Pneumonia is the leading cause of disease and death in foals in Texas and ranks 3rd as a cause of morbidity and 2nd as a cause of mortality in the United States. Rhodococcus equi is considered the most common cause of severe pneumonia. The disease is endemic at many horse-breeding farms with cumulative incidence often exceeding 40% of the foal crop. At farms where the disease is endemic, the costs resulting from veterinary care, long-term therapy, and foal mortality are very high. Also, foals that recover from the disease are less likely to race as adults. To date, there is no approved vaccine effective for the prevention of pneumonia caused by R. equi and the control of R. equi infections on many endemic farms currently relies on early detection of pulmonary lesions using thoracic ultrasonography and initiation of treatment with a macrolide in combination with rifampicin prior to development of clinical signs. Most recently, we have documented that mass antimicrobial treatment of subclinically affected foals with ultrasonographic pulmonary lesions has selected for antimicrobial resistance over time, with isolates of R. equi resistant to all macrolides and rifampicin now being cultured from up to 40% of the foals at a farm in Kentucky. Widespread macrolide and rifampicin resistance of R. equi isolates is a major emerging problem facing the equine industry and might also adversely impact human health. However, it is unknown if resistant isolates of R. equi are widespread across many horse breeding farms or if it is an isolated problem on a small number of farms. Our long-range goal is to limit the spread of resistant isolates of R. equi. However, achievement our long-range goal will only be possible if we have a good understanding of the epidemiology of macrolide and rifampicin resistance in isolates of R. equi at horse breeding farms. Specifically, we will determine the prevalence and genetic background of resistant R. equi isolates to macrolide and rifampicin in horse-breeding farms endemic for the disease, and if resistance increases throughout breeding season relative to management aspects of each of the farms.