ABSTRACT
Immune responses to Rhodococcus equi were assayed in mares and foals on 7 studs in south-eastern Australia using skin test reactivity to the intradermal injection of culture filtrate and an indirect fluorescent antibody test. The prevalence of positive skin-test reactions did not differ between studs with a history of R. equi disease and those without but there were more mares with high antibody titres on studs with a disease history. A leucocyte extract prepared from mares that were skin-test positive was evaluated for its ability to protect foals exposed to experimental or natural challenge: 2 foals receiving leucocyte extract became skin-test positive and had resolving lesions present at post-mortem examination. All foals challenged experimentally developed serum antibody but only in those that became skin-test positive were the lesions resolving. In a field trial of leucocyte extract, using 450 foals over 2 foaling seasons, no significant difference was detected in morbidity or mortality rates between treatment and control groups. It is suggested that, since R. equi is so ubiquitous and most horses show immunological evidence of exposure, the development of clinical disease may be related to individual inability to cope with this organism. This may be due to inherited immunological unresponsiveness or to environmental factors which increase the challenge or decrease resistance.
