Balance of RNA sequence requirement and NS3/NS3a expression of segment 10 of orbiviruses

Critical parameters of real time reverse transcription polymerase chain reaction (RT-PCR) diagnostics: Sensitivity and specificity for bluetongue virus

A new variant of bluetongue virus serotype 3, BTV3 ITL 2018 (here named: BTV3), was included in serial dilutions in the BT Proficiency Test 2020. Although the OIE-recommended panBTV real time RT-PCR test targeting genome segment 10 (Seg-10) detected this variant, we showed that reverse transcription (RT) at 61 °C instead of 50 °C completely abolished detection. Another Seg-10 panBTV real time RT-PCR test detected BTV3, irrespective of the temperature of RT. In silico validation showed that each of the OIE-recommended PCR primers using IVI-primers contain single mismatches at the -3 position for BTV3. In contrast, WBVR-primers of a second test completely match to the BTV3 variant. Our results suggest that single mismatches caused false negative PCR results for BTV3 at high RT temperature. Indeed, correction of both IVI-primers for BTV3 led to positive results for BTV3 but negative results for all other samples of the BT Proficiency Test 2020. Apparently, variability of the -3 position is sufficient for discriminative PCR detection, although the single mismatch in the IVI-reverse primer was the most important for this phenomenon. Extensive in silico validation showed that targets of both Seg-10 panBTV RT-PCR tests are not completely conserved, and the detailed effect of single mismatches are hard to predict. Therefore, we recommend at least two panBTV RT-PCR tests to minimize the risk of false negatives. Preferably, their PCR targets should be located at completely different and highly conserved regions of the BTV genome to guarantee adequate detection of future BTV infections.

Genomic and biological features of a novel orbivirus isolated from mosquitoes, in China

A novel orbivirus had been identified as a member of the Orbivirus genus, which was isolated from pooled Culex fatigans mosquitoes in Guangdong of China, named as the Fengkai virus (FKOV). The cytopathic effects (CPEs) on both Aedes albopictus cells (C6/36) and mammalian cell lines (Vero and BHK-21) emerged in the cell cultures inoculated above virus in. Experimental confirmation as the Orbivirus genus was conducted by the Real-time PCR and based on Ion Torrent Next-Generation in sequencing. The Identities of VP1, VP2 and VP3 in amino acid sequences between the Tibet orbivirus (TIBOV) and this strain were 98.6%, 42.9%, and 99.9%, respectively, which indicated that this strain shares the same genus (VP1, Pol) and species (VP3, T2) with TIBOV but was greatly different in VP2 and VP5 (10.3%) of TIBOV. The VP2 and VP5 diversities of both TIBOV and FKOV strains suggested both serotypes are distinct with each other. As natural evolution and circulation, this strain might expand its host ranges and infect human beings as a potential and severe pathogen.

Bluetongue virus serotype 27: Experimental infection of goats, sheep and cattle with three BTV-27 variants reveal atypical characteristics and likely direct contact transmission BTV-27 between goats

Bluetongue virus (BTV) hitherto consisted of 26 recognized serotypes, of which all except BTV-26 are primarily transmitted by certain species of Culicoides biting midges. Three variants of an additional 27th bluetongue virus serotype (BTV-27v01-v03) were recently detected in asymptomatic goats in Corsica, France, 2014-2015. Molecular characterization revealed genetic differences between the three variants. Therefore, in vivo characteristics were investigated by experimental infection of a total of 15 goats, 11 sheep and 4 cattle with any one of the three variants in separated animal trials. In goat trials, BTV-naïve animals of the same species were kept in a facility where direct contact was unhindered. Of the 15 inoculated goats, 13 and 14 animals were found positive for BTV-RNA and antibodies (Ab), respectively, until the end of the experiments. Surprisingly, BTV-Ab levels as measured with ELISA and neutralization test (SNT) were remarkably low in all seropositive goats. Virus isolation from whole-blood was possible at the peak of viremia until 49 dpi. Moreover, detection of BTV-27v02-RNA and Ab in one contact goat indicated that-similar to BTV-26-at least one of three BTV-27 variants may be transmitted by contact between goats. In the field, BTV-27 RNA can be detected up to 6 months in the whole-blood of BTV-27-infected Corsican goats. In contrast, BTV RNA was not detected in the blood of cattle or sheep. In addition, BTV-27 Abs were not detected in cattle and only a transient increase in Ab levels was observed in some sheep. None of the 30 animals showed obvious BT-like clinical signs. In summary, the phenotypes observed for BTV-27v01-v03 phenotypes correspond to a mixture of characteristics known for BTV-25 and 26.

Requirements and comparative analysis of reverse genetics for bluetongue virus (BTV) and African horse sickness virus (AHSV)

BACKGROUND

Bluetongue virus (BTV) and African horse sickness virus (AHSV) are distinct arthropod borne virus species in the genus Orbivirus (Reoviridae family), causing the notifiable diseases Bluetongue and African horse sickness of ruminants and equids, respectively. Reverse genetics systems for these orbiviruses with their ten-segmented genome of double stranded RNA have been developed. Initially, two subsequent transfections of in vitro synthesized capped run-off RNA transcripts resulted in the recovery of BTV. Reverse genetics has been improved by transfection of expression plasmids followed by transfection of ten RNA transcripts. Recovery of AHSV was further improved by use of expression plasmids containing optimized open reading frames.

RESULTS

Plasmids containing full length cDNA of the 10 genome segments for T7 promoter-driven production of full length run-off RNA transcripts and expression plasmids with optimized open reading frames (ORFs) were used. BTV and AHSV were rescued using reverse genetics. The requirement of each expression plasmid and capping of RNA transcripts for reverse genetics were studied and compared for BTV and AHSV. BTV was recovered by transfection of VP1 and NS2 expression plasmids followed by transfection of a set of ten capped RNAs. VP3 expression plasmid was also required if uncapped RNAs were transfected. Recovery of AHSV required transfection of VP1, VP3 and NS2 expression plasmids followed by transfection of capped RNA transcripts. Plasmid-driven expression of VP4, 6 and 7 was also needed when uncapped RNA transcripts were used. Irrespective of capping of RNA transcripts, NS1 expression plasmid was not needed for recovery, although NS1 protein is essential for virus propagation. Improvement of reverse genetics for AHSV was clearly demonstrated by rescue of several mutants and reassortants that were not rescued with previous methods.

CONCLUSIONS

A limited number of expression plasmids is required for rescue of BTV or AHSV using reverse genetics, making the system much more versatile and generally applicable. Optimization of reverse genetics enlarge the possibilities to rescue virus mutants and reassortants, and will greatly benefit the control of these important diseases of livestock and companion animals.