February 15-18, 2009 Washington State Convention Center Seattle ...

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MOLECULAR DIVERSITY AND EVOLUTION OF TAURA SYNDROME VIRUS (TSV)
Kathy Tang*, Joel Wertheim, Solangel Navarro and Donald Lightner
Department of Veterinary Science and Microbiology
University of Arizona
Tucson, AZ 85721 USA
fengjyu@u.arizona.edu
Taura syndrome virus (TSV) is a pathogen of Litopenaeus vannamei, one of the most important species in shrimp aquaculture.
The virus was discovered in Ecuador in 1992. It had spread to the US by 1994, to the SE Asia by 1998, to Belize by 2001, and
to Venezuela by 2003. In general, RNA viruses exhibit high mutation rates; and, subsequently, high variation in protein and
nucleotide sequences occurs among TSV isolates. With 83 isolates, from 16 countries, collected from 1992 to 2008, we found
up to 10 % genetic variation in nucleotide sequence, 8.0% for amino acid sequence. The sequence information is valuable in
that it can provide insight into the spread and evolution of TSV.
To determine the evolutionary history of this virus, we performed a phylogenetic analysis using BEAST (Bayesian Evolutionary
Analysis Sampling Trees) with these 83 TSV isolates. This revealed four distinct genetic lineages: Americas (including
South/Central America, Mexico and US), Belize, SE Asia, and Venezuela; stemming from the Ecuador isolate, the ancestral
genotype. In addition, the date that new isolates were discovered corresponded with the mutation-estimate. In the US, 6 cases
of TSV were found, three from Hawaii and three from Texas. The first two occurrences (1994, 1995) were found in Hawaii, in
which cases the virus was likely introduced from South/Central America. The third outbreak of TSV was in Texas in 1996, and
this isolate is closely related to the isolates from Mexico. The fourth case, also in Texas, occurred in 1998; but, in this case, the
isolate was most closely related to isolates from Ecuador. The fifth TSV outbreak occurred in Texas in 2004, and this isolate
clustered with those from SE Asia, strongly suggesting this virus was introduced either through the value-added processing
of imported shrimp commodities from SE Asia, or through the importation of post-larvae and/or brood-stock from the same
sources that supply producers in SE Asia. The sixth outbreak occurred in Hawaii in 2007, and the isolate was most closely
related to TSV isolates from Mexico.
Although the virus genetic diversity was low, the mean mutation rate is 2.5 x 10 -3 substitutions/site/year, approximately 3 times
higher than those of most animal viruses (an average of 7.0 x 10 -4 substitutions/site/year, from 50 animal RNA viruses). The
rate of substitution increased exponentially over the 16-year span of this study. This suggests that TSV evolves more rapidly
than most RNA animal viruses.