005. Breeding shrimp for disease resistance: Challenges and opportunities for improvement. In P. Walker, R. Lester and M.G. Bondad-Reantaso (eds). Diseases in Asian Aquaculture V, pp. 379-393. Fish Health Section, Asian Fisheries Society, Manila. Diseases in Asian Aquaculture V