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Why Controlling the White Spot Syndrome Virus (WSSV) is so Difficult?


WSSV affected shrimp


Most who have had the WSSV affect their shrimp farming operation know that this virus can wipe a farm out in days. The shrimp can appear healthy, still be consuming feed and within three days of the first appearance of symptoms, all or most die. Subsequent outbreaks tend not to be as serious in terms of overall mortalities but this is of little consequence when shrimp still die in large numbers at any size, fail to grow or are easily infected by opportunistic bacterial pathogens. What can a farmer do? Some background may help to appreciate why controlling the WSSV is so difficult.

  1. Culture conditions (as are most agricultural practices) are "unnatural". In the wild, shrimp do not grow in ponds in large numbers at high densities consuming pelleted feeds. They can spread out at very low relative densities. Interestingly enough the WSSV is often much less of an issue when shrimp are stocked in ponds at very low densities regardless of the biosecurity measures. Cultural conditions are stressful by their very nature and often conducive to the development of very high levels of viral particles in the water and weakened affected animals that are readily consumed by their stronger brethren, rapidly spreading the disease.

  2. The virus is known to occur in many different vectors. Vectors can be animals in which the virus replicates but not in a manner that kills them. They shed virus into the environment. Other vectors die much as the shrimp do (such as some crab species and other species of shrimp). These weakened and dying animals are eaten by vectors and healthy shrimp, readily infecting them and setting the stage for massive disease outbreaks.

  3. The virus is genetically plastic. This means that in any outbreak there may be many different genetic variants of the virus potentially with different abilities to infect and produce disease. There is a strong selection pressure against viruses that kill their hosts too quickly. This is a well-recognized principle and has been observed with many different pathogens. When a pathogen is too virulent, it tends to kill itself off. The best long-term strategy for survival is not to kill the host.

  4. Shrimp farming while it may appear to be simple is actually not. Shrimp are susceptible to many possible pathogens, especially when they are stressed and while the WSSV can be a killer it can also act in concert with other bacterial, fungal, protozoan and viral pathogens and likely some toxins to kill shrimp.

  5. Biosecurity failures in maturation and hatcheries. Efforts to eliminate the virus from broodstock requires stress testing and holding shrimp at cooler than ideal production temperatures. The virus is typically not detectable at 31 C or higher. Most companies do not routinely do this.

With these facts in mind a number of strategies have been developed which form the basis of standard biosecurity protocols on farm sites to try and mitigate the impact of the virus. These include, but are not limited to:

  1. Water disinfection protocols: Prior to the use of water in the ponds the water is treated with chemicals (typically disinfectants or pesticides) to kill any possible virus in the water as well as vectors.

  2. Crab fences: The movement of potentially infected crabs between ponds is prevented by the use of barriers that prevent ready movement between ponds.

  3. Specific Pathogen Free (SPF) animals: Truly SPF animals do not carry the virus into the ponds. They are not the source of the virus. SPF has statistical flaws as screening on a population basis can never give you greater than a 98% chance that a given pathogen is not present. Combining this with testing at typical production temperatures without closely following performance of all stockings creates a huge biosecurity gap that the virus readily exploits.

  4. Standard physical barrier approaches: This can include things such as disinfecting vehicles tires that move onto a facility. Typically they might also limit movement of non-disinfected tools such as nets, boats, etc. between ponds.

  5. Bird netting: Birds can readily move infected animals between ponds by gorging on dying shrimp and regurgitating them when they move to another pond. Properly designed netting can keep most of the birds out, eliminating this important route of viral transmission.

  6. Controlling the presence of other potential pathogens: There is strong evidence that suggests that the WSSV itself is not killing the virus directly. It is weakening the animals, depleting the ability of the immune system to keep it at bay and making them much more susceptible to secondary infection. Typically vibrio species, both obligate and opportunistic pathogens, are what is killing the shrimp. Minimizing the loads of these can impact the final outcome of the disease. The use of PRO4000X to degrade organic matter can help.

Birds picking off sick shrimp in Ecuador


This is only a partial list but encompasses some of the major steps typically taken. The question then is why don't these procedures always work? Why do animals still get infected? That there are at least four primary reasons for this.

  1. The first is that the disinfection protocols employed simply are inadequate. Chlorine is the most common disinfectant of water in use by shrimp producers. Failure to appreciate that chlorine reacts readily with organic material lessening its effectiveness is one component of its problematic use. There are literally dozens if not more disinfection by products that impact efficacy of disinfection protocols as well as potentially negatively impacting shrimp physiology. Using chlorine in a non-lined pond may be effective in killing the virus in the water but this viral reservoir is typically not the greatest threat. Using it in a lined pond may be more effective but the reality is that lined ponds may have damaged liners and that water diffuses underneath the liner acting as a more or less permanent reservoir for some potential vectors. There are several pesticides that are widely used as well purportedly to kill the vectors off. While they may be effective in lowering the overall level of vectors they do not apparently kill those forms (spores) of the vectors that are present in the pond sediments and even likely in the water. The proof of this is apparent in studies that have shown that post treatment PCR reactive material may no longer be present but that within days to a few weeks this changes and PCR reactive material abounds. Some of this could be explained by other mechanisms.

