The Office International des Epizooties, known by the acronym OIE, was founded almost 100 years ago. In May 2003 it broadened its scope and changed its name to the World Organization for Animal Health. Its stated mission is to ensure the transparent dissemination of animal health challenges with the goal of ensuring optimal animal health largely via education. The end result one would expect is a sustainable industry with effective controls in place for minimizing the impact of disease on aquaculture.
Although aquaculture has been practiced for a very long time it is only relatively recently that is has grown exponentially. Today many hundreds of fish species and a dozen or so species of crustaceans are being farmed with many more in various stages of R&D. Only recently the tonnage of the biomass produced via aquaculture surpassed that of the wild fishery. Several species of fish, including Rainbow trout (Oncorhynchus mykiss), Atlantic salmon (Salmo salar), tilapia (Oreochromis species), channel catfish (Ictalurus punctatus) and shrimp, including both tiger (Penaeus monodon) and white shrimp (P. vannamei) are being produced at levels that dwarf what is available through commercial fishing with no indication that production is slowing down. Diseases have large impacts on the sustainability of these production paradigms and OIEs mandate is intended to help mitigate this by preventing the movement of pathogens from infected areas to clean areas.
Many countries look to the OIE for guidance and direction as to how to address the presence of specific pathogens that the OIE has determined require regulatory oversight. Unfortunately, the process by which new pathogens are added is slow, too slow for this to be an effective system. By the time controls are adopted and put in place where feasible, production is usually seriously affected. Furthermore, because of rapid growth, many aspects of aquaculture are in a constant state of flux. Most shrimp farming takes place in underdeveloped countries. While oversight might be present to some extent it is rarely up to the task of keeping pathogens out of existing production systems and ensuring that they do not become the source of new infections. There are several reason for this.
What would appear to be a legitimate approach for generating primers for pathogen specific probes may be outdated and actually causing harm in some instances. An example of this is the presence of endogenous viral elements or EVEs. These are sequences of nucleic acid that incorporate into host DNA. When the population reproduces, these sequences are passed on. This is theorized to be an important step in how anti-viral tolerance/resistance comes about in crustaceans. If a primer nucleotide sequence overlaps with or is an EVE sequence and this is the standardized primer used to detect a given virus, then the presence of the viral fragment in host DNA would be interpreted as the actively pathogenic virus being present. This means that when populations are being screened for the presence of the specific viral pathogens that OIE has deemed are obligate pathogens they can be reported as carriers when they are not. The parvovirus (among others), IHHNV, responsible for RDS, runt deformity syndrome is one of the viruses that have been characterized as producing these sequences. Crustaceans, via accommodation, develop RNAi mediated anti-viral activity. This appears to be the case for many of the shrimp viruses and could actually be for any virus that is present in stocks, whether pathogenic or not. This suggests that it is critical to ensure that the primer sequences used to detect viruses in shrimp are critically evaluated on a regular basis to ensure that they are indeed picking up virulent virus and not EVE artifacts .
New viruses are being discovered regularly. DNA screening of crustaceans has revealed large number of what are putative viral sequences. Many as of yet uncharacterized viruses are likely present, some of which are likely to cause problems in farmed shrimp. Given the current state of affairs, it is impossible to ensure that when problems occur, even if OIE is notified immediately, that OIE can react quickly enough to prevent new viral disease outbreaks from becoming another WSSV, EHP or EMS.
Many (most if not all) of the countries that are members of the OIE routinely fail to report notifiable pathogens despite being obligated ethically to do so. They are not going to report new pathogens if they are loath to report existing pathogens. This allows them to avoid the repercussions that they would encounter in order to deal with this. These can be minor or severe in the sense that even frozen product can be rejected because it is carrying OIE listed pathogens. This attitude ensures that many will continue to spread a pathogen of importance because they are not looking for it. There are no consequences except to the poor shrimp farmers either way.
Every company that produces broodstock for export is required to test their populations for a list of known pathogens in order to be able to legally export/import the animals. The OIE aquatic animal health code is a very important document for understanding the relative responsibilities of OIE and individual nations as it relates to aquatic animal health (https://www.woah.org/en/what-we-do/standards/codes-and-manuals/aquatic-code-online-access/). As of this writing the list for shrimp (Table 1) includes the following pathogens:
There are a number of other pathogens that are not on the list and many workers in the field have pointed out that this along with the relative slow rate at which new pathogens are added undermines the credibility of OIE in terms of accomplishing its stated goals.
