Comparison of indirect based ELISA by employing purified LLO and its synthetic peptides and cultural method for diagnosis of ovine listeriosis

Listeriolysin O (LLO) is a dominant antigen target of anti-listerial immunity. The present study described the development of an indirect ELISA employing immunodominant non-cross-reactive synthetic peptides of LLO (LLO-1 and LLO-2) and its comparison with that of purified LLO based indirect ELISA using serum samples collected from 59 ewes which either had abortion or had a history of abortion. Initially isolation of Listeria was attempted from the samples collected from the ewes. A high seropositivity (54.2%) was observed against purified LLO; however, after adsorption of positive sera with streptolysin O (SLO), the seropositivity was 23.7%. Overall seropositivity with LLO-1 and LLO-2 peptides revealed comparatively less cross-reactivity in comparison to that of purified LLO. Three of the four animals culturally positive for Listeria monocytogenes were also positive serologically for ALLO by both the ELISAs even after adsorption of test sera with SLO. Antibodies against purified LLO and synthetic LLO-1 peptide based ELISAs detected antibodies even in samples from which non-pathogenic Listeria spp. were isolated; however, LLO-2 peptide did not reveal any ALLO antibodies from those samples which were culturally positive for non-pathogenic Listeria. In conclusion, LLO-2 peptide can serve as an ideal virulent marker for serodiagnosis of ovine listeriosis. Further evaluation of LLO-2 peptide based indirect ELISA with more number of samples is needed.