Lawsonia intracellularis is an obligate intracellular bacterium and the causative agent of proliferative enteropathy (PE) in pigs [1] and [2]. A commercial vaccine is available, and field trials have shown reduced clinical disease, increased daily weight gain and reduction in antibiotics consumption [3], [4] and [5]. However, neither disease nor transmission of infection is prevented by vaccination, and thus L. intracellularis remains a major problem in the swine industry [6].
The immunological effect of the vaccine still remains to be elucidated. Guedes et al. [7] detected specific serum IgG from 5 to 13 weeks and specific IFN-γ response (measured by ELISPOT) 4–13 weeks post oral vaccination. Kroll et al. [8] found fewer lesions in the intestine, reduced faecal shedding and higher average daily weight gain in vaccinated pigs compared to non-vaccinated pigs after challenge with virulent L. intracellularis. The vaccinated pigs did not shed L. intracellularis bacteria or seroconvert before challenge, and a significantly higher proportion of non-vaccinated pigs shed L. intracellularis compared to the vaccinated pigs after challenge. Seroconversion occurred from 2 weeks post challenge, without significant differences in the pattern of seroconversion in the groups of vaccinated and non-vaccinated pigs [