Isotype-specific Antibody Responses to Rhodococcus equi in Foals on a Horse-breeding Farm with a Persistent Incidence of R. equi Infection.

Isotype-specific antibody responses in 13 foals to Rhodococcus equi were studied during the first 20 weeks of life on a horse-breeding farm with a persistent incidence of R. equi infection. The levels of immunoglobulin (Ig) classes and IgG subclasses specific for R. equi antigens in the 13 foal sera were determined by ELISA and Western immunoblotting. Kinetics of the total amount of IgG and the number of WBC in the blood of 13 foals were also monitored. Soluble extract from R. equi strain ATCC 6939 was used as a routine EISA antigen. During the observation period, 8 of the 13 foals showed >0.3 OD value against ATCC 6939 antigen by a routine ELISA, which was tentatively fixed to be positive on the basis of readings made of healthy horse sera in previous studies. R. equi-specific IgGa was predominantly detected in these foals at 6 to 10 weeks after birth by ELISA with Tween 20-extracted ATCC 6939 antigens. Specific serum IgGb, and IgA were also detected, but they showed lower titers than IgGa antibodies in the foals. Specific IgG(T), IgGc and IgM was present consistently at low levels in most foals. On the other hand, IgGa and IgGb antibodies specific for 15-to 17-kDa antigens (VapA) were detected by immunoblotting at 5 to 7 weeks of age in most foals. Out of the 13 foals, 3 foals had a serum IgG concentrations less than 1,000 mg/dl (650 to 900 mg/dl) at the first week of birth. WBC counts tended to increase by 8 weeks with age in all foals. Virulent R. equi was isolated from the feces of foals during this study. This study suggests that R. equi-specific IgGa was predominantly measured by ELISA, and VapA-specific IgGa and IgGb were predominantly detected by immunoblotting in foals naturally exposed to R. equi on a farm with a persistent incidence of R. equi infection.