How to Tackle Major Shrimp Diseases

Andy Shinn, director of Fish Vet Group Asia, and a world\r\nrenowned expert on aquatic parasites, believes that the shrimp sector can do\r\nmore to limit the impact of those disease challenges that form the sector’s\r\nlargest constraint.

What are the current\r\nmajor shrimp diseases?

The top three disease challenges in shrimp are WSSV (white\r\nspot syndrome virus), AHPND (acute hepatopancreatic necrosis disease) and EHP (Enterocytozoon hepatopenaei).

These are all exacerbated by man - the Asian shrimp industry\r\nhas a mindset that must change. This includes taking responsibility for the\r\nhealth and welfare of the stocks in their care.

It is vital that fish farm operatives who are responsible\r\nfor farmed fish are trained in their health and welfare. This will help to ensure\r\nthat fish are free from disease and suffering whilst at the same time promote\r\ngood productivity and comply with legislation.

In a cultural context, this can be complicated and, while it\r\nperhaps unfair to overgeneralise, the following factors may contribute to\r\nincreased rates of certain diseases:

Farm management: the\r\nvast majority (ca. 80%) of Asian shrimp farms are of a small size which means\r\nthat farm budgets are limited. In many states there is no mandatory disease\r\ntesting of shrimp leaving hatcheries or being added to ponds.

Water management:\r\nthere is a vast range of farm systems in operation from open, extensive through\r\nto intensive, closed systems – for those sharing common water sources (e.g.\r\nrivers and lakes), the challenges of maintaining biosecurity are greater as\r\ngood site health is in part also dictated by the level of biosecurity practiced\r\nby a farm’s neighbours.

Biosecurity: the\r\nscale of culture systems are such that the implementation of biosecurity\r\nmeasures are either physically difficult or expensive.

Health surveillance: the\r\ngeneral rate of health surveillance of stocked populations is low - many\r\ngovernments offer a free diagnostic service which somewhat arguably shifts some\r\nof the responsibilities of health management; in many states there are low, or\r\nin some states there are no stock movement records. There is also no compulsory\r\ndestruction of diseased stocks which means the probability for pathogens to\r\npersist and to spread is higher.

Andy Shinn, director of Fish Vet Group Asia, is a world\r\nrenowned expert on aquatic parasites

How is the industry\r\nworking to tackle these challenges?

There are a number of preventative management initiatives\r\nthat have helped to reduce the challenge of disease such as:

1.      \r\nLining of ponds with polyethylene liners

2.      \r\nInstallation of shrimp toilets to collect and\r\nremove faecal waste uneaten feed, moults etc

3.      \r\nBetter pond bottom management through the use of\r\ntoilets and by observing the distribution of sludge following the draining of\r\nponds and then redeploying aerators to ensure proper pond bottom cleaning

4.      \r\nCleaning and maintenance of liners

5.      \r\nWater treatment

6.      \r\nInstallation of crab and bird predator nets

7.      \r\nStocking with shrimp that have been genetically\r\nselected to be more resistant to disease.

There is, however,\r\nmuch to do, such as:

1.      \r\nGreater awareness of the routes of shrimp diseases

2.      \r\nDisease testing prior to stocking and throughout\r\nthe production cycles

3.      \r\nIncreased biosecurity

4.      \r\nUse of functional feeds that include ingredients\r\nthat help promote general gut and shrimp health

5.      \r\nBreeding of specific pathogen resistant shrimp\r\nlines

6.      \r\nClosed system farms that are less susceptible to\r\ncontamination and good on site water management.

Unfortunately farm production economics all too frequently\r\noverrides health and biosecurity management.

We recently took part in a study, looking at the probability\r\nof a disease outbreak, the intensity of production and the investment in\r\nbiosecurity measures. Perhaps not surprisingly, in systems with a low intensity\r\nof production, with no or low biosecurity, there are high probabilities of\r\ndisease outbreaks, but these losses are generally low when outbreaks occur.

By comparison, for systems with high intensity production\r\nand with high levels of investment in biosecurity, there are low probabilities\r\nof disease outbreaks, however, the losses can be extremely high when disease\r\nevents do occur.

Do you think it's\r\npossible to eradicate any of the major shrimp diseases?

I think eradication is unrealistic. Some of the bacterial\r\nand viral agents are so ubiquitous in their infection, that their eradication\r\nis unlikely. The potential for viruses to mutate and for bacteria to acquire\r\ntransposable disease elements which may change their current pathogenecities are also a concern.

In most cases the best we can do is develop programmes of\r\nbetter management and control to minimise the magnitude and frequency of\r\ndisease episodes and loss.

We stand on the edge of a great sea of possibilities, with\r\nthe right tools we can glide across it or without them remain on the shore\r\nwondering what opportunities may have been missed

What diseases are on\r\nthe horizon?

Predicting the unpredictable is difficult, but we can learn\r\nfrom past experiences where certain threats may arise. We need to be prepared\r\nand to have in place the tools for surveillance and for the early detection of\r\nnew threats.

As global aquaculture operations continue to intensify,\r\ngreater environmental pressures placed on viral and bacterial communities may\r\nconsequentially lead to the emergence of new isolates with increased virulence.

With that said, we are aware of some more immediate threats\r\nsuch as SHIV (shrimp hemocyte iridescent) which is causing losses in China.\r\nAgain we need to be vigilant and to ensure our programmes of surveillance\r\ninclude the necessary tests to detect this pathogen in any given population.

How do you see\r\ndisease management evolving?

I see more sophisticated whiteleg shrimp farms and\r\nmanagement in the future. There will always be challenges in any biological\r\nsystem, but the implementation of new technologies and better management can\r\nhelp to reduce risks and create more sustainable systems.

There are a number of disciplines that I see as important in\r\nmoving this forward:

1.      \r\nData and the continued development of farm\r\nanalytics which increases farm efficiency.

2.      \r\nGenetic technologies to breed stronger shrimp\r\nthat are more robust and have a higher level resistance to disease.

3.      \r\nDiagnostic methods with increased sensitivity,\r\nspecifically detection of pathogens at lower levels which would thereby detect\r\nthe initial stages of infection and simultaneous diagnosis of multiple\r\ndiseases.

4.      \r\nIn-pond sensing – the use of eDNA approaches to\r\nmonitor the health of cultured populations and in the early detection of\r\npotentially pathogenic agents.

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5.      \r\nImproved welfare standards in commercial\r\nproduction - such as ending eyestalk ablation to stimulate maturation, and\r\nintroducing more humane methods of slaughter.

Source : The Fish Site