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

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.

Real time RT-PCR assays for detection and typing of African horse sickness virus

Although African horse sickness (AHS) can cause up to 95% mortality in horses, naïve animals can be protected by vaccination against the homologous AHSV serotype. Genome segment 2 (Seg-2) encodes outer capsid protein VP2, the most variable of the AHSV proteins. VP2 is also a primary target for AHSV specific neutralising antibodies, and consequently determines the identity of the nine AHSV serotypes. In contrast VP1 (the viral polymerase) and VP3 (the sub-core shell protein), encoded by Seg-1 and Seg-3 respectively, are highly conserved, representing virus species/orbivirus-serogroup-specific antigens. We report development and evaluation of real-time RT-PCR assays targeting AHSV Seg-1 or Seg-3, that can detect any AHSV type (virus species/serogroup-specific assays), as well as type-specific assays targeting Seg-2 of the nine AHSV serotypes. These assays were evaluated using isolates of different AHSV serotypes and other closely related orbiviruses, from the ‘Orbivirus Reference Collection’ (ORC) at The Pirbright Institute. The assays were shown to be AHSV virus-species-specific, or type-specific (as designed) and can be used for rapid, sensitive and reliable detection and identification (typing) of AHSV RNA in infected blood, tissue samples, homogenised Culicoides, or tissue culture supernatant. None of the assays amplified cDNAs from closely related heterologous orbiviruses, or from uninfected host animals or cell cultures.

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.

A new duplex real-time RT-PCR assay for sensitive and specific detection of African horse sickness virus

A new real-time reverse transcription-polymerase chain reaction (RT-PCR) assay for a simple and rapid diagnosis of African Horse Sickness (AHS) was developed. Primers and FAM-labeled TaqMan-MGB probes specific for African horse sickness virus (AHSV) were selected from the consensus sequence of the segment 8 of all 9 serotypes of AHSV reference strains. For the determination of the analytical sensitivity, an in vitro transcript (AHS_ns2T7) of the target region was constructed and tested. Furthermore, the AHS_ns2T7 transcript was used either as positive control or as a standard for quantifying target copies. A commercial heterologous Armored RNA was used as an internal positive control (IPC) for both RNA isolation and RT-PCR steps. The qRT-PCR AHS_ns2 was able to amplify the target sequence up to 0.71 copies/reaction. Its flexibility allowed to amplify a wide dynamic range of RNA copies from 1.5 to 0.001fg. Within this range, the Ct values varied from 18 to 38 cycles with SD values always lower than 0.5 confirming their strong and constant linear correlation with the RNA target. Furthermore the newly designed duplex real-time RT-PCR proved to be strictly AHSV-specific as it did not amplify close related viruses.

Development and optimisation of a duplex real-time reverse transcription quantitative PCR assay targeting the VP7 and NS2 genes of African horse sickness virus

Nucleotide sequences of 52 South African isolates of African horse sickness virus (AHSV) collected during 2004-2005 and including viruses of all nine AHSV serotypes, were used to design and develop a duplex real-time reverse transcription quantitative PCR (RT-PCR) assay targeting the VP7 (S8) and NS2 (S9) genes of AHSV. The assay was optimized for detection of AHSV in fresh and frozen blood of naturally infected horses. Assay performance was enhanced using random hexamers rather than gene-specific primers for RT, and with denaturation of double-stranded RNA in the presence of random hexamers. The assay was efficient with a linear range of at least five orders of magnitude. The analytical sensitivity of the assay was 132 copies of the target genes (4125 copies per ml of blood), and the assay was at least 10-fold more sensitive than virus isolation on BHK-21 cells. The assay was also highly specific because it did not detect related orbiviruses, such as bluetongue and equine encephalosis viruses.