Vlak
JM †
† Laboratory of Virology, Wageningen University and
Research, Wageningen, The Netherlands
Shrimp cultivation is threatened world-wide by a number of diseases caused
by viruses and bacteria. These diseases are the consequence of aquaculture
intensification, the lack of proper sanitation, world-wide trade and exchange
of ‘apparently healthy’ broodstock. In particular viruses also
form a threat to other arthropods, mainly crustaceans, in fresh, brackish and
salt water environments. Furthermore, global warming and increased trade in
crustaceans also has the potency to affect crustaceans in temperate zones with
viral diseases. From a virological perspective, the current scourges in shrimp
aquaculture are an interesting development, as a galaxy of new viruses is surfacing.
The biggest disease problem in shrimp aquaculture is formed by White Spot
Syndrome Virus (WSSV) or whispovirus. This virus causes among other symptoms
white spots under the outer skeleton of postlarval shrimp. Once infected with
the virus, a whole shrimp culture can die in a few days. These resulting ‘mushy’ shrimps
are far from a delicacy and no longer marketable. The virus emerged in the
early 1990’s near China and has since spread worldwide, with the exception
of Australia and Africa. WSSV is able to infect many crustaceans, including
crabs and crayfish, in salt- and fresh water environments. Recently, the virus
has been identified in Southern Europe in crabs.
The virus is ovoid in shape and currently the largest animal DNA virus, of which the entire genetic information has been resolved (ca. 300.000 base pairs). The tail-like appendage (nema) at one end of the virus particle is the most characteristic feature of this virus. The virus contains about 180 computational open reading frames or genes. Most of these genes do not have an assigned function yet. Only those encoding structural virion proteins and enzymes involved in nucleotide metabolism and DNA replication have been assigned. The other properties are unique so far; some of these may be related to the adaptation of the virus to crustaceans or the aquatic environment. Because of its unique properties WSSV has been assigned to a new virus family (Nimaviridae). Current research concentrates in particular on the functional analysis of WSSV genes, on the ecology and epidemiology of WSSV and to methods to prevent (vaccination) or mitigate WSSV infection in cultures shrimp or crayfish. Recently, biotechnologically produced WSSV proteins have been successfully used to vaccinate and protect shrimp against WSSV infections. The basis of this protection, however, is still enigmatic, since invertebrates – in contrast to vertebrates – are not thought to have a long-term immunological memory.
In addition to WSSV, two other new viruses have been discovered recently:
the Taura Syndrome Virus (TSV, Dicistroviridae) and Yellow Head Virus (YHV,
Roniviridae). They are an emerging problem in shrimp in Central and South America
and South-East Asia, respectively. Yet another set of new viruses have been
identified from shrimp, but they have not yet reached epidemic status. Apparently,
there is a whole galaxy of viruses hidden in the aquatic environment and many
of these will hold new virological surprises.
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