The immunological response of foals to Rhodococcus equi: a review.

Normal horses of all ages regularly show evidence of having responded immunologically to R. equi, thus adding serological support to epidemiological evidence that this organism is a normal intestinal inhabitant. More animals from "diseased" farms show a stronger antibody response when compared with foals from "healthy" farms. Various serological tests have been used to detect evidence of infection and to relate antibody level to severity of disease. Anti-R. equi IgG antibody levels, as measured by ELISA, are raised significantly during natural infection. Clinical severity of pneumonia can be correlated with lower specific antibody responses. Following experimental infection, immunological responses can be detected by complement fixation, indirect immunofluorescence, ELISA, lymphocyte blastogenesis and skin testing. Very little work has been carried out to evaluate vaccines against R. equi infection and results have not been encouraging. Success in treatment has been reported following passive immunisation. Administration of immune leucocyte extracts has had no effect on morbidity or mortality rates. The widespread distribution of this organism, together with the relative infrequency of disease caused by it, suggest that R. equi may initiate infection only in such circumstances as a very high infectious challenge, immunological immaturity or deficiency in the host and genetic predisposition.