Serotype-specific detection of African horsesickness virus by real-time PCR and the influence of genetic variations

Development and Validation of Three Triplex Real-Time RT-PCR Assays for Typing African Horse Sickness Virus: Utility for Disease Control and Other Laboratory Applications

The African horse sickness virus (AHSV) belongs to the Genus Orbivirus, family Sedoreoviridae, and nine serotypes of the virus have been described to date. The AHSV genome is composed of ten linear segments of double-stranded (ds) RNA, numbered in decreasing size order (Seg-1 to Seg-10). Genome segment 2 (Seg-2) encodes outer-capsid protein VP2, the most variable AHSV protein and the primary target for neutralizing antibodies. Consequently, Seg-2 determines the identity of the virus serotype. An African horse sickness (AHS) outbreak in an AHS-free status country requires identifying the serotype as soon as possible to implement a serotype-specific vaccination program. Considering that nowadays 'polyvalent live attenuated' is the only commercially available vaccination strategy to control the disease, field and vaccine strains of different serotypes could co-circulate. Additionally, in AHS-endemic countries, more than one serotype is often circulating at the same time. Therefore, a strategy to rapidly determine the virus serotype in an AHS-positive sample is strongly recommended in both epidemiological situations. The main objective of this study is to describe the development and validation of three triplex real-time RT-PCR (rRT-PCR) methods for rapid AHSV serotype detection. Samples from recent AHS outbreaks in Kenia (2015-2017), Thailand (2020), and Nigeria (2023), and from the AHS outbreak in Spain (1987-1990), were included in the study for the validation of these methods.

African Horse Sickness: A Review of Current Understanding and Vaccine Development

African horse sickness is a devastating disease that causes great suffering and many fatalities amongst horses in sub-Saharan Africa. It is caused by nine different serotypes of the orbivirus African horse sickness virus (AHSV) and it is spread by Culicoid midges. The disease has significant economic consequences for the equine industry both in southern Africa and increasingly further afield as the geographic distribution of the midge vector broadens with global warming and climate change. Live attenuated vaccines (LAV) have been used with relative success for many decades but carry the risk of reversion to virulence and/or genetic re-assortment between outbreak and vaccine strains. Furthermore, the vaccines lack DIVA capacity, the ability to distinguish between vaccine-induced immunity and that induced by natural infection. These concerns have motivated interest in the development of new, more favourable recombinant vaccines that utilize viral vectors or are based on reverse genetics or virus-like particle technologies. This review summarizes the current understanding of AHSV structure and the viral replication cycle and also evaluates existing and potential vaccine strategies that may be applied to prevent or control the disease.

Using a new serotype-specific Polymerase Chain Reaction (PCR) and sequencing to differentiate between field and vaccine-derived African Horse Sickness viruses submitted in 2016/2017

The outer capsid viral protein 2 (VP2) of African horse sickness virus, encoded by the most variable genome segment 2 (Seg-2), is the primary target for AHSV-specific neutralising antibodies and thus determines the virus serotype. Full length segment 2 sequences from more than 100 AHSVs isolated over the last 80 years were compared and single nucleotide polymorphisms (SNPs) identified between the reference strains and recent field viruses. Regions unique to each individual serotype were identified and primers designed to differentially amplify each of the nine serotypes. The sequences of resulting amplicons contained a significant amount of SNPs to discriminate between field viruses and reference strains or live attenuated viruses. The new serotype specific RT-PCR were subsequently used to determine the prevalence of different AHSV serotypes associated with samples submitted to the Agricultural Research Council - Onderstepoort Veterinary Research Institute during the 2016 / 2017 season. Subsequent sequencing of the PCR products were used to determine if the infections were caused by field or vaccine-derived strains. The serotypes of 70 AHSV positive diagnostic samples submitted to the ARC-OVR were determined. Serotypes 2 and 6 were the most prevalent, while Serotype 1 was the only serotype where sequences identical to the ALV or reference strains were detected in field samples. Based on this study, the incidence of vaccine-derived AHS infections submitted from southern Africa were low. This serotype-specific RT-PCR and sequencing assay could assist with the surveillance and control of equines movement nationally and internationally. It could also provide valuable scientific guidance on the policies and guidelines regulating vaccination and trade of equines in South Africa.

