Development of a Novel Reverse Transcription Loop-Mediated Isothermal Amplification Assay for the Rapid Detection of African Horse Sickness Virus

African horse sickness (AHS) is a disease of equids caused by African Horse Sickness Virus (AHSV) and is transmitted by Culicoides midges. AHS is endemic in sub‐Saharan Africa, but during the past century, outbreaks of significant economic importance and elevated mortality have been recorded in Northern African countries, the Iberian and Arabian Peninsula, the Middle East and the Indian subcontinent. Effective control combines the application of early warning systems, accurate laboratory diagnosis and reporting, animal movement restrictions, suitable vaccination and surveillance programs, and the coordination of all these measures by efficient veterinary services. Conventional reverse‐transcriptase (RT) PCR (RT‐PCR) and real‐time RT‐PCR (rRT‐PCR) assays have improved the sensitivity and rapidity of diagnosing AHS, resulting in the adoption of these methods as recommended tests by the World Organisation for Animal Health (OIE). However, currently these assays are only performed within laboratory settings; therefore, the development of field diagnostics for AHS would improve the fast implementation of control policies.

Loop‐mediated isothermal amplification (LAMP) is an isothermal, autocycling, strand‐displacement nucleic acid amplification technique which can be performed in the field. LAMP assays are attractive molecular assays because they are simple to use, rapid, portable and have sensitivity and specificity within the range of rRT‐PCR. This study describes the development of a novel RT‐LAMP assay for the detection of AHSV. The AHSV RT‐LAMP assay has an analytical sensitivity of 96.1% when considering an rRT‐PCR cut‐off value of C_T > 36, or 91.3% when no rRT‐PCR cut‐off is applied. Diagnostic sensitivity and specificity were 100%. This assay provides for a rapid and low cost AHS diagnostic for use in the field.

Molecular analysis of the 2011 HPAI H5N2 outbreak in ostriches, South Africa

The third outbreak of highly pathogenic avian influenza (HPAI) H5N2 in less than seven years affected ostriches of South Africa's Western Cape during 2011. Twenty farms tested PCR positive for the presence of HPAI H5N2 between March and November 2011. Three HPAI H5N2 (AI2114, AI2214, AI2512) and 1 H1N2 (AI2887) viruses were isolated during this period, but H6N2 and H1N2 infections of ostriches were also confirmed by PCR. HPAI H5N2 isolate AI2114 produced an intravenous pathogenicity index (IVPI) score of 1.37 in chickens whereas isolate AI2214 produced an IVPI score of 0.8. The former virus had an additional, predicted N-linked glycosylation site at position 88 of the hemagglutinin protein as well as an E627K mutation in the PB2 protein that was lacking from AI2214. Four variations at HA0 were detected in the PCR-positive cases. Phylogenetically, the branching order of outbreak strains indicated a lack of reassortment between outbreak strains that implied a single outbreak source and a wild duck origin for the progenitor outbreak strain. The 2011 outbreak strains had no genetic relationships to the previous 2004 and 2006 HPAI H5N2 outbreak viruses. Molecular clock analysis based on the N2 neuraminidase genes estimated a recent common ancestor for the outbreak tentatively dated at September 2010. Deep sequencing results of 16 clinical PCR-positive samples yielded data in the range of 573 to 12,590 base pairs (bp), with an average of 4468 bp of total genomic sequence recovered per sample. This data was used to confirm the lack ofreassortment and to assign samples into one of two epidemiologic groups to support epidemiologic tracing of the spread of the outbreak. One farm (no. 142), thought to have played a major epidemiologic role in the outbreak, was confirmed by deep sequencing to contain a mix of both epidemiologic virus groups.