Today there are more than a dozen companies competing for the global broodstock business. While some countries are recognizing that the best and most secure approach is to limit these imports and focus on local production most still import screened animals often with some form of limited quarantine. This ensures for the time being that they have access to the best animals, i.e., those strains that are domesticated, that grow faster, get bigger, convert better, tolerate stress, etc. Short term quarantine and random testing of a small percentage of the population for OIE pathogens opens the door for other pathogens to enter. The greatest degree of protection afforded to farmers entails the use of broodstock that have been produced according to CITES guidelines. They should have well established performance histories along with thorough repeat testing against all pathogens that there are primers for, regardless of OIE status, along with in-depth histopathology of target organs with the slides being read by a professional.
Screening of animals on a population basis is inadequate by itself for considering lines to be SPF. This is a statistical approach that while it may offer a high degree of statistical confidence that a given pathogen is not present is simply not adequate when one is dealing with millions and hundreds of millions of animals. At a 95% confidence interval, 50000 out of a million PLs can be carrying a pathogen. Individual screening of animals used to be prohibitive with the advent of new technology it is possible to test each individual, reducing this risk significantly (https://www.genics.com.au/). A multiple generation quarantine along with an in-depth examination of animals via histopathology for tell tale signs of the presence of uncharacterized pathogens must be a part of this process. Using the term SPF for animals without these cannot always typically provide the degree of protection needed to ensure sustainability. It ensures that problems will persist.
This is made even more challenging by the fact that some of the companies that produce what they claim are SPF animals are also claiming that this is because the animals are resistant to infection with the specific pathogens that they are screening for. They claim that the reason the population is SPF is because they are resistant to the pathogen being considered. While it is possible that SPF animals are “resistant”, this is not the reason for them being SPF. Determining that they are resistant or tolerant requires the ability to challenge naïve animals and appropriate controls with the pathogens of interest via the established route of infection under controlled conditions. Truly resistant animals, which is a rarity if it occurs at all, cannot be infected by any route regardless of how stressed they may be. Tolerant animals can be infected although they have the ability to tolerate higher levels of exposure prior to developing acute disease. Tolerance appears to be common. Resistance appears to be rare.
SPF is not a snapshot of the population. It is the result of a many cycle process that ensures that the pathogens of interest are not present. The moment this cycle is broken the animals can not be considered to be SPF. The reality is that when someone is buying SPF animals and they hold them in an environment where there are no or inadequate barriers to ensure that the pathogens of interest are not present and cannot infect animals that one can not consider these animals to be SPF.
For OIE to be truly effective in preventing the spread of existing and newly characterized as well as yet to be characterized pathogens, their approach needs change. Manuals need to be updated frequently in response to the presence of new pathogens. Failing to reveal the presence of pathogens of interest in a country should have repercussions. There are none and many countries do not report that they have WSSV, EHP, EMS, etc. New diseases need to be reported as soon as they are discovered and steps need to be taken to include the pathogens in the OIE list as quickly as is possible. The responsibilities of members are clearly outlined in the current manual, Chapter 1.1 (https://www.woah.org/en/what-we-do/standards/codes-and-manuals/aquatic-code-online-access/?id=169&L=1&htmfile=chapitre_notification.htm).
The fact that there are no repercussions for failing to report the presence of notifiable diseases ensures, in my opinion, that sustainable shrimp farming will continue to remain elusive well into the foreseeable future. As more researchers focus on specific pathogens, primer fine tuning is essential. Some viruses, notably RNA viruses, are much less stable than DNA viruses and have very high rates of mutation. They evolve quickly. It is critical that cutting edge tools and science be considered in determining which pathogens belong on the list, whether they should remain there and what can be done to eradicate them or at the very least mitigate their impacts to the maximum possible extent.
Organizations such as OIE are important for sustainable aquaculture. However, they need to recognize that they are constrained in this and that without the ability to ensure that endemic pathogens are reported and consequences for not reporting are commensurate with the offense, it is unlikely that they will be able to fulfill this role in the manner that the industry has to have if it is ever going to reduce the impact of disease on aquaculture.
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