Challenges of Using Non-specific Immune  Stimulant Tools forFarmed Shrimp

* By Stephen Newman, Ph.D.

The use of non-specific immune stimulants under field conditions can impact the ability of shrimp to tolerate exposure to pathogens. However, their effectiveness is related to a number of variables, including but not limited to the level of exposure to a pathogen or pathogens and the stress that the animals are placed under because of the production environment.

In the early 1990s, International Aquaculture Biotechnology Ltd. (IABL) researched, developed and  marketed  a  non-specific immune stimulant for shrimp. Our preliminary tests of the product demonstrated that animals that were exposed to a dilute suspension of the material prior to being challenged showed increased survival against both bacterial and viral pathogen laboratory challenges. The product was subsequently used on many billions of PLs in Ecuador and elsewhere. A variety of benefits were noted although they were not always consistent. Many variables impact shrimp health, and we still do not have a firm grasp on the extent and nature of them.

When the white spot syndrome virus (WSSV) entered Ecuador as the result of a deliberate introduction of the virus in infected PLs from Central America in the late 1990s, hundreds of millions of PLs in the field had been given the product. As the outbreak spread and viral loads increased, we noted that the primary benefit was a delay of the onset of mortality albeit most of the animals eventually die, often from secondary bacterial infections.

Simultaneously, we were conducting trials of many different compounds in the feed. This included several different beta glucans, peptidoglycans and lipopolysaccharides from a variety of yeast, fungi and bacteria. What we found was that none of the products performed the way that they did in controlled lab studies. Highly stressful production environments with a constant onslaught of obligate and opportunistic pathogens overwhelmed any benefits.

Since then, there have been many published observations on the use of a wide variety of non-specific immune stimulants in farmed shrimp. Experimental designs are highly variable and at least some of the conclusions reached are a result of flaws in the experimental design rather than a real-world reproducible effect that would result in a cost-effective benefit. The marketplace is crowded with companies selling non-specific immune stimulants today, despite little evidence that they provide a consistent cost benefit.

There have also been many reports of what had been claimed to be immunization although there is little if any evidence to suggest that shrimp mount anything that could be characterized as a classic immune response. The ecosystems that shrimp are farmed in are very different from their natural environments.

Much as with other agricultural endeavors, nothing about farming shrimp even remotely resembles how they live in the wild. We are farming a single species at densities never found in nature with animals confined to relatively small areas where they are largely reliant on what we feed them. For most paradigms, compound feeds that have been “developed” for farmed shrimp are increasingly being delivered via automatic feeders along with heavy aeration. These are important elements for success.

There are strains of the common white shrimp, Penaeus (also known as Litopenaeus vannamei), that are domesticated (i.e. genetically selected) and that display varying degrees of tolerance to the stressors encountered in these production environments. This does not mean that they cannot be and are not stressed. Stress can manifest itself in many ways including variable growth rates, high FCRs, disease outbreaks from both obligate and opportunistic pathogens, etc. Monoculture production systems, by their very nature, are stressful.

It has been well documented that stress has an overall negative impact on a given animal’s physiology. This includes their immune system. The notion is that if a product works in chickens or swine, it must of course work in shrimp. Nothing could be further from the truth. Shrimp do not form antibodies, they double grind their feed to ensure that the final particle sizes being ingested are the size of or smaller than bacteria, their digestive systems function at near neutral pH levels and the length of time that food is resident in their guts is measured in minutes.

They have little in common physiologically with mammals or birds. Feeding shrimp immune stimulants on a continual basis seems like a good idea, even though there are risks of depleting critical aspects of their immune systems. Their immune systems are not proliferative as are mammalian systems. There is no evidence that being exposed to immune stimulating substances (of which the cell walls of bacteria are one group, including lipopolysaccharides, beta glucans, and peptidogly cans) that their lymphocytes proliferate in response. Actually, it appears that they do the opposite. They are depleted. This limits the nature and intensity of any immune response in a healthy animal.

From our many trials we noted that it was more than likely a short-term impact that we were looking at, measured in weeks or a few months at best. It was easily overwhelmed if the shrimp were held in a manner that ensured high levels of obligate pathogens and, often, preventable stress. Our conclusions, based on extensive field trials in many countries, was that feeding farmed shrimp non- specific immune stimulants did not appear to give them a consistent benefit. However, there is a caveat in all of this. Domesticated animals reared in environments where there is little if any stress and that are free of obligate pathogens are much more likely to be able to gain a meaningful benefit from being exposed to these materials.