Characterization of pigeon paramyxoviruses (Newcastle disease virus) isolated in South Africa from 2001 to 2006

Pigeon paramyxovirus type 1 (PPMV-1), a variant of Newcastle disease virus that primarily affects doves and pigeons has been isolated in South Africa since the mid-1980s. Phylogenetic evidence indicates that pigeon paramyxovirus type 1 viruses were introduced into South Africa on multiple occasions, based on the presence of two separate lineages, 4bi and 4bii, that have been circulating in Europe and the Far East since the early 1990s. During 2006, a PPMV-1 virus was isolated from an African ground hornbill (Bucorvus leadbeateri) which became acutely infected with PPMV-1 and died, probably after scavenging off infected dove carcasses in the region, since a closely-related PPMV-1 strain was also isolated from doves collected nearby. The hornbill isolate had ICPI and MDT values characteristic of PPMV-1 strains. The threat of PPMV-1 to poultry production and biodiversity in southern Africa highlights the importance of monitoring the spread of this strain.

Discrimination between sheep-associated and wildebeest-associated malignant catarrhal fever virus by means of a single-tube duplex nested PCR

A single-tube duplex nested polymerase chain reaction (sdn-PCR) was developed for the detection of and discrimination between ovine herpesvirus-2 (OvHV-2) and alcelaphine herpesvirus-1 (AIHV-1). These viruses respectively cause sheep- and wildebeest-associated malignant catarrhal fever (SA-MCF and WA-MCF). In the first step of the sdn-PCR, two primers with high annealing temperatures based on conserved regions of the tegument genes were used for DNA amplification. In the second step, two primer sets based on variable regions of the respective OvHV-2 and AIHV-1 genes and with annealing temperatures > 11 degrees C below the primers used in the first step, were used. Internal regions of different sizes from amplicons produced in the first step were amplified. This single-tube test obviates the need for two separate assays to detect both viral types, thereby reducing time, labour and cost.

Phylogenetic analyses of genes from South African LPAI viruses isolated in 2004 from wild aquatic birds suggests introduction by Eurasian migrants

In 2004, South Africa experienced its first recorded outbreak of a highly pathogenic notifiable avian influenza (HPNAI) viral strain of the H5N2 subtype in ostriches in the Eastern Cape province. The traditional ostrich-farming areas in the Western Cape province report almost yearly outbreaks of low pathogenicity avian influenza (LPAI) in ostriches, which is attributed to introduction by wild birds and certain climatic patterns. During the winter of 2004, LPAI H3N8, H4N8, H5N2 and H5N1 avian influenza viruses were isolated from wild aquatic birds. All eight genes of the H3N8, H4N8 and H5N1 viruses were analysed. The results show that the H5N1 virus does not belong to the HPAI Z/Z+N genotype currently circulating in Asia, but that the most recent common ancestors are Russian H5N2 and H5N3 viruses. The N1 gene lacks the stalk deletion associated with virulence. Internal genes probably originate from a pool containing Chinese, Middle Eastern and Italian viruses. The South African H3N8 and H4N8 viruses appear to have derived their genes from an ecosystem where Asian H5N1, H6N9 and H9N2, Russian H4, and Danish H3N8 viruses have been circulating since 1997. All three viruses share recent nucleoprotein common ancestors with the German and Dutch HPNAI H7N7 viruses from 2003. The diverse pool of genes from which local viruses are derived suggests that reassortment occurred at the Siberian breeding grounds where migratory paths cross, or within the South African ecosystem. This data highlights the importance of surveillance in aquatic migratory birds, particularly members of the Charadriidae, for their potential roles in the introduction of avian diseases to South African poultry and especially ostriches in the case of avian influenza.

Discrimination between sheep-associated and wildebeest-associated malignant catarrhal fever virus by means of a single-tube duplex nested PCR

A single-tube duplex nested polymerase chain reaction (sdn-PCR) was developed for the detection of and discrimination between ovine herpesvirus-2 (OvHV-2) and alcelaphine herpesvirus-1 (AlHV-1). These viruses respectively cause sheep- and wildebeest-associated malignant catarrhal fever (SAMCF and WA-MCF). In the first step of the sdn-PCR, two primers with high annealing temperatures based on conserved regions of the tegument genes were used for DNA amplification. In the second step, two primer sets based on variable regions of the respective OvHV-2 and AlHV-1 genes and with annealing temperatures > 11 °C below the primers used in the first step, were used. Internal regions of different sizes from amplicons produced in the first step were amplified. This single-tube test obviates the need for two separate assays to detect both viral types, thereby reducing time, labour and cost.

