Bluetongue virus detection: a safer reverse-transcriptase polymerase chain reaction for prediction of viremia in sheep

Immuno-reactivity of recombinant non-structural protein 3 N-terminus (rNS3Nt) in indirect-ELISA for detection of bluetongue viral antibodies in serum samples

Bluetongue, an arthropod borne non-contagious disease of ruminants especially sheep, is caused by bluetongue virus (BTV). Detection of BTV antibodies in susceptible hosts is considered to be of significance in disease diagnosis and differentiation. In the present study, a partial NS3 gene encoding for non-structural protein-3 N-terminus (1MT117 aa) of BTV-23, produced as purified recombinant NS3Nt fusion protein (~32 kDa) using prokaryotic expression system (Escherichia coli), was evaluated as a candidate antigen in an indirect-ELISA (rNS3Nt-ELISA) to measure the serologic response to NS3 protein in small ruminants. The rNS3Nt fusion protein obtained in sufficient quantity and quality has good reactivity in detecting NS3 specific antibodies in field serum samples by indirect-ELISA. As NS3 protein is highly conserved, rNS3Nt-ELISA has potential for NS3 specific detection of antibodies in BTV affected animals irrespective of different viral serotypes. In comparison to structural protein (VP7) based c-ELISA kit and i-ELISA kit, the diagnostic sensitivity (85.1%, 86.2%) and specificity (92.5%, 93.2%) of rNS3Nt-ELISA were found to be relatively lower, respectively. Nevertheless, the study indicated the potential utility of rNS3Nt-ELISA as an alternate assay in routine sero-diagnosis of BTV infection and possible sero-surveillance of ruminants under DIVA strategy.

Assessment of listing and categorisation of animal diseases within the framework of the Animal Health Law (Regulation (EU) No 2016/429): bluetongue

A specific concept of strain was developed in order to classify the BTV serotypes ever reported in Europe based on their properties of animal health impact: the genotype, morbidity, mortality, speed of spread, period and geographical area of occurrence were considered as classification parameters. According to this methodology the strain groups identified were (i) the BTV strains belonging to serotypes BTV-1-24, (ii) some strains of serotypes BTV-16 and (iii) small ruminant-adapted strains belonging to serotypes BTV-25, -27, -30. Those strain groups were assessed according to the criteria of the Animal Health Law (AHL), in particular criteria of Article 7, Article 5 on the eligibility of bluetongue to be listed, Article 9 for the categorisation according to disease prevention and control rules as in Annex IV and Article 8 on the list of animal species related to bluetongue. The assessment has been performed following a methodology composed of information collection, expert judgement at individual and collective level. The output is composed of the categorical answer, and for the questions where no consensus was reached, the different supporting views are reported. The strain group BTV (1-24) can be considered eligible to be listed for Union intervention as laid down in Article 5(3) of the AHL, while the strain group BTV-25-30 and BTV-16 cannot. The strain group BTV-1-24 meets the criteria as in Sections 2 and 5 of Annex IV of the AHL, for the application of the disease prevention and control rules referred to in points (b) and (e) of Article 9(1) of the AHL. The animal species that can be considered to be listed for BTV-1-24 according to Article 8(3) are several species of Bovidae, Cervidae and Camelidae as susceptible species; domestic cattle, sheep and red deer as reservoir hosts, midges insect of genus Culicoides spp. as vector species.

High Throughput Detection of Bluetongue Virus by a New Real-Time Fluorogenic Reverse Transcription—Polymerase Chain Reaction: Application on Clinical Samples from Current Mediterranean Outbreaks

A real-time reverse transcription-polymerase chain reaction (RT-PCR) assay was developed for the detection of bluetongue virus (BTV) in blood samples. A combination of primers specific for a highly conserved region in RNA segment 5 (based on Mediterranean BTV sequences) and a DNA probe bound to 5′-Taq nuclease-3′ minor groove binder (TaqMan© MGB) was used to detect a range of isolates. This real-time RT-PCR assay could detect 5.4 × 10−3 tissue culture infectious doses (TCID50) of virus per milliliter of sample, which was comparable to our current BTV diagnostic nested RT-PCR assay. The assay detected all recent Mediterranean isolates (including serotypes 2, 4, and 16), BTV vaccine strains for serotypes 2 and 4, and 15 out of the 24 BTV reference strains available (all serotypes), but did not detect the related orbiviruses epizootic hemorrhagic disease and African horse sickness viruses. Following assay evaluation, the ability of this assay to identify BTV in recent isolates (2003, 2004) from ovine and bovine samples from an epizootic outbreak in Spain was also tested. Minor nucleotide changes (detected by sequencing viral genomes) within the probe-binding region were found to have a profound effect on virus detection. This assay has the benefits of being fast and simple, and the 96-well format enables large-scale epidemiological screening for BTV, especially when combined with a high-throughput nucleic acid extraction method.

