Evaluation of the pathogenicity of African Horsesickness (AHS) isolates in vaccinated animals.

BACKGROUND: The polyvalent African Horsesickness (AHS) attenuated live vaccine (ALV) produced by Onderstepoort Biological Products (OBP) Ltd., South Africa, has been associated with some safety concerns and alleged cases of vaccine failure or vaccine-induced disease. The risk of reassortment and reversion to virulence is a common concern associated with the use of ALVs, and a phenomenon reported for viruses with segmented RNA genomes. The purpose of this study was to determine whether or not reassortment of AHS vaccine strains could result in reassortants and reversion to virulence and therefore cause AHS in susceptible horses.
METHODS: Clinical or field isolates of AHS were obtained from horses with AHS symptoms or disease post vaccination. AHS-naïve horses were inoculated with these isolates and monitored for clinical reactions. Laboratory tests were performed at intervals to determine immune responses and viraemia. Viral RNA extraction and complete genome amplification of monovalent AHS-ALV vaccine strains and isolates collected post-vaccination was conducted. cDNA of the genome segments were run on PAGE to determine mobility patterns and genome segments 2, 3, 4, 5 and 6 sequenced for phylogenetic analysis.
RESULTS: No clinical symptoms typical of AHS were observed in inoculated horses and all showed a good immune response. A comparison of mobility patterns of the amplified cDNA genome on PAGE allowed the identification and differentiation of reassortants, which were confirmed by sequence and phylogenetic analysis of the nucleotide sequences.
CONCLUSION: This study, however, showed no indications that vaccine reassortants were pathogenic or lethal after inoculation in susceptible horses. Assumptions of virulence or reversion to virulence of vaccine reassortants post-vaccination in horses could not be substantiated.