Rhodococcus equi infections in humans: an emerging zoonotic pathogen

Abstract

Rhodococcus equi is a facultative intracellular gram-positive coccobacillus which is a well-known cause of foal pneumonia and/or enteritis in equine veterinary medicine. More than 300 cases of R. equi infection have been reported since the first description of human disease in 1968. Most patients who become infected with R equi are immunocompromised, such as those infected with human immunodeficiency virus (HIV), recipients of organ transplantation, and patients receiving cancer treatment. However, there are increasing reports of the immunocompetent hosts. The pathogenicity of R. equi has been attributed to the presence of plasmid-encoded virulence-associated proteins (Vap). To date, three host-associated virulence plasmid types of R. equi have been identified as follows: the circular pVAPA and pVAPB, related, respectively, to equine and porcine isolates in 1991 and 1995, and a recently described linear pVAPN plasmid associated with bovine and caprine strains in 2015. More recently, these three plasmid types have been re-found in the human isolates which were isolated during 1980s to 1990s. Not only horses, but also pigs, goats, cattle and their environment should be considered as a potential source of R. equi for humans. In this review, we shed light on the current understanding of R. equi as an emerging zoonotic pathogen.