Can probiotics for shrimp prevent or cure diseases?

Most people, when they hear or use the term probiotic, think of a yogurt with many different microbial species added. These “probiotics” are consumed orally, and the bacteria that survive the barrier of an acidic stomach (which is not the norm in shrimp, as they have an intestine with an almost neutral pH) travel through the digestive tract, where they colonize some part of the intestine. It is claimed that this alters the existing microbiome and that the added bacteria become a stable component of the microbiome. This is supposed to have a positive effect on the host’s health.

  There are mechanisms by which probiotics could, in theory, cure or prevent diseases. These include competition for essential nutrients, such as enzymatic cofactors (metals like iron or vitamins), or the production of antibiotics or antimicrobial peptides that inhibit specific strains.

  This very specific definition omits the wide range of other effects bacteria can have, from environmental impact through bioremediation to the stimulation of non-specific immunity in the host, etc. It is also controversial. The microbiome is a complex community of multiple species of bacteria, fungi, and protozoa that colonize both external and internal surfaces. Much of the existing evidence to date suggests that the addition of “probiotics” may (or may not) cause short-term alterations in the microbiome—at best. The apparent need to continuously dose probiotics clearly suggests that any alteration of the microbiome is temporary.

There are mechanisms by which probiotics could, in theory, cure or prevent diseases. These include competition for essential nutrients, such as enzymatic cofactors (metals like Fe or vitamins), or the production of antibiotics or antimicrobial peptides that inhibit specific strains.

Many bacteria degrade organic matter. Many also convert ammonia into metabolites such as nitrite, nitrate, and atmospheric nitrogen. They may have narrow activity ranges or other traits that make them poorly suited for use in commercial aquaculture products. Some are very effective, while others can be extremely finicky or simply unsuitable. Some companies offer strains of Nitrobacter and Nitrosomonas for nitrification. Many other bacteria can perform this function as well, including several species of Bacillus, which can be dried, have a shelf-stable spore form, do not require refrigeration, and are relatively inexpensive. Many companies sell mixtures of bacteria that compete with one another, which can reduce the overall effectiveness of the process. The inherent variability in production environments means that any standardized approach must take these variables into account.

  The cell walls of many different microbes are capable of stimulating the immune system. This is well documented in both fish and shrimp (as well as mammals). In shrimp, the impact is largely non-specific, since they do not produce antibodies—a component of the classical humoral response. The nature and intensity of the immune response depend on several variables, including the amount of material they are exposed to, its form, frequency of exposure, the stress level of the organisms, and the functioning of their immune system. These factors determine the intensity of the immune reaction.

Many laboratory studies are based on exposing organisms to controlled conditions that do not reflect the real world. They may be conducted in aquariums or microcosms with limited water exchange, which ensures that the organisms ingest the compounds repeatedly and through multiple pathways. Furthermore, in the case of spore-forming Bacillus species that germinate and grow in these systems to sufficiently high levels, it is the vegetative cells—not the spores—that are responsible for the observed impact. Spores germinate at rates that depend on the environment and the strain.

 Pathogens are generally divided into two categories: they are either obligate or opportunistic. Many bacteria isolated from diseased organisms are not the cause of the illness but rather take advantage of a weakened immune system; these are opportunists. Obligate pathogens can cause disease in strong, healthy organisms merely by being present at certain levels—they proliferate and cause illness. Highly virulent pathogens possess virulence determinants—traits that ensure they can harm healthy organisms. Examples include toxins (like the PirA and PirB toxins found in Vibrio parahaemolyticus strains), powerful enzymes present in sufficiently high concentrations to damage tissues, and the ability to sequester critical nutrients like iron (vibrios may contain genes that encode outer membrane proteins to bind iron, making it unavailable to the host).

  Most of the time, acute diseases in farmed shrimp result from multiple pathogens. This may involve a mix of opportunistic pathogens or a combination of obligate and opportunistic ones. In farmed shrimp, infections with pathogens like the white spot syndrome virus (WSSV) or the fungal agent responsible for Enterocytozoon hepatopenaei (EHP) are often accompanied by bacteremia, with vibrios being a primary component. There are many other species of bacteria that can cause problems. Most are opportunistic, but a few are obligate.

  There can be a synergy between multiple potential pathogens. The white feces syndrome in shrimp (a common problem in Southeast Asia) is caused by EHP and a Vibrio species acting together. EHP alone doesn’t cause mortality, but when coinfection occurs with a Vibrio, the result is white feces and acute mortality. As pathogen loads increase in an organism, it may retain its appetite but stop growing, which leads to significant size disparities at harvest and high feed conversion ratios. Their impact on the hepatopancreas—a critical organ for digestion and immunity—weaken the shrimp, making them highly susceptible to both opportunistic and obligate pathogens.