In ovoinhibition of fowlpoxvirus replication by a gall extract fromGuiera senegalensis

Several field isolates of fowlpoxvirus (FPV) from Burkina Faso, West Africa, were isolated and partly evaluated by molecular analysis. In addition, the in ovo antiviral activity against FPV of a gall extract from Guiera senegalensis was determined. Three viral isolates were obtained from suspected fowlpox cases after passage in embryonating chicken eggs and their poxviral identity confirmed by electron microscopy. All isolates were found to be pathogenic for chicks and all grew well in cell culture. Polymerase chain reaction and sequencing of amplicons revealed sequences identical with those of other FPV strains. The most studied isolate was then employed for use in an antiviral assay. An aqueous acetone extract from the galls of G. senegalensis was found to inhibit both virus-induced pock formation and to reduce viral titre in embryonating chicken eggs. The suggested mechanism of action is the activation of the alternative complement pathway and the inhibition of FPV-induced cholesterogenesis in ovo by constituents of the gall extract.

A phylogenetic study of South African Newcastle disease virus strains isolated between 1990 and 2002 suggests epidemiological origins in the Far East

Genetic comparisons were made of the fusion protein sequences of 155 Newcastle disease virus isolates collected in South Africa between 1990 and 2002. Their evolutionary relationships and origins are described. All of the lentogenic field isolates were shown to be derived from commercial vaccines. No true South African lentogenic wild type strain was identified. Furthermore, it was shown that almost all mesogenic isolates had avirulent F(0) cleavage site sequences. Three major epizootics occurred in South Africa during the period of this study. The first outbreak (1990/1991) was caused by viruses endemic to South Africa since the 1960's (genotype VIII) but were occasionally also isolated in 2000. Genotype VIIb viruses, implicated in the severe outbreaks during 1993/1994, persisted until 1999. Genotype VIId viruses, responsible for the most recent outbreak in 1999/2000, had their origins in the Far East like those of the two previous outbreaks.

PCR-based detection of the transovarial transmission of Uruguayan Babesia bovis and Babesia bigemina vaccine strains

Bovine babesiosis is responsible for serious economic losses in Uruguay. Haemovaccines play an important role in disease prevention, but concern has been raised about their use. It is feared that the attenuated Babesia bovis and Babesia bigemina vaccine strains may be transmitted by the local tick vector Boophilus microplus, and that reversion to virulence could occur. We therefore investigated the possibility that these strains could be transmitted via the transovarial route in ticks using a Babesia species-specific polymerase chain reaction (PCR) assay. DNA was extracted from the developmental stages of the tick vector that had fed on calves immunized with the haemovaccine. It was possible to detect Babesia DNA not only in adult ticks, but also in their eggs and larvae. In addition, it was shown that calves infested with larvae derived from eggs laid by ticks fed on acutely infected calves, were positive for Babesia using PCR. Caution should therefore be shown with the distribution of the haemovaccine in marginal areas. It is still advisable that suitable tick control measures be used to prevent transovarial transmission and the potential risk of attenuated Babesia reverting to virulence.

Development of a diagnostic one-tube RT-PCR for the detection of Rift Valley fever virus

Diagnosis of Rift Valley fever (RVF) is based on serology and virus isolation. The disadvantages of the former include poor sensitivity, high cost, risks associated with using infectious virus as antigen, the lengthy duration of ELISA as well as cross-reactivity with other Phleboviruses. We developed, optimised and evaluated a one-tube reverse-transcription-polymerase chain reaction (RT-PCR) for the detection of Rift Valley fever virus (RVFV) in ruminants. The PCR primers for this assay were designed to anneal to a region within the M segment of the virus genome, encoding glycoproteins G1 and G2. A PCR amplicon of 363 bp was obtained. The sensitivity of the assay was determined to be 0.25 TCID50. This test should allow for the early and rapid detection of RVFV in both serum and whole blood. In addition, it could facilitate the quantification of antigen for the manufacture of current vaccines.

