By Aqua-In-Tech - Diseases are due interaction between environments, the animal being reared (genetics, health, nutritional status, etc.) and the pathogen. In an ideal system these are in balance with the end result being minimal disease problems and increased profits.
However few, if any, cultural environments are ideal. Constant fluctuations in the environment and the interaction of genetic limitations and viral ecology assure that diseases will be an ever present problem in monoculture rearing environments.
Principles of minimizing the impact of diseases on animal populations are well established in other areas of aquaculture and agriculture. With each year we become able to detect even smaller levels of potential pathogens quicker, causing continual shifts in issues concerning carrier status and minimizing the stress on populations.
What is White Spot?
The term white spot is a description of the characteristic white spot appearance that has accompanied outbreaks of this viral disease around the globe.
The mere appearance of white spots is not necessarily indicative of the disease that is caused by this virus. Other things can cause white spots.
In P. vannamei, though there are white spots, in the disease in the field, they appear late in the infectious cycle and are much smaller than the classic spots noticed in other shrimp species. Classic spots from P. monodon are shown below.
The disease due to the WSSV in P. vannamei is not consistent in its impact. Some areas have been dramatically impacted with total crop failures while others seem to be living with the virus without acute mortality. Only time will tell if this is a permanent pattern. Thailand apparently experienced a similar situation before the problem increased dramatically. It is likely that as the virus spreads and becomes more firmly entrenched in the farm environments that more frequent and severe outbreaks will occur.
What causes the problem?
The disease is associated with a group of viruses that appear to be similar in genetic composition and are widely dispersed geographically. There is some evidence that suggests that not all of the variants are identical though this is the subject of ongoing research. The virus is very large, as viruses go, and has an envelope around it. It is very susceptible to iodine and chloroform and the transmission cycle appears to be easily broken.
Viruses require the host’s metabolic machinery to reproduce themselves and can not be eradicated with antibiotics. However, antibiotics can impact secondary bacterial infections and in theory might be useful if a bacterial infection is stressing animals leading to increased susceptibility.
What is PCR?
PCR is an acronym for Polymerase Chain Reaction. This technique is a valuable tool that enables the detection of minute quantities of DNA. Almost all organisms contain DNA as the primary genetic material, including viruses though some contain RNA. Using a piece of DNA that reacts with the viral DNA it is possible to “fish” for the presence of the virus DNA in a sample. When this piece of DNA reacts with the viral DNA the amount of this reactive DNA is amplified many times over, a billion fold or more. This makes PCR a very sensitive method for detected the DNA of any particular pathogen of interest. While PCR is just coming in to its own as a potential health management tool, it does however have some drawbacks.
It is technically exacting and prone to occasional errors. False positives (reactions that suggest that you have the virus when you do not) can cause serious problems in that animals that are not carrying the virus can be labeled as carrying it. False negatives are even worse as this can result in keeping stocks that should not be kept.
PCR tests need to be validated, sensitive, accurate and reproducible. Not all of the commercially available kits have been this thoroughly tested.
PCR does not distinguish between the DNA of a live or a dead virus.
PCR is not a quantitative technique. It does not tell you how much viral material there was to start with though it can be interpreted semi-quantitatively.
PCR depends upon the sequence of the primers (small pieces of DNA) to react only with the organisms DNA that they have been constructed against. This high degree of specificity is a strength of the assay. Though the primer will react with isolates that are not as virulent as are others or are avirulent.
When one is told that a population has been screened (say 150 animals out of a million) by PCR and all have come back negative, this does not mean that none of the animals in the population are carrying the virus. No technique is 100% unless every animal is screened and the technique is 100% accurate.
When sampling animals for the presence of a given pathogen, a specific number of animals are selected at random from the population for further examination. These numbers are based on well establish guidelines in fish health monitoring and certification. Supposedly at 150 out of 1,000,000 animals, you have a 98% chance of finding something that is there. Even if this were true, in a population of one million animals, you could still have 20,000 animals that were carriers. Since random sampling is not usually the case and the analytical techniques are not 100% effective at detecting the pathogen of interest this number could actually be much higher.
Do not rely on PCR as your only tool for protecting yourself against the virus. Like all other management tools it is just that and must be used in conjunction with other techniques to maximize its potential. Selecting larval suppliers that have a history of remaining free of the problem is one very useful tool as are others.
What can you do to lessen the impact of this disease on your farms and hatcheries?
There are some things that you can control and others that you can not.
What management techniques are going to be useful?
Do not buy nauplii or PL’s from a source that could be or is infected with the virus. It is likely that iodine and water washes remove and destroy the virus when used on eggs, nauplii and PLs. It is essential that this process be consistent. Hatcheries must maintain good biosecurity measures and examine each batch of animal. The hatchery needs to be constructed to prevent the introduction of the virus from the ocean.
Use the most sensitive and reliable diagnostic tests available to detect and monitor WSSV. These are going to be DNA based technologies such as PCR and in-situ-hybridization (ISH) of tissue lesions.
Sample hatcheries at least twice during the production cycle and retain samples for later testing (three weeks post shipping).
Only buy PCR screened and stress tested animals. Starting out with no or a very low virus load is important.
