Development and evaluation of recombinant epsilon antigen based indirect-ELISA for sero-monitoring of enterotoxaemia specific antibodies in small ruminants.

Epsilon toxin (Etx) is a major exotoxin secreted by Clostridium perfringens that causes Enterotoxaemia (ET) in small ruminants leading to 'sudden death' and is known to induce protective antibodies. This study describes an indirect-ELISA based on recombinant quadruple-point Etx-mutant protein (~ 36 kDa), a well-defined and non-toxic antigen to detect epsilon specific antibodies in sheep and goat sera samples. The assay was evaluated for diagnostic sensitivity and specificity, along with cross-reactivity, as well as compared with a commercial ELISA kit. Furthermore, rEtx-iELISA was used to assess antibody titers in randomly collected sheep (n = 584) and goat (n = 555) serum samples from different regions of Odisha, an ET endemic state in India. The rEtx mutant protein (125 ng/well) based indirect ELISA platform demonstrated the area under the curve (AUC) of 0.997, diagnostic sensitivity (DSn) of 98.7%, and diagnostic specificity (DSp) of 96.2% using a pre-established test serum panel with 95% CI of 93.2-99.2% (p < 0.0001) against an optimal cut-off criterion of 10.42 PP. A comparison of the assay with a commercial kit yielded a kappa value of 0.912. A higher (> 95%) degree of sero-positivity with an average protective flock immunity of > 56% (≥0.15 IU/mL) was noted in sheep and goat populations of Odisha state. The study indicated the potential diagnostic utility of recombinant Etx-mutant antigen based indirect-ELISA for sero-monitoring of ET-specific antibodies in small ruminants.