Immunoglobulin and specific antibody responses to Rhodococcus (Corynebacterium) equi infection in foals as measured by enzyme-linked immunosorbent assay.

Humoral immune response to intestinal Rhodococcus (Corynebacterium) equi in horses was studied by enzyme-linked immunosorbent assay. Anti-R. equi immunoglobulin M (IgM), IgG, and IgA antibodies were demonstrated in the healthy horse population. Adult horse levels of anti-R. equi IgM and IgG antibodies were reached by 5 to 9 weeks of age in two healthy newborn foals. R. equi was recovered from the foals in the range of 10(3) to 10(4) per g of intestinal contents. A 1-week-old foal was infected with R. equi by mouth daily for 9 weeks. The foal did not show any clinical signs of illness. Anti-R. equi IgM antibody values in the foal increased about 5 to 8 weeks after initial inoculation, similar to the naturally occurring immune response to intestinal R. equi. There were differences among the antibody responses to R. equi in healthy horses, foals with suspected infection, and infected foals. These results suggest that exposure to R. equi is widespread in the horse population and that intestinal R. equi is the most important source of antigenic stimulation for a naturally occurring immune response in horses.