  For probiotics to be effective through microbiome alteration—and therefore also affect the metabolome (the sum of all metabolites produced by the microbiome)—in preventing or curing diseases, several aspects must be considered. If they are to prevent diseases, they must keep potential pathogen loads below the threshold levels that cause acute disease. This would most likely require them to be constantly present. Most bacteria produce antimicrobial peptides and other compounds that allow them to compete for nutrients while inhibiting competitors.

  There are mechanisms by which probiotics could, in theory, cure or prevent disease. These include competition for essential nutrients, such as enzymatic cofactors (metals like iron or vitamins), or the production of antibiotics or antimicrobial peptides (AMPs) that inhibit specific strains. However, this requires that a sufficient quantity of probiotics be close enough to the pathogens to be effective. Since no commercial probiotic persists in the host or its immediate environment at high levels, it is unlikely that a probiotic could cure or prevent diseases caused by obligate pathogens through microbiome alteration.

  These microbial products, such as AMPs, do not act the same way as antibiotics. Antibiotics are administered to organisms in doses that ensure tissue levels are high enough to inhibit the targeted organisms. They do not act on viruses—only on bacteria and fungi. They are localized, and general tissue levels are not going to function the way antibiotics do. The observation that certain bacterial strains inhibit the growth of others through proximity does not mean this is what happens inside the host. Pathogens are often found in biofilms that protect and isolate them, and overall tissue loads are too low to act like antibiotics.

  The role of stress in susceptibility to disease is well documented. Stressors weaken organisms and make them less capable of fighting off infections. These factors include, among others, inadequate nutrition (excess or deficiency of essential nutrients), excessively high densities, poor water quality (sudden changes in salinity, pH too low or too high, etc.), low oxygen levels, high levels of metabolites (H₂S, CH₄, nitrate, nitrite, NH₃/NH₄, etc.), presence of toxic algal and bacterial strains, chronically low levels of pathogens, and frequent handling of shrimp as a result of partial harvests or transfers, etc.

  Healthy, unstressed organisms are in homeostasis with their environment. They can adapt to moderate environmental disturbances without negative impacts. Stressors affect their adaptive capacity and increase their susceptibility to both obligate and opportunistic pathogens.

  There are no magic solutions. Basic principles influence outcomes. Failing to keep obligate pathogens out of production systems increases the likelihood of disease outbreaks. Control begins with the broodstock. Controlling pathogens in a shrimp population doesn’t work unless the animals have been reared indoors under controlled conditions for at least several generations and repeated testing has not revealed anything concerning. Each organism must be analyzed individually. While it is possible for low levels of obligate pathogens to survive, they are unlikely to cause problems in a relatively stress-free, optimal environment. To be precise, there are potential pathogens that must be excluded by all means, as they can infect and kill organisms even at very low levels. Fortunately, these are very rare.

Analyzing healthy shrimp and observing differences between them and the sick ones does not mean that the microbiomes of the healthy organisms are responsible for their health.

  Population sampling should be conducted at the nauplius, zoea, mysis, and post-larval shrimp stages. The samples must be representative of the population. It can be useful to select visibly sick shrimp for testing. Live feeds must come from fully biosecure sources and must also undergo thorough testing. Too often, hatcheries use local sources of wild polychaetes or mass-produced artemia raised under non-sterile conditions, which leads to contamination of broodstock and subsequently nauplii, zoea, mysis, and post-larval shrimp. If tests reveal a biosecurity failure, the producer must inform the clients of the risks or destroy the batch and start over with clean organisms.

  The production environment influences risk. If neighboring farms are too close and there are no safeguards to prevent mixing of effluents and wastewater, the risk of introducing disease into a population with no prior immunity increases. Many have addressed this risk through widespread use of disinfectants—typically chlorine—to treat incoming water before fertilization and restocking. As stated in a previous article, this can actually make things worse.

  Pushing production paradigms to maximize output often results in shrimp stress. Too often, farmers focus solely on the bottom line: “How many tons of marketable product can I produce in a given pond?” Ideally, producers should find a balance between striving for maximum production and limiting stress, which is invariably a factor in this approach. Proper biosecurity and stress reduction are essential components for consistent success.

  Widespread misinformation does not help the sector. It leads people to believe they don’t need to pay attention to the basics. In the end, this ensures that cycles of severe disease persist, limiting profits and potentially creating new pathogens.

  Using every possible means, no matter how excessive or inappropriate, to address the situation in the ponds is not the right approach. Anyone with extensive experience in shrimp and fish farming can attest to this.

  Testing healthy organisms and observing differences between them and diseased ones does not mean that the microbiomes of healthy shrimp are responsible for their health. Even when challenged under controlled conditions, the observed differences are most likely due to a non-specific immune response.

  Ultimately, the focus should be on the fundamentals of biosecurity and stress reduction. Once these are adequately addressed, the benefits will be evident: higher survival, better growth, improved feed conversion ratios, and ultimately, greater profits.