Stressing PL’s with formalin has been found to weed out weaker animals though one should never stock PLs that are known to be carrying the virus. It has been noted that in P. japonicus, the virus may not display its pathogenicity till after PL6, making the screening of later stage animals essential.
Minimize the stress on the shrimp wherever possible
There are some ways that you can do this and many that you can not. Some of the things that you can do are:
- increase acclimation times before stocking
use non-specific immune stimulants (NSIS) and fortified mineral and vitamin diets to increase stress tolerance
Consider stocking during times of the year that you know there will not be experiencing severe stresses from sudden changes in temperature and salinity. It has been reported that these types of stresses can precipitate an epizootic in a population that carries the virus.
use good quality diets and continue the use of NSIS through out the life cycle
stock at lower densities
Monitor for the presence of vectors carrying WSSV in the ponds and control them.
Sample the phyto and zooplankton in the pond before stocking and test by PCR for WSSV. Positive ponds should be avoided.
Sample your ponds frequently. Assure that sick and dying animals and any unusual patterns of mortality are sampled as a routine by PCR and/or histopathology. At the first sign of a problem, harvest the shrimp if you can.
What are some of the ways that other countries manage this disease?
It is believed that one of the major methods for the movement of this virus (and others) has been the movement of infected PL’s. If this can be stopped, it should lessen the rate of spread of the virus. Many countries have taken steps to ensure this though how successful they will be will only be apparent with time. Once the virus gains a foothold it appears that it is there to stay. Fortunately as with all of the other viral diseases such as BP, IHHN, TSV, and MBV, the impact of the disease will lessen with time.
The Thai’s use a variety of tools to deal with the virus, some of which will be useful for P. vannamei. It is important to recognize the differences between the two forms of shrimp culture and understand that there are some difficulties associated with using the same tools to try and control the presence of the virus.
One recommendation made to and by Thai farmers is to use pesticides to kill the vectors before stocking the ponds. Usually a very potent pesticide is added to the water to kill any crustaceans and other vectors that might be present in the pond carrying the virus. This can not be done on the scale that would be required in most of the Americas. The costs would be quite high and the huge amounts of pesticides that would have to be dumped into the ecosystem are not desirable. This should be discouraged and only considered as a very last resort.
Filtering intake water into the pond is one viable approach to eliminating some vectors. Conventional filters may be problematic in the Americas due to the large demands for water. Though this demand can be moderated and relatively small amounts of water added to the system. Filters that use plant fibers to trap everything in conjunction with serial mesh filtration might be useful. Using 250 micron or smaller mesh filters including small mesh bags can be helpful as well.
Another recommendation is to avoid the exchange of water. There appears to be some merit in this in that recent outbreaks of WSSV in S. Carolina in the USA have been associated with the addition of water to ponds. Whether this stressed the shrimp and set off an epizootic or introduced vectors and virus into the ponds is not known. In intensive systems, aeration is used while it is not in semi-intensive systems. The ability to aerate the water mechanically without resorting water exchange might be very useful in those ponds where oxygen levels can not be managed without water exchange.
Animals that are positive for the virus by PCR are not purchased. In some cases it has been reported that screening stocked animals for the presence of the virus has been found to be a useful tool to follow the status of the disease in the population. This can be used to time harvests and to minimize the potential spread of problems to other ponds and/or neighbors.
What you can not control.
Probably the biggest single problem faced by shrimp farmers aside from the actions of their associates will be the sudden environmental fluctuations that accompany the rainy season. Sudden changes in salinity and temperature have been implicated in many outbreaks. As the disease moves from one area to another the viral load in the environment will increase to the point where the virus will be ever present. Ideally shrimp should be destroyed once they are ill to prevent high loads of the virus from entering the environment. Unfortunately this is usually not practical. Harvesting shrimp is and should be encouraged even if shrimp are too small to sell. Cutting losses and minimizing the spread of the virus are to the farmers advantage.
Use Immune Stimulants and optimize Nutrition
All of the data to date suggests that shrimp have relatively primitive immune systems that can not respond to vaccination. In fact it appears that you can not vaccinate shrimp in any sense of the word. Their immune response is short lived, non-specific in nature and provides a modest level of immunity. Though it is possible to exploit this with a variety of polysaccharides, there are very few reports of success using these compounds in the field. The compound with the most field data is a bacterial based material. Lab and field studies have shown a wide range of potential benefits, though like all other tools, these require that they be used as part of on overall management strategy geared towards minimizing the impact of the pathogen.
Animals that have lower levels of resistance due to inadequate nutrition are more susceptible to a variety of problems. Since shrimp in semi-intensive culture environments get between 50 and 70% of their nutrients from natural food source, it is usually difficult to assess what nutrients might be limiting. The role of vitamins A, B, C, D, and E and micro-nutrients such as Selenium are well documented in minimizing the effects of stress and should be routinely added to diets at higher than usual levels at times of stress.
Conclusions:
This virus disease is just one of the many that shrimp farmers will face in the years to come. Effective tools and techniques exist to determine how serious of a problem it can be and to moderate its impact at this time. We are fortunate to have the experiences of others to draw on. The use of PCR in conjunction with other management tools can affect how this disease impacts your bottom line.
Source: Aqua-In-Tech - Reproduced August 2005
