White Spot Syndrome

Shrimp with WSSWhite spot syndrome (WSS) is a virus infection of shrimps. That may not sound very important, but if you know that the aquaculture industry now produces nearly half of all the fish and shellfish we eat, maybe it is. The “blue revolution” of aquaculture has been particularly marked in Asia, where the majority of production is from intensive coastal shrimp farms. Aquaculture is growing at more than 10% per year while the output from wild fisheries has been essentially flat for the last decade (Managing to harvest? Perspectives on the potential of aquaculture. Philos Trans R Soc Lond B Biol Sci. 2005 360: 191-218). Back in 2000, the estimated first sale value of cultured fish and shellfish was US$56 billion. Approximately 90% of all the shrimp consumed in the USA is farmed and imported.

Although white spot syndrome virus (WSSV) does not infect humans and eating infected shrimp does not pose any risk to our health, the disease is highly lethal and contagious for shrimps, killing them quickly. Outbreaks of this disease have wiped out the entire populations of many shrimp farms within a few days. The first reported outbreak of the disease came from Taiwan in 1992, but in China in 1993 it led to a virtual collapse of the shrimp farming industry. This was followed by outbreaks in Japan and Korea in the same year, Thailand, India and Malaysia in 1994 and by 1996 it had severely affected most of Asia. It was first reported in the USA in late 1995. The disease is now a problem in all shrimp growing regions of the world except Australia.

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WSSV is a rod-shaped double-stranded DNA virus with a lipid envelope. The virus particles have been reported to be 240-380nm long and 70-159nm in diameter with a nucleocapsid of 120-205nm long and 95-165nm diameter. The virus sometimes has a tail-like appendage at one end of the virion. The International Committee on Taxonomy of Viruses (ICTV) has recognised WSSV as the type species of the genus Whispovirus, in the family Nimaviridae (A summary of taxonomic changes recently approved by ICTV. Arch Virol 2002 147: 1655-1663). The complete DNA sequence of WSSV genome has been assembled into a circular sequence of 292,967 base pairs (The white spot syndrome virus DNA genome sequence. Virology 2001 286: 7-22).

Transmission of the virus occurs both through oral ingestion and via the water in crowded farm pens (horizontal transmission) and by vertical transmission from infected female prawns in shrimp hatcheries. The virus is present in wild stocks of shrimp, especially in the coastal waters adjacent to shrimp farming regions in Asian countries, but mass mortalities of wild shrimps are yet to be observed. It is only in the crowded conditions of industrial shrimp farms that the disease seems to present a major problem.

There are currently no available treatments for WSS, although a large number of disinfectants are widely used in shrimp farms and hatcheries to prevent outbreaks. Stocking of uninfected shrimps, reducing environmental stresses and care to prevent contamination are important control measures. Although there have been many useful advances in disease diagnosis and treatment of aquatic diseases, the greatest impact has probably been through immunology, with advances in two major areas – diagnosis and vaccine development. The development of antibody probes to pathogens provides useful tools for rapid diagnosis using techniques such as immunohistochemistry, immunofluorescence and enzyme-linked immunosorbent assay (ELISA). Aquaculture vaccines are becoming much more sophisticated, with a trend for the development of subunit recombinant vaccines in preference to killed whole cell preparations, as these did not succeed for many important diseases and attempts to produce attenuated vaccines give rise to safety concerns. The most recent development is DNA vaccination, which is being developed commercially.

That shrimp on your barbie is big business.

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One Response to White Spot Syndrome

  1. elementaryteacher says:

    So it sounds like this could be called a “crowd disease” of shrimp, occurring mainly where the population density (in farms) is enough to support the spread of the disease. Am I understanding you correctly?

    Eileen
    Dedicated Elementary Teacher Overseas
    elementaryteacher.wordpress.com

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