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December 6, 2013
Washington State—Is Inbreeding Responsible for EMS?
There are a number of people who are asserting that the widespread occurrence of EMS (AHPNS) is a result of genetics. They blame inbreeding for weakening the immune system of the current farmed species of preference, Penaeus vannamei. While there is always the possibility that some inbreeding is occurring on local levels, the major suppliers of specific pathogen free broodstock are not selling inbred animals. They run professional programs where inbreeding coefficients are watched. There is no credible scientific data to support this theory. It’s speculation. The assumption is being made that inbreeding equates to the weakening of the immune system. Where is the data to support this? Due to the tremendous variability in the shrimp production systems, field observations mean little. Scientific research would be required to prove any relationship between weakening of the immune system and inbreeding.
The etiologic agent of AHPNS is a strain (or possibly strains—this still needs to be sorted out) of Vibrio parahaemolyticus, a bacterium that produces pathology by colonizing chitinous surfaces in the stomach and the gastric mill of shrimp and excretes a toxin that enters the intestinal tract and major digestive organ of the shrimp, the hepatopancreas (HP). It damages this organ to the point where it loses its integrity and becomes an open source for invasion of the hemolymph by opportunistic pathogens. The susceptibility to this bacterium has nothing to do with immunity. Shrimp with super strong immune systems would still be affected. Conceivably, shrimp could develop some tolerance to the toxin although this is theoretical with no evidence to support it at this time. Once the HP is destroyed the animal can no longer absorb nutrients, and even if it managed to fight off secondary infections for a while, it would eventually succumb.
Vibrio species are ubiquitous in the marine and brackish water ecosystems where shrimp are farmed. They are very promiscuous bacteria and exchange genetic material readily and continuously. This has resulted in a group of bacteria that is continually creating new strains with the science clearly showing that genes and gene function are in a state of constant flux. Cultural practices such as the widespread use of chlorine to “disinfect” shrimp ponds and a failure to follow even the simplest of biosecurity protocols likely creates niches that allowed some bacterial species to benefit from the disruption and the lack of control. This is possibly what led to the development of this particular pathogen. Other Vibrios produce disease in a similar fashion, and their ability to attach to chitin surfaces such as the stomach is a characteristic common to most strains of Vibrio.
The interests of the industry would be better served by dealing with facts rather than idle speculation. While there is still a dearth of published information on this pathogen, it is the subject of a great deal of basic and applied research. There is no evidence whatsoever to show that inbreeding of SPF P. vannamei, if it even occurred, had anything to do with the development of AHPNS and its spread.
Information: Stephen G. Newman, Ph.D., President and CEO, AquaInTech, Inc., Lynnwood, Washington 98037, USA (phone 1-425-787-5218, email firstname.lastname@example.org, webpages www.bioremediationaquaculture.com and www.sustainablegreenaquaculture.com).
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