Antigenic Variation in Virulence Determinants of Streptococcus zooepidemicus and Actinobacillus equuli Involved in Lower Airway Disease of the Horse and Strategies Towards Protective Immunisation

Ward, Chantelle Louise (1998). Antigenic Variation in Virulence Determinants of Streptococcus zooepidemicus and Actinobacillus equuli Involved in Lower Airway Disease of the Horse and Strategies Towards Protective Immunisation. PhD thesis The Open University.

Abstract

Lower airway disease (LAD) of the training Thoroughbred horse is characterised by poor performance and excess mucus and pus in the trachea. This condition, although often referred to as ‘the virus’, is significantly associated with Streptococcus zooepidemicus and Actinobacillus/Pasteurella species without evidence of viral infection.

The degree of antigenic variability in these bacteria was assessed, to judge the number of strains that might be required in future vaccines. Particular attention was paid to the putative virulence determinants of the M-like protein and capsule of S. zooepidemicus and the iron regulated proteins (IRPs), produced only in low available iron concentrations, of Actinobacillus equuli.

A range of Actinobacillus/Pasteurella species were isolated from the lower airway of horses, but more than half were accounted for by A.equuli. A. equuli bound and utilised equine holotransferrin for growth. The constitutive proteins of A. equuli were antigenically cross-reactive, but IRPs were more antigenically varied. Vaccines based on interference of IRPs would be likely to need more than one strain of A. equuli.

Hot acid extracts of S. zooepidemicus were antigenically cross-reactive, but there was less opsonogenic cross-reactivity in horse blood containing natural antibodies than rabbit antisera to extracts. A vaccine based on M-like protein would be likely to need more than one type of S. zooepidemicus, but no relationship could be found between ribotype / antigenic profile and opsonogenic behaviour, suggesting that M-like protein is not necessarily the principal anti-phagocytic mechanism for these isolates, at least in vitro.

S. zooepidemicus produces a hyaluronate capsule thought to be a virulence determinant, which is tightly bound by a protein (HAP). Immunisation with recombinant HAP significantly increased survival time and clearance of bacteria in mice challenged intraperitoneally with S. zooepidemicus. Recombinant HAP may have acted as a surrogate antigen for the capsule.

Introduction of bacteria into ponies intratracheally produced a dose response for S. zooepidemicus and A. equuli. However, so much intercurrent infection developed (due to S. zooepidemicus and Bordetella bronchiseptica) it was concluded that conventional ponies would be unsuitable for straightforward studies of vaccine efficacy in response to experimental challenge. Alternative ways forward are discussed.

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