First Report of Bluetongue Virus Serotype 1 Isolated from a White-Tailed Deer in the United States

In November 2004, tissues were collected from a hunter-killed white-tailed deer in St. Mary Parish, Louisiana. Bluetongue virus (BTV) was isolated from the tissues; however, the isolate could not be identified as any of the US domestic serotypes. Subsequent testing by virus neutralization using serotype-specific antiserum tentatively identified the isolate as BTV serotype 1 (BTV-1), which had not previously been found in the United States. Primers were designed based on the sequence of an outer capsid protein gene of a South African BTV-1 strain. Reverse transcription–polymerase chain reaction testing with the BTV-1 primers and product sequencing confirmed the Louisiana isolate as BTV-1. This is the first report of BTV-1 in the United States.

Diagnostic Tools for Bluetongue and Epizootic Hemorrhagic Disease Viruses Applicable to North American Veterinary Diagnosticians

This review provides an overview of current and potential new diagnostic tests for bluetongue (BT) and epizootic hemorrhagic disease (EHD) viruses compiled from international participants of the Orbivirus Gap Analysis Workshop, Diagnostic Group. The emphasis of this review is on diagnostic tools available to North American veterinary diagnosticians. Standard diagnostic tests are readily available for BT/EHD viruses, and there are described tests that are published in the World Organization for Animal Health (OIE) Terrestrial Manual. There is however considerable variation in the diagnostic approach to these viruses. Serological assays are well established, and many laboratories are experienced in running these assays. Numerous nucleic acid amplification assays are also available for BT virus (BTV) and EHD virus (EHDV). Although there is considerable experience with BTV reverse-transcriptase PCR (RT-PCR), there are no standards or comparisons of the protocols used by various state and federal veterinary diagnostic laboratories. Methods for genotyping BTV and EHDV isolates are available and are valuable tools for monitoring and analyzing circulating viruses. These methods include RT-PCR panels or arrays, RT-PCR and sequencing of specific genome segments, or the use of next-generation sequencing. In addition to enabling virus characterization, use of advanced molecular detection methods, including DNA microarrays and next-generation sequencing, significantly enhance the ability to detect unique virus strains that may arise through genetic drift, recombination, or viral genome segment reassortment, as well as incursions of new virus strains from other geographical areas.

Development and initial evaluation of a real-time RT-PCR assay to detect bluetongue virus genome segment 1

Since 1998, multiple strains of bluetongue virus (BTV), belonging to six different serotypes (types 1, 2, 4, 8, 9 and 16) have caused outbreaks of disease in Europe, causing one of the largest epizootics of bluetongue ever recorded, with the deaths of >1.8 million animals (mainly sheep). The persistence and continuing spread of BTV in Europe and elsewhere highlights the importance of sensitive and reliable diagnostic assay systems that can be used to rapidly identify infected animals, helping to combat spread of the virus and disease. BTV has a genome composed of 10 linear segments of dsRNA. We describe a real-time RT-PCR assay that targets the highly conserved genome segment 1 (encoding the viral polymerase--VP1) that can be used to detect all of the 24 serotypes, as well as geographic variants (different topotypes) within individual serotypes of BTV. After an initial evaluation using 132 BTV samples including representatives of all 24 BTV serotypes, this assay was used by the European Community Reference Laboratory (CRL) at IAH Pirbright to confirm the negative status of 2,255 animals imported to the UK from regions that were considered to be at risk during the 2006 outbreak of BTV-8 in Northern Europe. All of these animals were also negative by competition ELISA to detect BTV specific antibodies and none of them developed clinical signs of infection. These studies have demonstrated the value of the assay for the rapid screening of field samples.

