Comparison of two adjuvants in an experimental Actinobacillus equuli vaccine in horses

Abstract

Actinobacillus equuli is a Gram-negative bacterium that causes sleepy foal disease in neonatal foals. Mortality rates for foals infected with A. equuli tend to be higher than foals infected with other bacteria. The A. equuli bacterium is an oral commensal of adult horses and dams may produce detectable amounts of serum antibodies to A. equuli. Mare antibodies may pass to foals through colostrum, providing passive protection against A. equuli. Vaccinating dams with bacterial outer membrane proteins (OMPs) and Actinobacillus equuli Aqx toxin elicited measurable antibody response to both OMPs and Aqx, but this previous experimental vaccination had undesirable long-lasting side effects including injection site swelling and fibrosis. These reactions were thought to be a result of the adjuvant selected. The current project will replace the previously-used adjuvant with squalene or double mutant Escherichia coli heat labile toxin (dmLT). Squalene has been used successfully in BHV-1 vaccines, which suggests it is a candidate for avoiding negative injection site reactions. For this study, we will use previously established procedures to isolate and purify OMPs and Aqx and combine the purified proteins with the selected adjuvant. The adjuvant-protein combination will be administered to mares and the immune response will be assessed.  Illness behavior and injection site reactions will also be evaluated.  Follow-on studies will evaluate successful candidate vaccines for colostral transfer to foals.