Volume 1-6  August 2020

The Covid-19 (C19) global pandemic is having a global impact.   We are still in the early days of this unfortunately.   Aquaculture markets have been severely disrupted.   The supply chains have been impacted, irretrievably in some cases.   Fortunately, there are at least 2 dozen vaccine candidates that are close to or have entered phase 3 testing, the final step in evaluating the ability of a vaccine to safely prevent C19.   Additionally, there are a number of drugs and chemicals that testing under rigorous, science-based testing show promise of mitigating the impact of disease.  These will ensure that the global marketplace will rebound. 

Typical rural shrimp farm in China

Chinese inspectors recently detected SARS-CoV-2 (the virus that causes C19) viral RNA when they sampled shipments of frozen shrimp from three Ecuadorean companies.   The result was the immediate ban of imports from these three companies while the problem is addressed.   The possibility of the product itself being deemed adulterated, even though there were no positive tests of any of the shrimp, is a very real consideration that could result in the product being destroyed and any product from these companies currently in cold storage being destroyed.   The total number of positive tests were minute.  Small enough that one could argue that these could be false positives from issues related to the sampling leading to the observations.   However, given the extensive occurrence of the virus in Ecuador, it is not unexpected that there could be some viral contamination of packaging materials specifically.    Processing plants should not have any risk of viral contamination of the shrimp themselves.  It is not likely that any of the PCR positive tests were from viable virus.     Several months of exposure to the -20 C or colder temperatures in a reefer with nothing to protect a fragile virus that may or may not have been viable to start with is a guarantee of this.

This is however a moot point.  China has the right, just as any country does, to dictate what risks they consider acceptable when they import products from elsewhere.    With a few changes in SOP's Ecuador should be able to ensure that there are no positives at all.   C19 cannot infect farmed shrimp or fish and there should be no concern of this.

NEW VIRUS OUT OF CHINA

I reported in an earlier newsletter on the SHIV virus.   This has been renamed the Decapod iridescent virus 1 (DIV1).   It is highly virulent and likely transmitted as a result of failed biosecurity efforts to keep it out of broodstock.   It is still having a large impact and is only one of many pathogens that are endemic among farmed shrimp in China (not to single China out-every shrimp farming country has to deal with endemic pathogens).

China has almost 1.5 billion people to feed and as is the case in every country, there are areas where farmed animals are going to be reared at very high densities, an important contributor to the ready spread of pathogens.   This is one reason why so many viruses affecting shrimp seem to originate in China.   Biosecurity is not adequate (for the most part, globally) and the widespread lack of understanding over what SPF actually means (it is a process and not a label for subsets of animals) and the limitations of PCR (both by no means confined to China) are a significant part of this as well.   Official estimates are that  China produces up to 1.5 million MTs of shrimp per year.   Estimates by workers in the field are that it is probably closer to a third of this, although no one knows for certain as there is a huge inland shrimp farming industry.   It is important to note that there have been several instances where it appears that a pathogen may have originated in China, but actually had been imported into China where it was subsequently exported from.

In a recent issue of Aquaculture Frontier magazine there is a summary of what is being tentatively named, the Hepatopancreas and Digestive Tract Necrosis Virus (HINV).   The photo below is from the magazine and shows affected PLs. 

Table 1. Brief description of the recently described Hepatopancreas and Digestive Tract Necrosis Virus (HINV)

As is typical of diseases of this nature, there is much more to be learned.   What does appear to be factual at this point in time is in Table 1.   Note that Chinese shrimp farmers (as do most shrimp farmers everywhere) face biosecurity challenges and that affected shrimp have been reported to be carrying a variety of viral and bacterial pathogens.   Infection with multiple pathogens is a common factor among many disease outbreaks.   The presence of this new virus is not particularly surprising as recent research reported that RNA viruses are quite common in invertebrates.  Almost 1500 different presumptive, as of yet uncharacterized RNA viruses, have been found in some 200 species of invertebrates.   There will be many more of these if we do not tighten up biosecurity.

Small farm in rural Vietnam

As I have written about many times, biosecurity is a pivotal weak point in the process of shrimp farming.  Few practice it in a manner that ensures that pathogens are not coming in with the PLs. 