An improved, high-throughput method for detection of bluetongue virus RNA in Culicoides midges utilizing infrared-dye-labeled primers for reverse transcriptase PCR

A new rapid (less than 6h from insect-to-results) high-throughput assay that is sensitive and specific for detecting BTV RNA in Culicoides biting midges is reported. Homogenization and extraction of nucleic acids from individual Culicoides specimens were performed in a 96-well plate format using specialized beads in a homogenization buffer compatible with cell culture and RNA extraction. A portion of homogenate (10%) from each specimen was retained for confirmatory infectious virus isolation, while the remaining 90% was used for RNA extraction. The RNA was used in a single step reverse transcriptase PCR (RT-PCR) reaction with infrared (IR)-dye-labeled primers. The RT-PCR products were visualized in agarose gels with an infrared scanner. The adaptation of IR-dye-labeled primers in combination with a one step RT-PCR resulted in a detection limit of 0.5 pfu of purified BTV RNA. All 24 serotypes of BTV prototype strains and none of the 8 serotypes of the closely related epizootic hemorrhagic disease virus (EHDV) prototype strains were detected.

Differentiation of Italian field and South African vaccine strains of bluetongue virus serotype 2 using real-time PCR

The current outbreaks of bluetongue (BT) disease in sheep in the central parts of the Mediterranean basin are being combated by extensive vaccination to control further spread of the virus and to suppress its long-term maintenance in the field. To be able to monitor the success of this campaign, and to be able to identify new foci of the disease, it is necessary to harness diagnostic methods, both rapid and sensitive, for differentiating reliably field from vaccine strains of bluetongue virus (BTV). A new method is described for their differentiation using fluorescence resonance energy transfer (FRET) probes with real-time PCR. The method is based on the principle that the melting temperature of a DNA duplex gives information about the sequence, and allows even double-base alterations in the amplicon to be identified. The RT-PCR, the generation of melting curves, and fluorescence detection were all performed using the LightCycler system (Roche Diagnostics, Mannheim, Germany).

Spatial distribution of bluetongue virus and its Culicoides vectors in Sicily

During the recent Mediterranean epizootic of bluetongue, an extensive programme of serological and vector (Culicoides biting midges (Diptera: Ceratopogonidae)) surveillance was carried out across Sicily. This paper presents the analysis of 911 light trap catches collected at the times of peak Culicoides abundance (summer to autumn 2000-2002) in 269 sites, in order to produce detailed maps of the spatial distribution of the main European vector, Culicoides imicola Kieffer and that of potential novel vectors. Whereas C. imicola was found at only 12% of sites, potential novel vectors, Culicoides obsoletus group Meigen, Culicoides pulicaris Linnaeus and Culicoides newsteadi Austen were present at over 50% of sites. However, the spatial distribution of C. imicola showed the closest correspondence to that of the 2000 and 2001 bluetongue (BT) outbreaks and its presence and abundance were significant predictors of the probability of an outbreak, suggesting that it was the main vector during these years. Although C. imicola may have played a role in transmission in several sites near Paternó, it was absent from the majority of sites at which outbreaks occurred in 2002 and from all sites in the province of Messina. All three potential novel vectors were widespread across sites at which outbreaks occurred during 2002. Of these, C. newsteadi was an unlikely candidate, as it was significantly less prevalent in outbreak vs. non-outbreak sites in Messina. It is hypothesized that the yearly distribution and intensity of outbreaks is directly attributable to the distribution and abundance of the vectors involved in transmission during each year. When C. imicola operated as the main vector in 2000 and 2001, outbreaks were few in number and were restricted to coastal regions due to low abundance and prevalence of this species. In 2002, it is hypothesized that BTV transmission was handed over to more prevalent and abundant novel vector species, leading to numerous and widespread outbreaks and probably to overwintering of the virus between 2001 and 2002. Based on catch ranges in outbreak vs. non-outbreak sites, it is tentatively suggested that nightly catches of 400 or more C. obsoletus and 150 or more C. pulicaris allow BTV transmission at a site, and provide a strategy for a fuller examination of the relationship between BTV transmission and the abundance and distribution of different vector species.

