Dr. Chu-Fang Lo

Institute of Zoology, National Taiwan University, Taipei,Taiwan, ROC

Corresponding e-mail: gracelow@ntu.edu.tw

Introduction

The first report of a crustacean virus was in the crab Macropipus depurator by Vago in 1966, but it was not until the 1990’s that large numbers of studies began to focus on the viral pathogens of shrimps (and especially cultured shrimps). Crustacean viruses are currently known to come from a wide range of virus families, including Baculoviridae, Birnaviridae, Bunyaviridae,  Herpesviridae, Piconaviridae, Parvoviridae, Reoviridae, Rhabdoviridae, Togaviridae, Iridoviridae, Nodaviridae, Nimaviridae, and the crustacean viral diseases listed by the OIE (Office International Des Epizooties; the World Organization for Animal Health) include Taura Syndrome (TS), White Spot Disease (WSD), Yellowhead Disease (YHD), Tetrahedral Baculovirosis (Baculovirus penaei [BPV] infection), Spherical Baculovirosis (Penaeus monodon-type baculovirus [MBV] infection), Infectious Hypodermal and Haematopoietic necrosis (IHHN), Infectious Myonecrosis (IMN). Spawner-isolated Mortality Virus Disease (SMVD) was removed from the OIE Aquatic Animal Health Code (2005) because the etiology is not well defined. Infection with Mourilyan Virus (MoV), White Tail Disease (WTD), and Infection with Hepatopancreatic Parvovirus (HPV) are listed as emerging diseases. It is now easy to test for the presence of almost all of these diseases using standardized diagnostic tests and commercial kits. These tests are vital for monitoring and controlling the spread of these pathogens internationally.

Current Research on WSSV

Background

WSSV (Family: Nimaviridae, Genus: Whispovirus) is very unique: its infection strategy does not match the infection models of any other known virus, so it must be investigated ab initio. All three of the WSSV isolates that have been sequenced have a genome of about 300 kbp, and genetic comparisons have shown a high degree of genetic similarity. The availability of the complete WSSV sequence facilitates the global molecular characterization of the virus by genomic and proteomic approaches, and has recently led to the discovery of many important WSSV genes, including latency associated genes, immediate genes, structural genes and many other non-structural genes. 

“Vaccination” Trials

VP28 and VP19 are well known major envelope proteins of the WSSV virion, and VP28 has been shown to bind to the host shrimp cells as an attachment protein to facilitate the entry of the virus into the cytoplasm. “Vaccination” of shrimp with VP28 led to significantly lower cumulative mortality after WSSV infection, while a group “vaccinated” with VP19 showed no improvement over the control group. This result has implications for how the shrimp immune system works, and at the very least it suggests that the shrimp immune system can differentiate between VP19 and VP28. This is also among the first evidence of memory in the invertebrate immune system, and suggests that “vaccination” might some day be an effective way of controlling shrimp viral disease. Assuming that specific memory actually exists in invertebrates, the underlying mechanisms are still unknown, and this is an important area of study. Possible mechanisms might include: special combinations of non-specific mechanisms, differential recognition receptors, and receptor polymorphism.