Stability of the WSSV ORF94 VNTR genotype marker during passage in marine shrimp, freshwater crayfish and freshwater prawns

Pathogenicity of white spot syndrome virus (WSSV) after multiple passages in mud crab, Scylla olivacea

White spot syndrome virus (WSSV) is a highly virulent shrimp pathogen with a broad host range. Among the hosts, though mud crab, Scylla olivacea is reported to be more susceptible to WSSV than S. serrata and S. paramamosain, a detailed study on the pathogenicity and genome stability of the virus after multiple passages has yet to be reported. Firstly, to test the pathogenicity of the virus, WSSV was intramuscularly injected into healthy shrimp, Penaeus vannamei. Experimentally infected P. vannamei showed the first mortality at 36 h post-injection (hpi), followed by 100 % cumulative mortality in 7 days post-injection (dpi). However, S. olivacea injected with the WSSV inoculum derived from infected shrimp showed the first mortality at 48 hpi and a cumulative mortality of 70 % at the end of the ten days experiment. Subsequently, WSSV was sequentially passaged five times in Scylla olivacea to find out any change in the virulence of the virus in each passage. S. olivacea groups injected with 1st, second, third and fourth passages derived from the crab recorded the first mortality between 48 and 56 hpi and the cumulative mortality of 60 to 70 % at the end of the ten days experiment. Injection of WSSV inoculum in P. vannamei derived from multiple passages in S. olivaceae revealed the retention of the pathogenicity of the virus. Shrimp groups injected with WSSV derived from different passages showed first mortality between 24 and 36 hpi and cumulative mortality of 100 % between 6 and 7 dpi. The average viral load in the shrimp groups injected with WSSV inoculum derived from shrimp was 3.6 × 108, whereas in shrimp injected with the inoculum derived from 1st, third and fifth passages from crab showed 4.0 × 108, 4.7 × 108 and 4.3 × 108 copies per 100 ng DNA. Histological examination of the gill and stomach tissue of shrimp injected with inoculum prepared from shrimp as well as the inoculum derived from 1st, third and fifth passages in S. olivacea revealed characteristic pathological manifestations of the WSSV infection in gill and stomach tissues such as hypertrophied nuclei, Cowdry A-type inclusions as well as massive basophilic intranuclear inclusions. Further, to study the genome stability, the primers targeting highly variable regions of the WSSV genome (ORF94, ORF125, ORF75, variable region (VR) 14/15 and VR 23/24) were used to amplify WSSV derived from different passages and the amplified PCR products were sequenced. The sequence analysis revealed the WSSV genome stability after multiple passages in mud crab, S. olivacea.

Characterization of putative proteins encoded by variable ORFs in white spot syndrome virus genome

Background

White Spot Syndrome Virus (WSSV) is an enveloped double-stranded DNA virus which causes mortality of several species of shrimp, being considered one of the main pathogens that affects global shrimp farming. This virus presents a complex genome of ~ 300 kb and viral isolates that present genomes with great identity. Despite this conservation, some variable regions in the WSSV genome occur in coding regions, and these putative proteins may have some relationship with viral adaptation and virulence mechanisms. Until now, the functions of these proteins were little studied. In this work, sequences and putative proteins encoded by WSSV variable regions were characterized in silico.

Results

The in silico approach enabled determining the variability of some sequences, as well as the identification of some domains resembling the Formin homology 2, RNA recognition motif, Xeroderma pigmentosum group D repair helicase, Hemagglutinin and Ankyrin motif. The information obtained from the sequences and the analysis of secondary and tertiary structure models allow to infer that some of these proteins possibly have functions related to protein modulation/degradation, intracellular transport, recombination and endosome fusion events.

Conclusions

The bioinformatics approaches were efficient in generating three-dimensional models and to identify domains, thereby enabling to propose possible functions for the putative polypeptides produced by the ORFs wsv129, wsv178, wsv249, wsv463a, wsv477, wsv479, wsv492, and wsv497.

Electronic supplementary material

The online version of this article (10.1186/s12900-019-0106-y) contains supplementary material, which is available to authorized users.

White spot syndrome virus (WSSV) genome stability maintained over six passages through three different penaeid shrimp species

White spot syndrome virus (WSSV) replicates rapidly, can be extremely pathogenic and is a common cause of mass mortality in cultured shrimp. Variable number tandem repeat (VNTR) sequences present in the open reading frame (ORF)94, ORF125 and ORF75 regions of the WSSV genome have been used widely as genetic markers in epidemiological studies. However, reports that VNTRs might evolve rapidly following even a single transmission through penaeid shrimp or other crustacean hosts have created confusion as to how VNTR data is interpreted. To examine VNTR stability again, 2 WSSV strains (PmTN4RU and LvAP11RU) with differing ORF94 tandem repeat numbers and slight differences in apparent virulence were passaged sequentially 6 times through black tiger shrimp Penaeus monodon, Indian white shrimp Feneropenaeus indicus or Pacific white leg shrimp Litopenaeus vannamei. PCR analyses to genotype the ORF94, ORF125 and ORF75 VNTRs did not identify any differences from either of the 2 parental WSSV strains after multiple passages through any of the shrimp species. These data were confirmed by sequence analysis and indicate that the stability of the genome regions containing these VNTRs is quite high at least for the WSSV strains, hosts and number of passages examined and that the VNTR sequences thus represent useful genetic markers for studying WSSV epidemiology.