Detection of low copy numbers of HIV-1 proviral DNA in patient PBMCs by a high-input, sequence-capture PCR (Mega-PCR)

An internally controlled high-input PCR method, termed HIV-1 Mega-PCR was developed to lower the detection limit of HIV-1 DNA polymerase chain reaction (PCR) and to improve its value as a complementary diagnostic test. It is based on PCR amplification of two target sequences in the gag gene of HIV-1 following the selective capture of the targeted sequence and removal of unselected DNA from up to 500 microg of DNA. Efficient selection and amplification was monitored by inclusion of two mimic plasmids. The method was evaluated with buffy coat cells from healthy blood donors which were spiked with blood from 106 different HIV-1-infected individuals, and with 107 HIV-1 seronegative control buffy coats. All specimens from HIV-infected individuals were positive by a PCR protocol using 1 microg of patient DNA. Amplification of 1 microg DNA of the 106 spiked, diluted samples resulted in 68 double positive, 14 single positive, and 24 double negative reactions. In the Mega-PCR, the average input was 260 +/- 84 microg DNA containing an estimated 1.1 +/- 0.6% of spiked patient DNA. Of the 106 samples tested by Mega-PCR, 102 were positive and three negative. One failed to select the mimic plasmid. Among the 107 negative buffy coat controls, none was false-positive and four exhibited a failure of the internal reaction control. Application of HIV-1 Mega-PCR to clinical specimens from seroreverting newborns of HIV-infected mothers and seroindeterminate, PCR-negative specimens revealed no indication for HIV infection, whereas three samples from confirmed, HIV-1-infected but PCR negative individuals showed evidence of the presence of HIV-1 DNA. Mega-PCR lowers the detection limit of an individual analysis to approximately 0.01 HIV-1 DNA copies/microg of applied DNA and may help to confirm or exclude HIV-1-infection in difficult situations diagnostic.

Identification of a novel bluetongue virus vector species of Culicoides in Sicily

The vectors of bluetongue virus are certain species of Culicoides biting midges, and in the Mediterranean area Culicoides imicola has long been considered to be the only field vector. In Sicily an entomological and serological surveillance programme has been in operation since the autumn of 2000, which has shown that the prevalence and abundance of C. imicola is lower than in many other Italian regions. Moreover, in 2002, there were outbreaks of bluetongue in the absence of C. imicola, and in these regions bluetongue viral RNA was detected by means of a nested reverse-transcriptase PCR in wild-caught, non-blood-engorged, parous Culicoides pulicaris. Furthermore, bluetongue virus serotype 2 was isolated on five occasions from extracts of non-blood-engorged parous C. pulicaris by using embryonated hens eggs and BHK-21 cells as assay systems. These findings suggest that in parts of Italy and possibly in other areas of Europe, where C. imicola is absent or rare, C. pulicaris may act as a fully competent vector of bluetongue virus.

Culicoides mohave (Diptera: Ceratopogonidae): new occurrence records and potential role in transmission of hemorrhagic disease

Biting midges of the genus Culicoides are important in the transmission of viral diseases affecting wild and domestic ungulates, including bluetongue (BLU) and epizootic hemorrhagic disease (EHD). The primary known vector for these viruses is C. sonorensis Wirth & Jones, however, it has been speculated that other species of Culicoides may also be involved. One potential candidate is C. mohave, a poorly studied species found in inland desert areas of the southwestern United States. In 2000 and 2001, we collected C. mohave and C. sonorensis at six sites in a previously unsurveyed area in the Sonoran Desert of southwestern Arizona and used PCR to detect nucleic acids associated with BLU and EHD viruses. C. mohave was abundant at two low-elevation sites on the study area, but uncommon or absent elsewhere. C. sonorensis commonly occurred along with C. mohave at one site, but was much less abundant. All C. mohave pools were negative for BLU viral RNA, however, 35% yielded positive results for EHD. All C. sonorensis were negative for both BLU and EHD. Our results suggest that C. mohave is a potential vector of EHD virus in this area, however additional studies are needed to determine its ability to transmit EHD.