Sero-epidemioloical survey on African horse sickness virus among horses in Khartoum State, Central Sudan

BACKGROUND

African horse sickness virus (AHSV) is an infectious non contagious insect-transmitted double-stranded (ds) RNA orbivirus of the family Reoviridae. AHSV causes an often fatal hemorrhagic infection with high mortality among selected breeds of Arabian horses. This study was conducted to avail some information with regard to the prevalence and associated risk factors of AHSV among ecotype breeds of horses in central Sudan.

METHODS

Sera were collected from 320 horses, which were selected randomly from four localities and employed in the study. A competitive enzyme-linked immunosorbent assay (cELISA) was used to screen sampled sera for AHSV-specific immunoglobulin G (Ig G) antibodies.

RESULTS

Seropositivity to AHSV Ig G was detected in 275 out of the 320 horse sera, thus accounting for a prevalence rate of 85.9%. Potential risk factors to AHSV infection were reported to be associated with horse breed (OR = 5.0, CI = 0.07-2.104, p-value = 0.039) and activity of the horse (OR = 3.21, CI = 0.72-1.48, p- value = 0.008).

CONCLUSIONS

The high prevalence of AHSV in Khartoum State of Central Sudan necessitates the need for continuous surveillance for AHSV infection to prevent a possible disease outbreak in this region of the African continent.

Delineation of the population genetic structure of Culicoides imicola in East and South Africa

Background

Culicoides imicola Kieffer, 1913 is the main vector of bluetongue virus (BTV) and African horse sickness virus (AHSV) in Sub-Saharan Africa. Understanding the population genetic structure of this midge and the nature of barriers to gene flow will lead to a deeper understanding of bluetongue epidemiology and more effective vector control in this region.

Methods

A panel of 12 DNA microsatellite markers isolated de novo and mitochondrial DNA were utilized in a study of C. imicola populations from Africa and an outlier population from the Balearic Islands. The DNA microsatellite markers and mitochondrial DNA were also used to examine a population of closely related C. bolitinos Meiswinkel midges.

Results

The microsatellite data suggest gene flow between Kenya and south-west Indian Ocean Islands exist while a restricted gene flow between Kenya and South Africa C. imicola populations occurs. Genetic distance correlated with geographic distance by Mantel test. The mitochondrial DNA analysis results imply that the C. imicola populations from Kenya and south-west Indian Ocean Islands (Madagascar and Mauritius) shared haplotypes while C. imicola population from South Africa possessed private haplotypes and the highest nucleotide diversity among the African populations. The Bayesian skyline plot suggested a population growth.

Conclusions

The gene flow demonstrated by this study indicates a potential risk of introduction of new BTV serotypes by wind-borne infected Culicoides into the Islands. Genetic similarity between Mauritius and South Africa may be due to translocation as a result of human-induced activities; this could impact negatively on the livestock industry. The microsatellite markers isolated in this study may be utilised to study C. bolitinos, an important vector of BTV and AHSV in Africa and identify sources of future incursions.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-1277-4) contains supplementary material, which is available to authorized users.

Rapid molecular detection methods for arboviruses of livestock of importance to northern Europe

Arthropod-borne viruses (arboviruses) have been responsible for some of the most explosive epidemics of emerging infectious diseases over the past decade. Their impact on both human and livestock populations has been dramatic. The early detection either through surveillance or diagnosis of virus will be a critical feature in responding and resolving the emergence of such epidemics in the future. Although some of the most important emerging arboviruses are human pathogens, this paper aims to highlight those diseases that primarily affect livestock, although many are zoonotic and some occasionally cause human mortality. This paper also highlights the molecular detection methods specific to each virus and identifies those emerging diseases for which a rapid detection methods are not yet developed.

