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May 5, 2015
Professor Timothy FlegelShrimp Diseases in Asia
On February 28, 2015, at the Thirteenth Meeting of the Asia Regional Advisory Group on Aquatic Animal Health, sponsored by Network of Aquaculture Centres in Asia-Pacific (NACA), Professor Timothy William Flegel presented a paper on shrimp diseases in Asia. Following are excerpts from Flegel’s paper that appeared in the “new” Aquaculture Magazine:
Whitespot syndrome virus (WSSV) and type-1 yellowhead virus (YHV-1) are still the most lethal viral pathogens for Penaeus monodon and P. vannamei, although yellowhead (YHV-1) has been confined to Thailand. A new, lethal variant of yellowhead (YHV-8), however, has been found in China, and it is recommended that a disease card for it, together with a specific detection method be posted at the NACA website. Also from China, another new virus called “covert mortality nodavirus” (CMNV) was recently reported. We have found that it also occurs in approximately 40% of Thai shrimp farms, and we have recently received RT-PCR positive material from India. Its range and impact on culture in the region have not yet been determined. Again, it is recommended that a disease card, including the specific RT-PCR detection method be posted at the NACA website and that member countries work together to study the prevalence and impact of this virus.
For P. vannamei, the next most important viral threat is infectious myonecrosis virus (IMNV, still confined to Indonesia). Taura syndrome virus (TSV) and infectious hypodermal and hematopoietic necrosis virus (IHHNV) are not serious threats to tolerant shrimp stocks. P. vannamei sometimes exhibits abdominal segment deformity disease (ASDD), associated with a retrovirus-like agent.
For P. monodon, the next most important viral pathogen is Laem Singh virus (LSNV) and an integrase-containing element (ICE) that are together associated with monodon slow growth syndrome (MSGS), but, so far, only in Thailand. Less important are hepatopancreatic parvovirus (HPV) and monodon baculovirus (MBV), but only when wild-caught monodon are used for postlarval production without implementation of proper preventative measures.
The most important non-viral disease threat for both species since 2009 has been called (unadvisedly) “early mortality syndrome” (EMS). It should be referred to as “acute hepatopancreatic necrosis disease” (AHPND) because it is characterized by massive sloughing of hepatopancreatic epithelial cells followed by death. The causative agent has unique isolates of Vibrio parahaemolyticus that carry a plasmid that contains two toxin genes capable of acting together to kill shrimp. Two interim PCR detection methods (AP1 and AP2) were introduced on the NACA website in December 2012 based on detection of the plasmid, and AP2 turned out to be the best with about 3% false positive results. Despite this weakness, the method was used successfully to reveal a high prevalence of AHPND bacteria in live broodstock feeds (polychaetes and bivalves), in broodstock and in postlarvae used to stock rearing ponds. A new PCR method, AP3, was released at the NACA website in June 2014. It gave no false positive or false negative results with 104 bacterial isolates tested. It is recommended that the AP3 method be used to identify sources of AHPND bacteria and that positive shrimp or other materials be excluded from shrimp production facilities. It is also recommended that the practice of feeding live marine animals to broodstock shrimp be strongly discouraged unless they have been proven free of AHPND bacteria and other pathogens.
Possible preventative measures against pathogen entry with live feeds require treatments that result in their death, including (in declining order of desirability) gamma irradiation (sterilization), pasteurization or freezing. The last of these methods (freezing) was the standard practice for polychaetes fed to shrimp broodstock, and it is still the practice in North and South America. The widespread habit of feeding live polychaetes, however, has apparently arisen based on associated increases in nauplii production, while ignoring all biosecurity concerns. It would probably be better to accept decreased nauplii yields in order to insure the integrity of SPF broodstock. This is especially important for the risk of exposure to previously unknown pathogens. Another approach to solve the problem of disease transmission from living polychaetes has been to produce SPF animals in closed culture facilities.
In addition to AHPND, other pathogens associated with the hepatopancreas, like the microsporidian Enterocytozoon hepatopenaei have become prominent in broodstock and farmed shrimp. The rapid regional spread of AHPND and the simultaneous increase in prevalence of infections by the distinctly different, endemic pathogen E. hepatopenaei, suggests that the current situation in Asia may have resulted from an industry wide decrease in rigor of biosecurity measures in shrimp hatcheries and rearing ponds. This could have arisen due to the dramatic reduction in disease outbreaks in cultivated shrimp since the widespread adoption of specific pathogen free (SPF) P. vannamei in Asia since 2001. Even with production based on use of SPF stocks, any decline in biosecurity measures would have left the industry vulnerable to the emergence of any new pathogen.
From 150 ponds in an ongoing Thai study of 200 ponds randomly selected before stocking, the prevalence of ponds with AHPND was in the range of 24%, while prevalence of the microsporidian Enterocytozoon hepatopenaei (EHP) was 49%.
For all the pathogens described above, the most effective control measures for reducing the risk of disease are to use postlarvae derived from domesticated SPF shrimp stocks (with a pathogen exclusion list that includes all major viruses and parasites, including E. hepatopenaei), cultivated in biosecure settings under management practices aimed at optimum (not maximum) production.
Information: Professor T.W. Flegel, Chalermprakiat Building, Faculty of Science, Mahidol University, Rama 6 Road, Bangkok 10400, Thailand (phone +662-201-5870, fax. +662-247-051, mobile +6681-403-5833, email firstname.lastname@example.org, webpage http://www.mahidol.ac.th/en/).
Sources: 1. Aquaculture Magazine. Editor Greg Lutz (email email@example.com). Current Status of Shrimp Diseases in Asia. Timothy William Flegel. Volume 41, Number 2, Page 54, April/May 2015. 2. Bob Rosenberry, Shrimp News International, May 5, 2015.
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