Bluetongue virus diagnosis of clinical cases by a duplex reverse transcription-PCR: a comparison with conventional methods

A duplex reverse transcription polymerase chain reaction (RT-PCR) assay for the detection of bluetongue virus (BTV) in clinical samples was developed. This assay, which detects the highly conserved S10 region of BTV, was assessed for sensitivity and application as a rapid and dependable diagnostic tool by comparison with standard assays of virus detection, such as virus isolation in embryonated chicken eggs and cell culture. Simultaneous detection of BTV and host beta-actin RNAs minimizes the possibility of false negative results. The sensitivity of the assay was found to be equal to five cell culture infectious dose (CCID(50)) units and its specificity was confirmed as no RT-PCR product was detected with RNAs from two closely related orbiviruses, i.e. epizootic haemorrhagic disease virus (serotypes 1, 2 and 318) and African horse sickness virus, serotype 9, or RNAs from uninfected BHK-21 cells and blood samples from uninfected sheep or goats. In this study, 36 blood samples from naturally infected mixed flocks of sheep and goats were examined. Seventeen animals were identified as BTV-positive by RT-PCR, whereas only 13 were found positive by virus isolation in embryonated chicken eggs and nine by cell culture assays. These results indicate that the duplex RT-PCR could be a useful technique for monitoring BTV infection in the field.

Laboratory and field studies of an antigen capture ELISA for bluetongue virus

An improved bluetongue antigen capture ELISA (BTACE) technique was evaluated for its ability to detect the full range of 24 bluetongue (BLU) serotypes. The BTACE detected all 24 serotypes in cell culture fluids, including eight serotypes where the representative strains originated from both Australia and also from the South African reference collection. The amount of infectious virus required to obtain a positive BTACE result varied between 100-1000 TCID50. This was approximately 10-fold more sensitive than the antigen capture test described previously (Hosseini, M., Hawkes, R.A., Kirkland, P.D., Dixon, R., 1998. J. Virol. Methods 75, 39-46.). The BTACE method was compared with conventional passage in cell culture to detect the presence of virus in the tissues of embryonated chicken eggs (ECEs) which had been inoculated intravenously with the blood of sheep and cattle infected experimentally with the eight Australian serotypes of BLU (1, 3, 9, 15, 16, 20, 21, and 23). The BTACE method was at least as sensitive as the conventional cell culture detecting virus in ECEs, obviating the need for prolonged cell culture passage to detect the virus. A comparison of the amount of antigen detected in different embryo tissues indicated that liver homogenates gave the highest positive to negative ratios in the BTACE and were selected as the specimen of choice. In studies of sheep infected with all 24 South African reference BLU serotypes this new BTACE was able to detect viraemia with all serotypes. Finally, the BTACE was validated in surveillance programs for BLU in both New South Wales, Australia and in Yunnan Province, People's Republic of China. Blood samples from sentinel cattle were inoculated into ECEs. Homogenised ECE livers were tested by BTACE and those positive were passaged subsequently in cell culture for virus isolation and identification. This protocol led to the efficient isolation of field isolates of many serotypes. The high sensitivity and broad reactivity of the method indicates that it should be valuable for BLU diagnosis and surveillance programs.

The effects of pharmacological and lentivirus-induced immune suppression on orbivirus pathogenesis: assessment of virus burden in blood monocytes and tissues by reverse transcription-in situ PCR

We investigated the effects of pharmacological and lentivirus-induced immunosuppression on bluetongue virus (BTV) pathogenesis as a mechanism for virus persistence and induction of clinical disease. Immunologically normal and immunosuppressed sheep were infected subcutaneously with BTV serotype 3 (BTV-3), a foreign isolate with unknown pathogenicity in North American livestock, and with North American serotype 11 (BTV-11). Erythrocyte-associated BTV RNA was detected earlier and at greater concentrations in sheep treated with immunosuppressive drugs. Similarly, viral RNA and infectious virus were detected in blood monocytes earlier and at higher frequency in immunosuppressed animals: as many as 1 in 970 monocytes revealed BTV RNA at peak viremia, compared to <1 in 10(5) monocytes from immunocompetent sheep. Animals infected with BTV-3 had a higher virus burden in monocytes and lesions of greater severity than those infected with BTV-11. BTV RNA was detected by in situ hybridization in vascular endothelial cells and cells of monocyte lineage, but only in tissues from immunocompromised animals, and was most abundant in animals infected with BTV-3. In contrast, reverse transcription-in situ PCR showed BTV RNA from both viral serotypes in high numbers of tissue leukocytes and vascular endothelial cells from both immunosuppressed and, to a lesser extent, immunocompetent animals. Collectively, these findings show that BTV infection is widely distributed during acute infection but replication is highly restricted in animals with normal immunity. These findings also suggest that in addition to virulence factors that define viral serotypes, immunosuppression could play a role in the natural history of orbivirus infection, allowing for higher virus burden, increased virus persistence, and greater potential for acquisition of virus by the arthropod vector.