Serotype specific primers and gel-based RT-PCR assays for 'typing' African horse sickness virus: identification of strains from Africa

African horse sickness is a devastating, transboundary animal disease, that is ‘listed’ by the Office International des Epizooties (OIE). Although attenuated, inactivated and subunit vaccines have been developed for African horse sickness virus (AHSV), these are serotype-specific and their effective deployment therefore relies on rapid and reliable identification of virus type. AHSV serotype is controlled by the specificity of interactions between neutralising antibodies, and components of the outer-capsid, particularly protein VP2 (encoded by AHSV genome segment 2 (Seg-2)). We report the development and evaluation of novel gel based reverse transcription-PCR (RT–PCR) assays targeting AHSV Seg-2, which can be used to very significantly increase the speed and reliability of detection and identification (compared to virus neutralisation tests) of the nine serotypes of AHSV. Primer sets were designed targeting regions of Seg-2 that are conserved between strains within each of the AHSV serotype (types 1 to 9). These assays were evaluated using multiple AHSV strains from the orbivirus reference collection at IAH (www.reoviridae.org/dsRNA_virus_proteins/ReoID/AHSV-isolates.htm). In each case the Seg-2 primers showed a high level of specificity and failed to cross-amplify the most closely related heterologous AHSV types, or other related orbiviruses (such as bluetongue virus (BTV), or equine encephalosis virus (EEV)). The assays are rapid and sensitive, and can be used to detect and type viral RNA in blood, tissue samples, or cultivated viral suspensions within 24 h. They were used to identify AHSV strains from recent outbreaks in sub-Saharan African countries. These methods also generate cDNAs suitable for sequencing and phylogenetic analyses of Seg-2, identifying distinct virus lineages within each virus-type and helping to identify strain movements/origins. The RT-PCR methods described here provide a robust and versatile tool for rapid and specific detection and identification of AHSV serotypes 1 to 9.

PCR detection of African horse sickness virus serogroup based on genome segment three sequence analysis

A nested reverse transcriptase (RT) polymerase chain reaction (RT-PCR), for rapid detection of African horse sickness virus (AHSV) double-stranded ribonucleic acid (dsRNA) in cell culture and tissue samples, was developed and evaluated. Using an outer pair of primers (P1 and P2), selected from genome segment three of AHSV serotype 6 (AHSV-6), the RT-PCR-based assay resulted in amplification of a 890 base pair (bp) primary PCR product. RNAs from the nine vaccine strains of AHSV, and a number of AHSV field isolates including the Central African isolates of AHSV-9 and AHSV-6, propagated in cell cultures, were detected by this assay. A second pair of nested primers (P3 and P4) was used to produce a 240-bp PCR product. The RT-PCR described below detected as little as 0.1 fg of AHSV RNA, which is equivalent to six viral particles. The nested amplification confirmed the integrity of the primary PCR product and increased the sensitivity of the PCR assay by at least 1000-fold. Application of this RT-PCR assay to clinical samples resulted in direct detection of AHSV dsRNA from blood and a variety of tissue samples collected from equines infected experimentally and naturally. The specificity studies indicated that the primary or the nested PCR products were not amplified from, closely related orbiviruses including, bluetongue virus (BTV) prototypes serotypes 1, 2, 4, 10, 16 and 17; epizootic hemorrhagic disease of deer virus (EHDV) prototypes serotypes 1 and 2; EHDV-318, Sudanese isolates of palyam serogroup of orbiviruses; total nucleic acid extracts from uninfected Vero cells; or unfractionated blood from horses and donkeys that were AHSV-seronegative and virus isolation negative. The RT-PCR provides a valuable tool for study of the epidemiology of AHSV and can be recommended for rapid diagnosis during an outbreak of the disease among susceptible equines.