The effect of antibiotic treatment on Haemophilus parasuis colonization, disease and immunity

Abstract

Glasser's disease is an important source of economic losses in commercial swine production systems. The success of controlled exposure in the field suggests that early exposure (colonization) with pathogenic H. parasuis protects pigs from Glasser's disease. However, little is known about the immune response that is generated during colonization and how antimicrobials can modulate the immune response to H. parasuis. The overall goal of this PhD dissertation was to investigate the influence of antimicrobial treatment on H. parasuis colonization and infection, and its effect on the development of immune responses against H. parasuis in swine. First, a species-specific ELISA assay and an experimental model of H. parasuis colonization were developed to study the immune response to H. parasuis colonization by a pathogenic strain in conventional pigs. Moreover, the effect of the antimicrobial enrofloxacin in reducing H. parasuis colonization in weaned pigs was assessed. It was demonstrated that enrofloxacin was able to affect H. parasuis colonization by temporarily reducing the bacterial load in the upper respiratory tract. However, enrofloxacin was unable to eradicate the organism. Then, it was demonstrated that the inoculation of pigs with a low dose of pathogenic H. parasuis protected pigs against secondary infection. Most importantly, the timing of enrofloxacin administration in relation to H. parasuis exposure was relevant to develop protection to a subsequent H. parasuis challenge. The protection observed after H. parasuis inoculation was not affected by enrofloxacin given 3 days before inoculation. In contrast, enrofloxacin given 3 days after inoculation with H. parasuis interfered with protection against challenge. Based on the lack of H. parasuis isolation after enrofloxacin treatment, it appeared that the antibiotic quickly inactivated and removed H. parasuis. For the given infectious dose of pathogenic H. parasuis used, more than 3 days of live bacteria exposure is necessary for pigs to seroconvert and be protected against reinfection. Enrofloxacin-induced abridgement of infection on day 3 post inoculation resulted in the absence of serum IgG response and protection.