| White spot syndrome virus (WSSV) has proven to be by far the most serious virus to affect the penaeid shrimp industry in the past decade due to its sudden onset, high mortality and ability to infect over 40 species of crustaceans. However, there have been reports of WSSV infected shrimp that do not show clinical symptoms, indicating possible non-virulent strains or other genetic differences. Therefore, seven geographic isolates of the White Spot Syndrome Virus (WSSV) from China (CH 1995), Nicaragua (N2000), Honduras (H2000), Ecuador (2 strains - E-L1999, E-LT2002) and Mexico (2 strains - M-M2001, M-LP2001) were characterized in terms of virulence and genomic make up.; Virulence comparison was accomplished by challenge experiments. Four quantified dilutions (10-6, 10-7, 10 -8, 10-9) of viral filtrate prepared from WSSV infected shrimp tissue from each isolate was injected into ten specific pathogen free (SPF) juvenile (0.25--1.5 g) Litopenaeus vannemei per dilution. The amount of virus present in each filtrate preparation was determined by real time PCR. The estimated dose of filtrate needed to kill 50% of injected shrimp (ED50) was determined using an EPA probit program. The CH 1995 and M-M2001 isolates were the least virulent with an ED50 of 894 and 468 mug of tissue, respectively. The most virulent isolate was M-LP2001 with an ED50 of 1 mug of tissue. Real time PCR data showed that the amount of virus differed by an order of magnitude (6 to 64 pg viral DNA/mul), but had no correlation to the ED50 tissue values.; The WSSV genome was compared among the seven isolates by examining banding patterns generated from twenty-nine primer pairs that covered 10% of the 305 kb WSSV genome. Primer pairs were designed based on published open reading frames (ORFs). Differences in expected band patterns were seen for some of the isolates with thirteen primer pairs (129, 129/200, 147, 147/400, 178, 178/200, 325, 325/200, 399, 465F/477-82R, 477F/477-82R, 477-82, VR14/15), which covered six distinct areas of the genome. Based on banding pattern differences alone tentative relationships may be postulated. Isolates N2000 and E-LT2002 show no differences in band patterns, and likely share a common origin. Isolates H2000, E-L1999, M-M2001 and M-LP2001 also form a closely related group differing in only two or three of the ten band patterns. CH 1995 shares only five band patterns with this group and likely originates from a different source. (Abstract shortened by UMI.)... |
