ABSTRACT. We recently demonstrated the presence of virulence-associated protein antigen (VapA)-positive Rhodococcus equi in Jeju Island, Korea. These bacteria contained one of two distinct plasmid types, a 90-kb type II plasmid, which has been found in isolates from the native Kiso horses of Japan, and a new variant, a 90-kb type V plasmid. However, the genotypic characters of the VapA-positive R. equi from Jeju native horses and their environments are poorly understood. Ninety-eight isolates from soil samples and 89 isolates from fecal samples were collected from five farms that breed or have bred Jeju native horses, and were tested for the presence of VapA by immunoblotting and PCR. Of the 98 soil isolates and 89 fecal isolates, seven and 13 were VapA-positive R. equi, respectively. In 2003, two Jeju foals died suddenly and were brought to the Faculty of Veterinary Medicine, Cheju National University, for postmortem examination. Pure cultures of R. equi were isolated from the lung lesions of both foals. Rhodococcus equi is one of the most important bacterial pathogens in one-to three-month-old foals. Infections caused by this organism are characterized by chronic, suppurative bronchopneumonia, which is often accompanied by ulcerative enteritis and mesenteric lymphadenitis [1,6,9]. Since the first isolation of Rhodococcus equi from the lung abscesses of Swedish foals by Magnusson in 1923 [4], the disease has been described in many areas of the world [1,6]. The discovery of virulence-associated antigens and virulence plasmids has clarified some aspects of the virulence of R. equi [14,18]. Virulent R. equi, which are characterized by a virulence-associated 15-17-kDa protein antigen (VapA) and a virulence plasmid of 85-90 kb, have been isolated from the lesions of infected foals and the disease has been reproduced experimentally [22,23]. However, avirulent R. equi, which have no virulence-associated antigens or plasmids and do not cause disease in foals, are distributed widely in horses and their environments [10,23]. Epidemiological studies have revealed that horse-breeding farms with endemic infections are heavily contaminated with virulent R. equi [9,17].The restriction-enzyme digestion patterns of virulent virulence plasmids from foals have allowed the plasmids of virulent isolates to be categorized into 12 closely related types and have revealed geographic differences in the distributions of these virulence plasmids, not only throughout the world, but also within Japan [7,11,15,16,19,20,22]. In a recent study, we isolated virulent R. equi containing the 90-kb type II plasmid that has been found in isolates from the native Kiso horses of Japan, from fecal and environmental samples of the native Jeju horse and thoroughbred horse farms on Jeju Island, Korea, and a new variant, the 90-kb type V plasmid [20]. However, only one virulent R. equi was isolated from soil samples collected from a Jeju native horse farm [20].The purposes of this study were to investigate the prevalence of virulent R. equi in Jeju native ...
