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There have been numerous reports of vaccinated horses that contract fatal African horse sickness due to African horse sickness virus (AHSV) serotypes that are included in the current commercial vaccine used in southern Africa, which emphasizes the importance of thorough characterization of the equine immune response to AHSV. In particular, there are concerns about possible interference between vaccine strains in the polyvalent vaccine which led us to hypothesise that the administration of individual AHSV serotypes could induce a better immunity to individual serotypes than that achieved with the current polyvalent vaccines. There is also little published information describing the half-life of maternally derived neutralising antibody in foals to the nine AHSV serotypes. This is important for revising and developing vaccination protocols for foals of vaccinated mares. Given the lethality of both natural and experimental AHSV infections in horses, several aspects of immunity induced by different types of AHSV vaccines were evaluated. The neutralising antibody response of horses (foals) immunized with a commercial modified live virus (MLV) AHSV vaccine was evaluated and compared to the immune response elicited to monovalent MLV AHSV serotypes. Foals were immunized with either the polyvalent AHSV vaccine, or one of four monovalent vaccines containing individual AHSV serotypes 1, 4, 7 and 8. There were marked differences in the immunogenicity of individual virus serotypes contained in the vaccine. Foals more consistently seroconverted to AHSV 1 and responses to other serotypes were highly variable, and often weak or not detected. The serotype-specific responses of foals given the monovalent MLV vaccines were similar to those of foals given the polyvalent preparation suggesting that there is no apparent enhanced immune response through the administration of a monovalent vaccine as opposed to the polyvalent vaccine. Furthermore, the immunogenicity of individual AHSV serotypes contained in the commercial MLV vaccine varies remarkably. Neutralising antibody titres to the 9 known serotypes of AHSV were determined in a cohort of brood mares that were regularly vaccinated with the MLV AHSV vaccine, and the passive transfer and rate of decay of maternal antibody to the individual virus serotypes in their foals were measured. Similar to the data obtained from immunized foals, there was marked variation in the neutralising antibody response of the mares to individual AHSV serotypes even after repeated vaccination. This was mirrored in the duration of maternally-derived antibodies in their respective foals. In an effort to further characterize cellular immune (CMI) responses to AHSV, the immunity in horses induced by an experimental canarypox virus vectored recombinant (ALVAC®-AHSV4) vaccine was characterised. The detection of VP2/VP5 specific IFN-γ responses was assessed by enzyme-linked immune spot (ELISpot) assay and clearly demonstrated that all ALVAC®-AHSV4 vaccinated horses developed significant IFN-γ production compared to unvaccinated horses. Flow cytometry demonstrated that this vaccine induced mainly CD8+ T-cells, able to recognize multiple T-cell epitopes throughout all of VP2 and only the N-terminus portion of VP5. In summary, the antibody and cellular response of horses to different AHSV vaccines was evaluated and compared. The results are relevant to the design of more efficacious AHSV vaccines and to identification of protective immunity in horses to this virus. / Thesis (PhD)--University of Pretoria, 2013. / gm2014 / Veterinary Tropical Diseases / unrestricted

This thesis comprises an introductory review of the literature, followed by reports of two experiments which are presented in the form of scientific papers. For this reason, there may be some repetition between chapters, particularly in terms of experimental procedure. To avoid unnecessary repetition, a single list of references is given at the end of the thesis. For the sake of completeness , several appendices are attached to Chapters Two and Three which would not ordinarily be included in a scientific paper. The literature review looks at the passive transfer of immunity from the mare to the foal, the consequences of failure of passive transfer of immunity and different methods of testing whether the transfer of passive immunity has occurred. The review concludes with a discussion of vaccination programmes against African horse sickness. Trial One evaluated different tests for determining whether the transfer of passive immunity from mare to foal has occurred in order to determine which of these tests should be used preferentially. A single radial immunodiffusion test was used as the reference standard. A series of samples .was taken from a group of foals and tested using four methods: single radial immunodiffusion , glutaraldehyde coagulation, zinc sulphate turbidity and protein refractometer tests. Trial Two explored African horse sickness vaccination programmes, focusing on when to vaccinate foals for the first time. A series of samples was taken from a group of foals from birth until two months after their second set of African horse sickness vaccinations (one year old). The samples were tested for the presence of African horse sickness antibodies for each of the nine serotypes to determine when maternal immunity fades and to evaluate the effect of each vaccination on the level of immunity. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2005.