Visitas: 214
By Ph.D. Stephen Newman
Some countries deny that the strain of Vibrio parahaemolyticus (VP) that causes acute hepatopancreatic necrosis syndrome or disease (AHPNS or AHPND) is present in their stocks, further increasing the chances of its spread. It is likely that this will continue to be the case for highly lethal vibriosis (HLVD/GPD/TPD). The environmental component of AHPND and GDP and recommendations for controlling these diseases are discussed in this part.
An Environmental Component Early efforts to control the acute hepatopancreatic necrosis syndrome (AHPNS) pathogen focused heavily on the use of chlorine to kill it and any potential vectors. This in retrospect turns out to have been misguided. Where the practice continues the disease continues. Chlorine wipes out huge swatches of the natural microbial flora in the environment. This appears to give the strains of Vibrio parahaemolyticus (VP) that cause AHPNS a competitive edge.
Type VI secretion system (T6SS), a mechanism that these strains use to kill other bacteria, ensures that VP dominates as it reproduces very quickly with some reports suggesting ten minutes or less, under ideal conditions. The strains responsible for glass post-larval disease (GPD) will likely behave similarly.
Maintaining a “healthy” microbiome appears to be critical for minimizing the impact. There are field observations linking the use of chlorination in pond preparation to the spread of this strain of VP at the expense of many other bacteria. This might explain why co-cultivation of shrimp with Tilapia both directly in shrimp ponds or preferably in adjacent ponds where the water from these ponds is used in the shrimp ponds reduces the severity and disease incidence of AHPNS.
The microbiome ensures that even though the pathogen may be present it does not dominate. Once the problem is present to any significant degree it is not easy to deal with reactively in a manner that is consistent with limiting its impact. Large amounts of water ex change may lessen the loads, but this may be at best temporary.
No country has been successful in totally eradicating AHPNS although the extent to which it impacts a given domestic industry is highly variable. Early indications suggest that GPD may be more of the same. In Thailand there are examples where farmers stopped using chlorine while their neighbors who routinely used chlorine continue to experience losses from AHPNS. Stocking densities appear to play a role in susceptibility as does the amount of biomass.
High densities can stress animals that are not fit for these conditions and make it much easier for the pathogen to spread between animals. Large animals have much more HP tissue and even when damaged there may remain sufficient healthy tissue for the animals to thrive.
“It stands to reason that large animals with much larger HPs would be able to tolerate exposures that would damage the HPs of smaller animals to the point where they die. What happens with GPD is likely similar.”
Genetics also plays a role in susceptibility in addition to the size of the animals. Broodstock held outdoors in non-biosecure facilities are likely to become infected. This may not be detectable using traditional approaches to testing. Once they spawn this can spread the infection.
It can be present in the hatchery at a low level that standard approaches to screening may not see. Once the post larval shrimp (PLs) are stocked the levels gradually increase to the point where acute disease is present. The use of disinfectants that significantly disrupt the microbiomes also appears to be a risk factor. Strains of VP that contain the PIRa and PIRb plasmids and that contain the Tc toxins have a mechanism (T6SS) that allows them to kill off competitors in environments including those where the bacterial populations have been damaged by the use of chlorine.
Conclusions
In conclusion, eradication of the strains of VP that cause AHPNS and GPD is challenging. Many experts in the field will tell you that you must live with them. This is of course potentially problematic. Moreover, a farmer is likely better off with a balanced microbiome where these pathogens may be present at low levels than a microbiome that is not in balance that allows these strains to proliferate to high levels. Suggested approaches towards controlling them are:
1. Broodstock must be held in a biosecure manner, and every effort made to ensure that they are not fed contaminated feeds or inadvertently infected by carelessness. Clean broodstock produce clean PLs. Screening broodstock animals individually is essential. Population screening will not eliminate the vibrios.
2. Care must be taken when using chlorine to treat ponds and reservoirs, etc. Not using chlorine is a step that many would be reluctant to consider. The dogma has been that this is necessary to keep pathogens out of production systems. With AHPNS this does not seem to work for the most part. It probably will not work either for GPD.
Avoiding the use of chlorine and other disinfectants is important. Farmers are probably better off with healthy microbiomes where the VP may be present, but it is not given the opportunity to proliferate in a damaged microbiome.
3. Developing strains of shrimp that are tolerant and even resistant to the effect of the toxins is important in the long run. Shrimp have a great deal of genetic potential that can be exploited.
4. Understanding the nature of the pathology is important to be able to determine if the characteristic pathology is present in animals. There can be a wide range of damage depending on the toxin levels. With GPD, it appears that a virus may also cause a very similar disease with the described Tc toxin containing VP being absent. This needs to be looked at much closer.
5. Enriching for VP using standard approaches may be needed to verify its presence.
6. Minimizing the stress that animals are under is essential for mitigating the impact of most diseases that impact shrimp. Weakened shrimp will generally be much more susceptible to a wide variety of diseases. Minimizing stress, among other things, entails using strains of shrimp that have been bred to tolerate the high densities that are now becoming the norm for production in Vietnam and potentially elsewhere.
7. Ensuring that oxygen levels are kept at or close to saturation levels through aeration is essential.
8. Minimizing the stress of feeding by the use automatic feeders will also lessen the overall stress by ensuring that animals can access feed in a consistent fashion and that they can consume most if not all of the feed that they need.
9. Sampling animals weekly for health and having trained personnel look at the HP for the characteristic pathology should be a part of the overall strategy. Regular testing of moribund animals is important. The goal should be to gain an edge on the pathogen.
10. High densities of shrimp that are not well adapted to these conditions increase the chances of the disease spreading.
11. The use of bioremediation via targeted delivery of Bacillus species using PRO4000X to reduce organic matter accumulation will reduce the food sources available for VP and other potential pathogens.
The preponderance of evidence points towards EMS/AHPNS being an environmental disease. More than likely GPD will be similar. Chlorination damages the microbiome and allows strains of VP that possess toxin generating plasmids and produce type 6 secretion systems (T6SS) to proliferate at the expense of all other bacteria. This allows these strains to dominate. If this happens early in the production process, we see large levels of early mortality.
If it happens later in the cycle, depending on the toxin loads and the overall health of the population, one can see a range of impacts from minimal to populations that gradually stop eating and at harvest moderate to high mortalities (sometimes just from the stress of being harvested), damage from secondary pathogens (which makes the shrimp non exportable and/or not consumable), etc.
Table 1, modified from the International Technical Seminar/Workshop “EMS/AHPND” June 2015, summarizes known risk factors with some management suggestions. These are going to be the same for GPD.
If the VP strains that cause AHPNS or GPD are part of a healthy microbiome their presence in and of itself does not invariably result in acute disease. Pathology ranges from minimal to severe depending on the amount of toxin exposure. Healthy, strong, stress-free animals may be damaged to some extent but they can still end up being part of a profitable crop. Note that as with similar types of diseases, what is occurring is complex.
What works under some environmental conditions may not be as successful under other environmental conditions.
Note that there is a great deal of ongoing research on these pathogens. Comprehensive reviews have been published on AHPNS. GPD is certain to also be the subject of considerable research. There are no magic bullets. The best approaches to dealing with these pathogens are to create production environments that favor the shrimp and that allow for a balance. Lowering the loads is critical although elimination is not likely.
Maintaining a healthy dynamic microbiome that prevents these pathogens from dominating is critical for minimizing their 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