What can be done to address this? 

An important first step is to recognize that biosecurity is not achieved by subsampling a population for a given pathogen.    Specific pathogen free (SPF) animals are not the result of this type of testing.   Many sellers of broodstock however ignore this.   They believe that if they do not find a given pathogen by PCR in a population sub-sample that this means that it is not present.     For the most part however, this is simply not true. The American Fisheries Societies Blue Book (Click Here) lays out what the minimum assumed pathogen prevalence levels (APPL) are at various levels of sampling (based on fish).  For the lowest possible APPL of 2%, 150 animals need to be tested individually and not pooled.   No one does this.   They pool samples, reducing the sensitivity of the test.   The APPL with 60 samples is 5% and with 30 is 10%.  Given the nature of the sampling process and the tendency to pool samples,   it is likely that many companies that have not actually gone through the process of ensuring that their animals are indeed SPF are selling broodstock that still carry those pathogens that they claim are not present.     Samples are not always taken of the right tissues or even when animals are in environments that are consistent with the ability of a given pathogen to grow.   A classic example of this is the WSSV virus.   It goes dormant when water temperatures are above 31 C or so (note that this is not black and white-there are many factors in play).   If broodstock are not held at lower temperatures for a few days, they may test negative by PCR.  Yet, they will infect their offspring and farmers will have WSSV issues in environments that are conducive to it.

Shrimp carapace with WSSV pathology

The first animals that were deemed to be SPF were not shrimp.   The concept was developed for use with swine and poultry and the methods are clearly defined.    They entail building a facility that is not open to the environment and holding animals for at least two generations (2 years) under strict quarantine conditions.   For marine species, the seawater should be artificial and no living feed or any frozen from live animals feed material can be used.   Animals should be tested by PCR individually-not pooled and any animals that are positive  destroyed.    Clean animals are allowed to reproduce, and the testing cycle is repeated.   Animals should not be used for broodstock until they have been clean for at least two full cycles.    All moribund and dead animals must be examined for the presence of the entire spectrum of pathogens by a board-certified pathologist who can identify the presence of lesions indicative of a pathological process.   Any signs of a disease in a population sets off additional testing and/or destruction of the affected populations.   When broodstock are used from this process, prior to use, they should each be tested individually for the spectrum of know pathogens.   Some say this is too expensive.   This is false economy.   The losses to the farming community far outweigh the costs of this testing.   It will however increase the costs of broodstock being sold.   Although with reasonable fecundity, the final impact on the overall cost of the PLs will be minimal and the reduction of risk far outweighs any increase in costs.  

It is the not the animals that are SPF, SPF is the process to used generate these animals.   When done properly the SPF production system results in SPF broodstock that in turn result in SPF nauplii and PLs.    Failure to use established methods to develop SPF animals is a big problem.   Another serious problem is selectively testing for specific pathogens.   The entire range of known pathogens affecting a given species should be tested for.    When you test for a few of them you can never say with certainty that others are not present.   

The term SPR (specific pathogen resistant) is also thrown about loosely.  This can only be established by testing.   Survival in the field in the presence of these pathogens does not allow one to state that the offspring of these animals are SPR for a specific pathogen.   Disease processes are complex.   The unfortunate reality  is that the development of resistance is typically not simple or straightforward.   Realistically, for laboratory evaluation, it would require the use of test animals that were clean and free of any interfering organisms.  No such animals exist at this moment.    There are no germ-free shrimp.    As we learn more about the microbiome, there is solid evidence that its composition may make inferences of resistance problematic.  

So, the bottom line is that for shrimp to be truly free of pathogens entails the use of processes that few broodstock producers actually engage in.    Some companies have invested in the needed infrastructure.   Most have not.   In SE Asia where there are a proliferation of hatcheries and someone may preferentially buy from a friend, a neighbor or a relative, few realize that SPF status is lost as soon as the animals are not in the original facility.   Taking short-cuts does not change this.  No amount of foot baths is going to lessen the risk.   The only way that it will change is for this practice to stop. 

Farmers in Ecuador are using PRO4000X and seeing dramatic results that they are not seeing with other similar products.  These are before and after photos demonstrating the result.