Association between antimicrobial treatment of subclinical pneumonia in foals and selection of macrolide- and rifampicin-resistant Rhodococcus equi strains at horse-breeding farms in central Kentucky

OBJECTIVE To compare soil concentrations of macrolide- and rifampicin-resistant Rhodococcus equi strains (MRRE) on horse-breeding farms that used thoracic ultrasonographic screening (TUS) to identify foals with subclinical pneumonia combined with subsequent administration of macrolides and rifampin to affected foals (TUS farms) versus soil concentrations on farms that did not (non-TUS farms), determine whether the combined use of TUS and antimicrobial treatment of subclinically affected foals was associated with soil concentration of MRRE, and assess whether there were temporal effects on soil concentrations of MRRE during the foaling season. SAMPLES 720 soil samples and 20 completed questionnaires from 20 horse-breeding farms (10 TUS farms and 10 non-TUS farms) in central Kentucky. PROCEDURES A questionnaire was used to gather information from participating farms about their 2019 foaling season. Soil samples were collected during January, March, May, and July 2019 for bacterial culture and antimicrobial susceptibility testing to identify any isolates of MRRE. Results were compared for TUS farms versus non TUS farms. Linear mixed-effects modeling was used to evaluate for potential associations between the soil concentration of MRRE and the use of TUS. RESULTS Overall, the sum of the mean soil concentrations of MRRE was significantly higher for TUS farms (8.85 log(10)-transformed CFUs/g) versus non-TUS farms (7.37 log(10)-transformed CFUs/g). CONCLUSIONS AND CLINICAL RELEVANCE Our findings indicated that farms that use TUS to identify foals with subclinical pneumonia for antimicrobial treatment select for antimicrobial-resistant R equi strains. Because prognosis is worse for foals infected with resistant versus nonresistant strains of R equi, prudent use of antimicrobials to treat foals with subclinical pulmonary lesions attributed to R equi is recommended.