SELECTION TO IMPROVE DISEASE RESISTANCE IN SHRIMP

Genetic disease resistance is very important in all aquaculture species.  Without genetic resistance animals are more susceptible to pathogens and when they become infected the morbidity and mortality is higher.  In most cases the genetic resistance is not perfect, i.e. the genetic resistance is less than 100%.  

Genetic selection has been effective in improving the resistance to several pathogens in shrimp.  Today many strains have high resistance to TSV.  In TSV challenge tests the survival is typically more than 80%.  Improving the resistance to TSV occurred quickly and only a few generations of selection were required to increase the TSV resistance to a high level.  Resistance to WSSV and AHPND has progressed at a slower rate.  The slower rate of improvement can be due a low frequency of the resistance alleles in the population and/or due to the resistance being determined by many genes with small effects.  After many generations of selection by different groups strains of Penaeus vannamei have been developed with some resistance to WSSV and AHPND.  The following table shows the survival of four different strains at the end of challenge tests designed to measure the resistance to WSSV and AHPND.

The slow rate of improvement in WSSV and AHPND resistance is following the typical pattern of improving genetic disease resistance.  Generally, the improvement is made in small incremental steps with the rate of improvement depending on the design of the selection program.  Programs can be designed that will improve survival between 1% and 6-7% per generation.  Consequently, increasing genetic resistance of shrimp so the survival is increased from 1% to 50% may require between 7 and 50 generations of selection.  In the next few years quantum leaps in genetic disease resistance may be possible by using the new genetic techniques that are on the horizon.