Review: Walker PJ, Mohan CV (2009). Viral disease emergence in shrimp aquaculture: origins, impact and the effectiveness of health management strategies. Reviews in Aquaculture, 1: 125-154.

Feature Paper: Walker PJ, Mohan CV (2009). Viral disease emergence in shrimp aquaculture: origins, impact and the effectiveness of health management strategies. Reviews in Aquaculture, 1: 125-154.

Author Abstract: Shrimp aquaculture has grown rapidly over several decades to become a major global industry that serves the increasing consumer demand for seafood and has contributed significantly to socio-economic development in many poor coastal communities. However, the ecological disturbances and changes in patterns of trade associated with the development of shrimp farming have presented many of the pre-conditions for the emergence and spread of disease. Shrimp are displaced from their natural environments, provided artificial or alternative feeds, stocked in high density, exposed to stress through changes in water quality and are transported nationally and internationally, either live or as frozen product. These practices have provided opportunities for increased pathogenicity of existing infections, exposure to new pathogens, and the rapid transmission and transboundary spread of disease. Not surprisingly, a succession of new viral diseases has devastated the production and livelihoods of farmers and their sustaining communities. This review examines the major viral pathogens of farmed shrimp, the likely reasons for their emergence and spread, and the consequences for the structure and operation of the shrimp farming industry. In addition, this review discusses the health management strategies that have been introduced to combat the major pathogens and the reasons that disease continues to have an impact, particularly on poor, small-holder farmers in Asia.

Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.

Review: For a couple weeks we've been looking at coral reefs and I thought that we would shift gears this week and look at fisheries resources (specifically, shrimp aquaculture) and the challenges that face removing stress from overfishing by aquaculturing resources.

Still, shrimp aquaculture has a lot more to do with coral reef management than many people realize. To collect most shrimp in the wild, nets are dragged across the bottom, destroying benthic habitat and catching many other organisms, usually disgarded as "by-catch." For every one pound of wild caught shrimp, about 8-9 pounds of other animals get caught and die in the process (ones caught by the nets dragging across the bottom to catch shrimp), with most disgarded rather than being kept for other food sources. Additionally, the benthic cover is destroyed, which usually means corals are destroyed. And while most commercially harvested shrimp do not exist directly on coral reefs, they still exist in mixed sandy bottom and coral (and other invertebrate and algae) habitats, so animals are still killed in the process of catching wild shrimp.

Therefore, shrimp aquaculture, in general, is good. There are exceptions, where mangrove swarms are cut down to make room for shrimp aquaculture facilities, or where fish pond densities are so close that disease is rampant and overflow of waste water is discharged into the natural environment, thereby degrading surrounding coastal reefs. But on the whole, aquacultured shrimp is better for the environment than wild caught shrimp. Please note that this is not true for all aquacultured products, so for those who enjoy seafood, do your research if you care about the impact of your source.

This paper addresses the situation of shrimp aquaculture and some of the diseases that have emerged in the attempt to culture shrimp in captive environments.

The main stressors that are cited as causes of shrimp disease in aquaculture settings are: stress from "displacement from natural environments, artificial or alternative feeds, being stocked in high densities, exposed to stress through changes in water quality, and being transported nationally and internationally."

This paper then reviews the recently emerging viral diseases that have become rampant in shrimp aquaculture and led to the production of special strains of disease-free shrimp, increasing the cost and complexity of shrimp aquaculture. As of 2006, shrimp aquaculture in Asia alone is an $8 billion industry so it is vital to maintain the future production of shrimp.

The major shrimp viral diseases in aquaculture identified by the paper are: "GAV, gill-associated virus; IHHNV, infectious hypodermal and haematopoietic necrosis virus; IMNV, infectious myonecrosis virus; MBV, monodon baculovirus; MSGS, monodon slow growth syndrome; TSV, Taura syndrome virus; WSSV, white spot syndrome virus; YHV, yellow head virus." Their origin is show on the map below from the paper, but they are now global due to the international trade of aquacultured shrimp products, and some diseases have been shown to survive freezing.

Besides the 8 viral diseases noted above and in the figure, there have been more than 20 viruses found in shrimp aquacutlure. However, the eight in the figure above are the most devistating to production, with WSSV being the worst of the worst, and is responsible for "much of the economic impact of disease on production globally." It is a devastating disease because not only does it kill shrimp and lower fecundity (reproductive outcome) of survivors, it affects multiple species in production (whereas some viruses are unique to certain species and carriers in others). From being discovered in China in 1992, it had spread to the United States by 1995 from imported frozen shrimp. By 1997 it had spread into the wild shrimp stocks of the southeastern US.

The paper continues to describe all the other major shrimp diseases and details the history of their spread from their sources of original identification (on the map above) so I won't reiterate the great review that can be read freely by following the full paper link at the beginning of this review.

However, a couple other noteworthy points in the paper involve their discussion of why viruses have emerged in shrimp aquaculture and the future of shrimp production.

1. Viruses can emerge when not causing major disease outbreaks in nature through shifts in the natural environment, thereby creating an imbalance in the artificial aquaculture environment that benefits diseases and lessens the immune protection of captive shrimp. This can be addressed in part through multiple generations of shrimp in captivity, since captive-raised shrimp are better able to withstand disease than wild-caught shrimp, which often host multiple diseases.

2. High shrimp densities in aquaculture allows for diseases to spread like wildfire in a population, but a balance must be struck between aquaculture facility size and the ability to effectively feed large populations at low enough costs to make production worthwhile.

The authors also point out two major kinds of shrimp pathogens:

1. "Those for which shrimp are the natural host in which they have existed in for, perhaps, millions of years"

2. "Those that have been introduced to shrimp as a result of a recent cross-species transmission"

WSSV, TSW, and IMNV have all emerged through the latter category, in some cases through complex life histories among aquatic snails and terrestrial insects before spreading to shrimp in aquaculture environments.

In 1996 shrimp diseased cased a 40% loss in production capacity worldwide, but some new diseases have nearly bankrupted production of certain shrimp species. While losses have decreased since the mid-1990s, some diseases continue to affect production worldwide.

The industry has adapted in part by shifting production species of shrimp, with the three major species today being Penaeus vannamei, P. chinensis, and P. monodon.

In Asia, the major barrier to disease eradication continues to be the multitude of "low-income, small-holder farmers (<1 ha holding)," who represent a significant sector of the Asian industry but their lack of education and resources to "implement comprehensive disease management practices are major barriers to achieving reliable production."

Many international organizations (including the FAO, NACA, UNEP, WB, and WWF) have worked towards creating "better management practices" to ensure adequate production in order to maintain food security, expecially among the poor.

Industrial nations have mostly switched to high-security facilities with expensive, disease-free and disease-resistent cultures of shrimp, with workers maintaining strict health standards (such as not being allowed to eat crustaceans outside of the workplace) to ensure production capacity.

The take-home message is that disease emergence is to be expected in any new intensive agricultural or aquaculture facility, but today most "aspects of the biology and epidemiology of infection are now known for most of the major pathogens and technological advances have enabled the production of SPF [specific-pathogen free] broodstock and provided sensitive detection tools that allow screening of seed [stock] to eliminate pathogens."

What is also interesting about the paper is that the authors note that the industry is facing "challenges presented by global warming," which threatens not only coral reefs but a host of natural and production environments.

Next we'll look at a paper about public speaking, which is one of the greatest fears of people around the world. Besides the paper, I'll also share some of my perspectives as I've given over 500 hours of public speaking engagements.