So, the next time somebody tells you all about the great immune stimulant that they have that will protect your farmed shrimp from endemic pathogens, consider this. Is this yet another example where farmers are being told that they can grow shrimp under highly stressful conditions without ensuring that obligate pathogens are not present. To be clear, I am saying that there are not circumstances under which these materials provide some benefit. We have extensive field data from billions of shrimps that supports this with a product that was used on animals once.

Pathogens need to be kept out of production systems using specific pathogen free (SPF) animals from pathogen free nucleus breeding centers. Unfortunately, many animals that are sold as SPF still carry opportunistic pathogens and some even the pathogens that they are purportedly free from. Stay away from all pathogen exposed animals (APE), which stipulates that the best approach to generate strong animals is to allow them to be exposed to all pathogens, known and unknown.

The development of tolerance (defined as requiring higher loads of exposure to produce disease) and resistance (defined here as an absolute, yes or no in terms of susceptibility) to pathogens is not necessarily achievable via this approach. You are better off paying attention to the basics. Make sure that each brood animal is tested for all known obligate pathogens. Follow what happens in the ponds to ensure that this, or the nauplii or PLs are not the source of the problem. Aerate your ponds. Do not use too much or too little.

Use compound feeds that are designed for shrimp that contain highly digestible proteins and adequate levels of vitamins and minerals. Use automatic feeders that allow you to feed small amounts of feed either on demand or as you want. If you do all of this, then the use of a non-specific immune stimulant has a greater chance of being beneficial. There are clear short-term impacts established both in the field and the lab that show that exposing animals prior to stocking will afford them some measure of immunity against viral and bacterial obligate pathogens.

A common approach today is to include these materials in the feed. Inclusion of living bacteria in feed is problematic as feed manufacturing methods typically result in the generation of high levels of pressure and heat that kill most of the bacteria. Bacillus spores are widely sold for bioremediation and can be included in the feed with reasonable survivals under most conditions. There is not enough time for the Bacillus spores to germinate before they are defecated (in shrimp).

The feces are nutrient rich, and the bacteria will have a place to grow when the spores do germinate. How many spores are needed to consume the feces is not defined. The level in the feed depends on the method of manufacture of the feed and the inclusion rate of the dried spores blended with an excipient. To calculate the potential lifetime dosage of bacteria per gram of feed consumed it depends on the initial dose (say 1 kg per MT of feed), the method of feed manufacture as this can impact spore survival rates through the milling process.

For example, when you add a 4 billion CFU per gram spore containing product to the feed at 1 kg per MT, if 100% of the spores survived there would be 4,000 spores in a gram of feed, 70% survival would be 2,800 and 50% would be 2,000 spores.

This is what would pass out in the feces assuming that the spores are evenly distributed in the feed. This is likely not going to be enough to have much of an impact as the niche that the feces end up in contains many other bacteria. The lifetime dosing of a 30-gram shrimp with an FCR of 1.5 depends on what percentage of the spores germinate. Starting with the same dose per gram of product as above, 100% survival would be a total of 180,000, 70% is 126,000 and 50% is 90,000. This is over the entire life cycle of the shrimp assuming that the material is fed daily.

While there may be a benefit, by itself, it is not likely to provide the optimum benefit that the direct addition of the spores to the environment can produce. Delivering much larger numbers to “need to be treated areas” of the pond bottom is a much more powerful tool. This can deliver many billions of spores at once into a given area (such as where the feces collect). The challenge is to ensure that the frequency of application and dosage levels are sufficient to ensure maximum degradation of the organic matter. The presence of large numbers of Bacillus that germinate from the spores is more likely to result in a non-specific immune response in shrimp ingesting high levels.

In conclusion, the use of non-specific immune stimulants under field conditions can impact the ability of shrimp to tolerate exposure to pathogens. However, their effectiveness is related to a number of variables, including but not limited to the level of exposure to a pathogen or pathogens and the stress that the animals are placed under because of the production environment. Bacillus spores, used for bioremediation, once they germinate, can produce non-specific immunity but the levels that are typically present in feeds are likely not high enough for this impact to be all that is required to maximize the impact.

* Stephen G. Newman has a bachelor’s degree from the University of Maryland in Conservation and Resource Management (ecology) and a Ph.D. from the University of Miami, in Marine Microbiology. He has over 40 years of experience working within a range of topics and approaches on aquaculture such as water quality, animal health, biosecurity with special focus on shrimp and salmonids. He founded Aquaintech in 1996 and continues to be CEO of this company to the present day. It is heavily focused on providing consulting services around the world on microbial technologies and biosecurity issues.
sgnewm@aqua-in-tech.com
www.aqua-in-tech.com
www.bioremediationaquaculture.com
www.sustainablegreenaquaculture.com.