Researchers in Pretoria have conducted a study and subsequent report of the complete genome sequences of plaque selected isolates of each of the 3 virus strains included in a South African commercial trivalent African horse sickness attenuated live virus vaccine.
Examining the genome
African horse sickness (AHS) is caused by AHS Virus (AHSV), the genome of which is comprised of 10 segments. In South Africa, a polyvalent AHS attenuated live virus (ALV) vaccine is manufactured by Onderstepoort Biological Products (OBP) Ltd. This vaccine is supplied in 2 separate vials each of which contains different AHSV serotypes: Bottle I is trivalent and contains serotypes 1, 3 and 4 while Bottle II is tetravalent (x 4) and contains serotypes 2, 6, 7 and 8 (2).
The research team studied the full genome sequences of AHSV1, AHSV3 and AHSV4 which were isolated from Bottle I of the AHS-ALV vaccine. The individual serotypes were independently isolated, and each was passaged between one and three times. AHSV dsRNA was extracted from virus-infected cells and sequencing templates were prepared.
The full genome sequences of the attenuated AHSV serotypes 1, and 4 viruses, which are precursors of the respective AHS-ALV virus strains, are available for public scrutiny from GenBank as accession numbers FJ183364-FJ183373 and KM820849-KM820858 respectively. The pairing nucleotide sequence identity of AHSV1-OBP vaccine and Attenuated serotype 1 was 99.643%, whilst that of AHSV4-OBP vaccine with Attenuated serotype 4 was 99.625%.
The full genome sequences of the AHSV serotype 3 from which AHSV-3-OBP are available for public scrutiny from GenBank as accession numbers KM886354-KM886363. The pairing identity between AHSV3-OBP and AHSV serotype 3, over nine of their ten genome segments was in excess of 99.7% whilst that for the segment carrying the gene encoding the highly conserved VP1 protein was appreciably lower at 94.86%. Further analysis of these sequences clearly showed that AHSV3-OBP is a reassortant comprised of nine genome segments likely derived from an AHSV serotype 3 ancestor closely related to the vaccine.
Comment : Having the sequences of the vaccine viruses means that when we do outbreak investigations, we can categorically determine whether we’re dealing with field virus, or a virus in which the vaccine is playing a role. These findings therefore provide the basis for investigations into any future outbreaks, and the potential role of vaccine viruses.
Note also that this addresses Bottle I only of the vaccine. Study is underway on Bottle II, the findings for which will be published in due course.
Equine Research Centre, Faculty of Veterinary Science, University of Pretoria – Alan J Guthrie, Carina W Lourens, Camilla T Weyer, Christopher Joone, Misha le Grange, Peter G Howell
Department of Veterinary Tropical Diseases, Faculty of Veterinary Science, University of Pretoria – Peter Coetzee, Estelle H Venter, N James MacLachlan
Source: Equine Research Centre