Evidence that Chlamydia abortus vaccine strain 1B produces indistinguishable placental lesions to wild type strains

Abstract

Chlamydia abortus is the most commonly diagnosed cause of abortion in small ruminants around the world [1]. Control of chlamydial abortion is achieved in several European countries using an attenuated live 1B C. abortus vaccine strain, which can be distinguished from virulent wild-type strains by PCR-RFLP analysis [2]. Application of this method has provided molecular evidence that the 1B strain can cause abortion in ewes [3, 4]. The objective of this study was to define the distribution of lesions and bacterial load in cotyledons from ewes vaccinated with the 1B strain compared to normal wild-type infections. A Chlamydia-free flock of 75 multiparous adult ewes were vaccinated twice, two years apart, each prior to mating, with the commercial 1B vaccine. In the second lambing season following the last vaccination, placentae (n=116) were collected and analysed by C. abortus real-time qPCR [3]. Only two of the placentae, both from the same ewe, were found to be positive. Viable organisms were isolated from these placentae and confirmed by RFLP-PCR [3] to be vaccine-type. All cotyledons from these placentae were analysed by histopathology and immunohistochemistry [5], and compared with those from wild-type infected placentae. The lesions in the vaccine-type infected placentae were indistinguishable from the wild-type infected placentae in terms of their severity, load and distribution. These results suggest that the 1B vaccine strain of C. abortus can cause chlamydial abortion in ewes producing typical placental lesions to wild-type infected animals, and could be circulating with the potential to cause natural infection and disease.