Eliciting antigen-specific egg-yolk IgY with naked DNA

Immunization with naked DNA was used to elicit chicken egg yolk antibodies (IgY). Layer hens were inoculated with plasmid DNA encoding the enhanced green fluorescent protein, the fusion protein of Newcastle disease virus, and VP2 of African horse sickness virus. IgY was extracted from egg yolks by polyethylene glycol precipitation. Specific antibodies were present in the yolks of eggs from hens immunized with each of the three different plasmids. This approach to raising polyclonal antibodies obviates the need to produce and purify large quantities of proteins for immunization and can potentially yield large amounts of diagnostically or therapeutically useful reagents.

Immune responses in a horse inoculated with the VP2 gene of African horsesickness virus

The ability of a DNA vaccine to elicit an immune response in a horse was evaluated. The outer capsid protein VP2 of African horsesickness virus is known to elicit protective immunity in horses. Reverse transcribed DNA of the gene encoding VP2 was placed under the transcriptional control of the cytomegalovirus immediate-early enhancer/promoter and was injected on several occasions intramuscularly into a horse. Low antibody levels could be detected by ELISA. Antibodies directed against VP2 alone were shown by Western blot while low levels of neutralizing antibodies were detected by a 50% plaque reduction assay. In contrast to a relatively poor humoral response, a significant lymphoproliferative response in the presence of whole virus proteins, as well as a cytotoxic cellular reaction against virus-infected syngeneic target cells was shown.

The use of chicken IgY in a double antibody sandwich ELISA for detecting African horsesickness virus

An indirect sandwich ELISA that can detect as little as 8 ng of African horsesickness virus (AHSV) was developed. Viral antigen was captured from suspension using an immobilized monoclonal antibody specific for an epitope on VP7, a protein that is a major constituent of the virus core. Egg-yolk derived chicken IgY directed against AHSV (serotype 3) was used as the secondary antibody. Since IgY and mouse IgG do not cross-react serologically, the secondary antibody was not labelled, but was instead detected with enzyme-coupled sheep antibodies directed against avian immunoglobulins. The assay recognized all nine AHSV serotypes, but not the Cascara isolate of equine encephalosis virus, a related orbivirus that also infects horses. In addition to being able to detect and quantify whole AHSV, the ELISA could show the presence of VP7 produced by recombinant baculoviruses.

Identification of an antigenic peptide specific for bluetongue virus using phage display expression of NS1 sequences

BACKGROUND

NS1 is a non-structural protein associated with the replication of bluetongue virus (BTV), an orbivirus (Reoviridae) that infects sheep and cattle. NS1 is potentially useful as a diagnostic antigen since the presence of specific antibodies is an indication that the virus has replicated in the host. It is, however, not antigenically unique and cross-reacts serologically with the analogous protein of the related epizootic haemorrhagic disease virus (EHDV).

OBJECTIVES

To investigate phage display of peptides derived from the gene encoding NS1 as a way of identifying unique antigenic regions that can be mimicked by synthetic peptides.

STUDY DESIGN

A cDNA clone of a large portion of the gene encoding NS1 of bluetongue virus was fragmented by partial DNase digestion. The fragments were ligated into the filamentous phage display vector, fUSE 2. Peptides expressed on the surface of the phages as part of the gene III proteins were selected from the library using antibodies affinity-purified from an antiserum to NS1. The peptides were identified by sequencing the phage DNA and alignment with the sequence of the target gene.

RESULTS

Two antigenic regions were identified, one of which could be effectively mimicked by a 28 residue synthetic peptide. This peptide did not cross-react with an antiserum directed against NS1 of EHDV.

CONCLUSION

The strategy of screening gene-derived phage display libraries with antibodies from an immune serum is expected to be useful in the development of highly specific peptide-based diagnostic assays.