Seroepidemiological survey of Rhodococcus equi infection in adult horses in Mongolia.

ELISA antibody titers against Rhodococcus equi strain ATCC 6939 were measured in serum samples collected from 644 horses aged 1-21 years at 13 provinces and the capital of Mongolia, of which 631 (98%) samples showed values of approximately zero. The mean antibody titers in the 14 areas ranged from -0.08 to -0.01. Nine samples (1.4%) exhibited titers of 0.1-0.2, and surprisingly, four horses (0.6%) each aged 2, 5, 8, and 9 years demonstrated positive values in the range of 0.3-0.8. Considering the age resistance characteristic of this disease, the positive horses were hypothesized to exhibit nonspecific reactions against R. equi antigens, potentially attributable to underlying infections, such as Corynebacterium pseudotuberculosis infection, or others as unidentified factors. Further investigation of these positive sera could potentially reveal previously unknown infectious diseases.