Immunity to and immunotherapy for Rhodococcus equi.

This research article studies the immune responses to Rhodococcus equi in mares and foals in Australia, testing whether a leucocyte extract from mares could protect foals from the bacteria. The study suggests that the occurrence of clinical disease may depend on individual resistance, possibly due to genetic or environmental factors.

Research Context and Methodology

• The research focused on Rhodococcus equi, a bacterium that commonly affects horses and can cause serious diseases. The immune response to this bacterium was studied in mares and foals across seven horse studs in southeastern Australia.
• Two main methods were used to measure the immune response: a skin test reaction to an injected culture filtrate, and an indirect flourescent antibody test.
• The researchers looked for differences in reactions between studs with a history of R. equi disease and those without.

Discoveries from Testing and Analysis

• The results indicated that prevalence of skin-test positive reactions was not different between studs with and without a history of R. equi disease. However, there were more mares with high antibody titres in studs with a disease history.
• These results suggest that most horses are exposed to R. equi and show immunological evidence of it, but not all develop the disease. The researchers propose that the development of clinical disease may be tied to individual abilities to manage the bacterium, possibly due to inherited differences in immune response or environmental factors that could amplify the challenge or decrease resistance.

Evaluation of Immunotherapy

• A leucocyte extract (a type of white blood cell extract) was created from mares who tested positive in the skin test. This was tested on foals to ascertain if it could provide protection against R. equi.
• The outcome indicated that two foals which received the leucocyte extract became skin-test positive and had resolving lesions at the post-mortem examination stage.
• All foals that were challenged experimentally developed serum antibody, but only in those that tested skin-positive were the lesions resolving.
• A larger field trial using the leucocyte extract on 450 foals over two foaling seasons did not show a significant difference in disease or death rates between foals that received the treatment and the control group.

Conclusions and Implications

• The study implies that the occurrence of clinical disease from R. equi may be more dependent on individual immune response capacities, with both inherited unresponsiveness and environmental factors playing a role.
• The immunotherapeutic potential of leucocytes extract did not show significant protective effects against R. equi in this study, suggesting more research is required to find effective prevention methods.