  2. Specific pathogen free (SPF) animals are not truly SPF. There are strict protocols that must be followed to generate SPF animals. These entail quarantine for more than one production cycle and critical examination and PCR testing of the broodstock. WSSV can go dormant in an animal, typically at the ideal production temperature of 31 C. Note that the virus has been detected at elevated temperatures, just not commonly. Testing must be conducted on animals held at 25 C or so for at least 24 hours (longer is better) in order to allow the virus to replicate. Testing at higher temperatures can and often does result in false negatives. Animals that carry the virus may be PCR negative and yet at some point in time become positive. As soon as PLs are produced in open systems they are potentially exposed to the virus. Screening PLs is a numbers game and it is possible to have a very low level of infection even when testing using standard approaches.

  3. Failure to screen every individual brood animal. Historically this has not been economically viable. AFS blue book guidelines allow for statistical sampling protocols. These were developed for fish and not for a hatchery tank that contains millions of PLs. Recent innovations have allowed for animals to be tested for many different possible pathogens at a fraction of the cost of individual testing. Genics (https://www.genics.com.au) can show you how to go about doing this. At a 98% level of confidence, out of a thousand broodstock, 20 can still be actively carrying the virus. Out of one million PLs in a hatchery tank, 20000 can be still be carriers. These levels can easily be disastrous.

  4. Dogma preached by salesman and others that products that are being sold in the market place with purported (or even real) anti-viral activity will kill off the virus. Some natural materials are widely used for this purpose often at the expense of the development of a meaningful and reality based biosecurity program. The WSSV is an enveloped virus and there are many compounds that will react with this envelope. The challenge is that most of these compounds are too large to enter the hemolymph when ingested and thus act primarily in the gut. Highly purified compounds cost too much to use routinely.

WSSV lesions on shrimp carapace


What Can A Farmer Do?

  1. Make sure that your PL provider is using truly SPF broodstock. Ideally, these are animals that are produced in closed systems indoors preferably in an area where there are no farms nearby. Biosecurity protocols in the hatchery must be consistent with this and the hatchery should destroy animals that are ill rather than consolidating low survival tanks.

  2. Develop alternative environmental disinfection protocols. Recognize that there may be specific attributes of your production system that require the use of protocols that are not consistent with standard protocols. This could include removal of topsoil from pond bottoms, filling ponds partially post drying and using very high levels of disinfectants, draining the ponds and repeating the process. This allows one to use better possible disinfectants at higher levels and lower costs. For lined ponds, fix the leaks every cycle.

  3. Recognize that limiting viral loads is critical. The virus can mutate to becoming less virulent and this may be a reason why there typically are lower levels of specific mortality due to the WSSV over the course of repeated production cycles. Use every tool at your disposal to keep the virus out of your production system. Learning to live with it may be a viable solution in some instances but it sets a potentially troubling precedent for the eventuality of future viral outbreaks.

  4. Stop looking for magic bullets. While I would never rule out the possibility that some tools are useful, such as those that act in concert with other biosecurity measures to lessen overall environmental viral loads, there is little (if any) peer reviewed data that demonstrates a reproducible impact on the disease process in the field. Lab studies may show efficacy but they rarely translate into a similar effect in the field. The field is not an aquarium kept at a constant temperature indoors in a stress free environment. Anecdotal field trial results rarely tell the whole story.

  5. Consider poly-culture with fish that will eat dying animals keeping other shrimp from eating them. Some have reported that stocking with Tilapia or other fish that are omnivorous can reduce the impact of the virus. Eliminating shrimp to shrimp passage of the virus can have a powerful impact on the outcome.

  6. Stock at densities that are more consistent with your ability to control overall viral loads. You might be better off stocking fewer animals, getting higher survivals of very large shrimp then repeatedly trying to out maneuver the virus.

White shrimp infected with the WSSV often die from secondary vibriosis infections.


Conclusions:


The WSSV is endemic in shrimp farming. Many areas purport to be free of it when in fact they are not. I have seen many instances where shrimp are being grown at low densities (under 15 per square meter) with no evidence of the virus being present. As biomass levels increase, some cull the shrimp for an early harvest or to separate slower growing animals and move them to other ponds. This stress can set off an outbreak. This is compounded when the temperature of the ponds that they are being moved to is even a few degrees below 31 C. The best way to control this virus is to keep it out of your animals from the onset. The technology exists to ensure this although at an added cost. From a strict cost benefit perspective these approaches pay for themselves.


Shrimp farming will continue to expand well into the foreseeable future. Much of this will be in third world countries using technology and approaches that are inconsistent with the levels of biosecurity that sustainable farming requires. Improving biosecurity can benefit producers in terms of controlling existing viruses such as the WSSV but more importantly it can reduce the risks of introducing as of yet uncharacterized and localized viruses. Failure to do so will result in new diseases some of which could cripple the industry. There are already several as of yet poorly characterized viruses present in global stocks that are impacting production. The handwriting is on the wall!



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