Production of Rhodococcus equi-specific horse polyclonal antibodies and evaluation of their therapeutic efficacy and prophylactic potentials in a mouse model

Rhodococcus equi (R. equi), a Gram-positive facultative intracellular pathogen, is the most widely recognised cause of pneumonia in foals. Several antimicrobial drugs are effective against R. equi in vitro, however, many of these agents are ineffective in vivo because R. equi is a facultative intracellular pathogen that can survive and replicate in macrophages. The present study was planned to prepare R. equi-specific hyper-immune horse polyclonal antibodies and to evaluate their therapeutic and prophylactic efficacy against R. equi-specific infections in mice models. Four horses were immunised with a locally prepared formalin-inactivated R. equi vaccine adjuvanted with mineral oil adjuvant. Each horse was immunised with 3 doses of the prepared vaccine (3 ml/dose) at 2-week intervals. One week after the last vaccination dose, serum was collected by plasmapheresis. The titre of R. equi-specific antibodies was determined by ELISA and reached up to 104 ELISA units/ml two weeks after the primary immunisation and up to 105 ELISA units/ml one week after the 2nd immunisation dose. The therapeutic and prophylactic efficacy of the prepared R. equi-specific antibodies was evaluated in a mice model, where 87.5% of mice challenged with virulent R. equi bacteria (0.2 ml/mice of 4×108 CFU/ml) were successfully treated just after the appearance of the clinical signs of the infection by I.V. injection of 0.2 ml of R. equi-specific polyclonal antibodies. Also, the prophylactic efficacy of the prepared R. equi-specific antibodies for induction of immediate protection was ensured where 100% of passively immunised mice could resist challenges with virulent R. equi bacteria. The R. equi-specific polyclonal antibodies titre used to evaluate its protective and prophylactic efficacy was 105 ELISA units/ml.